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The science behind the Big Bang theory

The first suggestion of the Big Bang was in 1912. Astronomer Vesto Slipher “conducted a series of observations of spiral galaxies (which were believed to be nebulae) and measured their Doppler Redshift. In almost all cases, the spiral galaxies were observed to be moving away from our own,” according this phys.org article . Later in the 1930s, Edwin Hubble used the world’s largest telescope and showed that the distant galaxies all appeared to be receding from us. That means that the farther away they are, the faster they are retracting. The first and most confident evidence we have came from 1964 when scientists at the Bell Labs discovered the cosmic microwave background radiation, confirming there was a Big Bang. This revolutionized cosmology, how we see the universe, and how we view our place within it.

Scientists have come up with several possible explanations for what happened before the Big Bang, if anything at all, and it is entirely possible that there was no previous era. Assuming this is true, it means that matter, energy, space, and time just began abruptly.

What happened after the Big Bang?

What happened immediately after the explosions is called the Planck Era: the earliest known period of time. According to the theories of physics, one second after the Big Bang, the heat of the universe caused atoms to collide with enough force to create a ten billion degree soup of neutrons, protons, electrons, positrons, photons, and neutrinos. Essentially, cosmic inflations created a soup of sub atomic particle plasma. It appears that this is what gave rise to dark matter and likely the phase in which matter gains superiority over anti matter.

Within the first 300 seconds of the existence of the universe, the elements hydrogen, helium, and some lithium form from the protons and neutrons. This process is called nucleosynthesis and is a theory that accurately predicts the abundances of elements and isotopes found in the early samples of the universe, like some of oldest stars. This verification is a strong indication that our model of the universe is accurate. 300,000 years later, when the nearly uniform soup cooled, atoms formed other nuclei. Photons ceased to scatter through space, turning the prior opaque universe into one with visible light. Those same photons, the actual after glow of the Big Bang known as  cosmic background radiation , can still be observed today.

The cosmic background radiation and the Big Bang

Space agencies have launched three missions to study this cosmic background radiation, taking baby pictures of the universe only 400,000 years after its birth. The first two probes map the primordial hot and cold spots in cosmic background radiation, measuring temperature differences that are nearly uniformly distributed across the universe. A third mission, with instruments sensitive to temperature variations of a few millions of a degree, made the most accurate maps of the microwave background radiation yet.

The dark ages before stars were born

About one million years after the Big Bang, we enter a period call  the Dark Ages , which are known as the final frontier of cosmology. Little is known about this period, except that is was the period before the first stars were born. The Dark Ages are thought to have lasted about 100 million years. However, due to the limit of current observations, the oldest object we can see are at a time when the universe was only several hundred million years old. Two future projects that have already begun construction, are specifically designed to shed some light on this era and hopefully bring the Dark Ages to an end.

Then, ten billion years after the Big Bang, dark energy, a mysterious force starts to accelerate. After that, 13.8 billion years after the Big Bang, we reach our time today.

The end of the universe

The ultimate fate of the universe all hinges on dark energy. If the universe continues to grow at about the same pace, 30 billion years from now, all the galaxies would be pulled from our view and all the evidence of the Big Bang would be completely lost forever. This would result in all the last stars burning out in about 100 trillion years, so we have plenty of time left.

Dark energy could also intensify resulting in a Big Rip scenario. This would happen in approximately 50 billion years from now. Dark energy would effectively tear everything apart, from superclusters to atoms. On the contrary, if dark energy slowed down, then this would give gravity the upper hand and lead to a collapse 30 billion years from now, ultimately resulting in a Big Crunch. 

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What is the Big Bang Theory?

The Big Bang Theory explains how the universe began.

The birth of the universe

• Big Bang theory FAQs

Modelling the Big Bang

• How old is the universe?

What are gravitational waves?

• Expansion vs explosion

The expansion of the universe

Jwst and the big bang.

• The "Big Bang Theory" TV show

Additional resources

The Big Bang Theory is the leading explanation for how the universe began. Simply put, it says the universe as we know it started with an infinitely hot and dense single point that inflated and stretched — first at unimaginable speeds, and then at a more measurable rate — over the next 13.7 billion years to the still-expanding cosmos that we know today.

Existing technology doesn't yet allow astronomers to literally peer back at the universe's birth, much of what we understand about the Big Bang comes from mathematical formulas and models. Astronomers can, however, see the "echo" of the expansion through a phenomenon known as the cosmic microwave background .

While the majority of the astronomical community accepts the theory, there are some theorists who have alternative explanations besides the Big Bang — such as eternal inflation or an oscillating universe.

Related: What happened before the Big Bang?

Around 13.7 billion years ago, everything in the entire universe was condensed in an infinitesimally small singularity, a point of infinite denseness and heat. 

Suddenly, an explosive expansion began, ballooning our universe outwards faster than the speed of light . This was a period of cosmic inflation that lasted mere fractions of a second — about 10^-32 of a second, according to physicist Alan Guth’s 1980 theory that changed the way we think about the Big Bang forever. 

When cosmic inflation came to a sudden and still-mysterious end, the more classic descriptions of the Big Bang took hold. A flood of matter and radiation, known as "reheating," began populating our universe with the stuff we know today: particles, atoms, the stuff that would become stars and galaxies and so on.

This all happened within just the first second after the universe began, when the temperature of everything was still insanely hot, at about 10 billion degrees Fahrenheit (5.5 billion Celsius), according to NASA . The cosmos now contained a vast array of fundamental particles such as neutrons, electrons and protons — the raw materials that would become the building blocks for everything that exists today.

This early "soup" would have been impossible to actually see because it couldn't hold visible light. "The free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds," NASA stated. Over time, however, these free electrons met up with nuclei and created neutral atoms or atoms with equal positive and negative electric charges. 

This allowed light to finally shine through, about 380,000 years after the Big Bang.

Sometimes called the "afterglow" of the Big Bang, this light is more properly known as the cosmic microwave background (CMB). It was first predicted by Ralph Alpher and other scientists in 1948 but was found only by accident almost 20 years later .

Related: Peering back to the Big Bang & early universe

This accidental discovery happened when Arno Penzias and Robert Wilson, both of Bell Telephone Laboratories in New Jersey, were building a radio receiver in 1965 and picked up higher-than-expected temperatures, according to a NASA article . At first, they thought the anomaly was due to pigeons trying to roost inside the antenna and their waste, but they cleaned up the mess and killed the pigeons and the anomaly persisted.

Simultaneously, a Princeton University team led by Robert Dicke was trying to find evidence of the CMB and realized that Penzias and Wilson had stumbled upon it with their strange observations. The two groups each published papers in the Astrophysical Journal in 1965.

Big Bang theory FAQs answered by an expert

We asked Jason Steffens, assistant professor of physics and astronomy at the University of Nevada, Las Vegas, a few frequently asked questions about the Big Bang Theory. 

Jason Steffens is an assistant professor of physics and astronomy at the University of Nevada, Las Vegas.

Has the Big Bang Theory been proven?

This isn't really a statement that we can make in general. The best we can do is say that there is strong evidence for the Big Bang Theory and that every test we throw at it comes back in support of the theory. Mathematicians prove things, but scientists can only say that the evidence supports a theory with some degree of confidence that is always less than 100%.

So, a short answer to a slightly different question is that all of the observational evidence that we've gathered is consistent with the predictions of the Big Bang Theory. The three most important observations are:

1) The Hubble Law shows that distant objects are receding from us at a rate proportional to their distance — which occurs when there is uniform expansion in all directions. This implies a history where everything was closer together.

2) The properties of the cosmic microwave background radiation (CMB). This shows that the universe went through a transition from an ionized gas (a plasma) and a neutral gas. Such a transition implies a hot, dense early universe that cooled as it expanded. This transition happened after about 400,000 years following the Big Bang.

3) The relative abundances of light elements (He-4, He-3, Li-7, and Deuterium). These were formed during the era of Big Bang Nucleosynthesis (BBN) in the first few minutes after the Big Bang. Their abundances show that the universe was really hot and really dense in the past (as opposed to the conditions when the CMB was formed, which was just regular hot and dense — there's about a factor of a million difference in temperature between when BBN occurred and when the CMB occurred).

Is there any occurrence that contradicts the Big Bang Theory?

Not that I know of. There are some issues that arise with the simplest model of the Big Bang, but those can be resolved by invoking a physical process that is still consistent with the basic premise of the Big Bang Theory. Specifically, the fact that the CMB temperature is the same everywhere, that the universe does not appear to have any curvature, and that density fluctuations from quantum mechanical predictions do not produce galaxy clusters of the right size and shape today. These three issues are resolved with the theory of inflation — which is part of the broader Big Bang Theory.

When was the Big Bang Theory established?

Who came up with the idea?

Hubble was really the person who set up the observations. Evidence continued to mount, especially in the 1970s with the detection of the CMB. The term "Big Bang" was first used in the late 1940s by the astronomer Fred Hoyle — eventually, it caught on in the 1970s.

Because we can't see it directly, scientists have been trying to figure out how to "see" the Big Bang through other measures. In one case, cosmologists are pressing rewind to reach the first instant after the Big Bang by simulating 4,000 versions of the current universe on a massive supercomputer. 

"We are trying to do something like guessing a baby photo of our universe from the latest picture," study leader Masato Shirasaki, a cosmologist at the National Astronomical Observatory of Japan (NAOJ), told our sister website Live Science . 

With what is known about the universe today, the researchers in this 2021 study compared their understanding of how gravitational forces interacted in the primordial universe with their thousands of computer-modeled universes. If they could predict the starting conditions of their virtual universes, they hoped to be able to accurately predict what our own universe may have looked like back at the beginning. 

Other researchers have chosen different paths to interrogate our universe's beginnings. 

In a 2020 study, researchers did so by investigating the split between matter and antimatter. In the study, not yet peer-reviewed, they proposed that the imbalance in the amount of matter and antimatter in the universe is related to the universe's vast quantities of dark matter, an unknown substance that exerts influence over gravity and yet doesn't interact with light. They suggested that in the crucial moments immediately after the Big Bang, the universe may have been pushed to make more matter than its inverse, antimatter, which then could have led to the formation of dark matter .

The age of the universe

The CMB has been observed by many researchers now and with many spacecraft missions. One of the most famous space-faring missions to do so was NASA's Cosmic Background Explorer (COBE) satellite, which mapped the sky in the 1990s.

Several other missions have followed in COBE's footsteps, such as the BOOMERanG experiment (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck satellite .

Planck's observations, first released in 2013, mapped the CMB in unprecedented detail and revealed that the universe was older than previously thought: 13.82 billion years old, rather than 13.7 billion years old. The research observatory's mission is ongoing and new maps of the CMB are released periodically.

Related: How old is the universe?

The maps give rise to new mysteries, however, such as why the Southern Hemisphere appears slightly redder (warmer) than the Northern Hemisphere. The Big Bang Theory says that the CMB would be mostly the same, no matter where you look.

Examining the CMB also gives astronomers clues as to the composition of the universe. Researchers think most of the cosmos is made up of matter and energy that cannot be "sensed" with our conventional instruments, leading to the names " dark matter " and " dark energy ." It is thought that only 5% of the universe is made up of matter such as planets, stars and galaxies .

While astronomers study the universe's beginnings through creative measures and mathematical simulations, they've also sought proof of its rapid inflation. They have done this by observing gravitational waves , tiny perturbations in space-time that ripple outwards from great disturbances like, for instance, two colliding black holes or the universe's birth.

According to leading theories, in the first second after the universe was born, our cosmos ballooned faster than the speed of light . (That, by the way, does not violate Albert Einstein's speed limit. He once said that light speed is the fastest anything can travel within the universe — but that statement did not apply to the inflation of the universe itself.)

As the universe expanded, it created the CMB and a similar "background noise" made up of gravitational waves that, like the CMB, were a sort of static, detectable from all parts of the sky. Those gravitational waves, according to the LIGO Scientific Collaboration , produced a theorized barely-detectable polarization, one type of which is called "B-modes."

In 2014, astronomers said they had found evidence of B-modes using an Antarctic telescope called "Background Imaging of Cosmic Extragalactic Polarization," or BICEP2.

"We're very confident that the signal that we're seeing is real, and it's on the sky," lead researcher John Kovac, of the Harvard-Smithsonian Center for Astrophysics told Space.com in March 2014.

But by June, the same team said that their findings could have been altered by galactic dust getting in the way of their field of view. That hypothesis was supported by new results from the Planck satellite.

By January 2015, researchers from both teams working together "confirmed that the Bicep signal was mostly, if not all, stardust," the New York Times said .

However, since then gravitational waves have not only been confirmed to exist, they have been observed multiple times. 

These waves, which are not B-modes from the birth of the universe but rather from more recent collisions of black holes, have been detected multiple times by the Laser Interferometer Gravitational-Wave Observatory (LIGO), with the first-ever gravitational wave detection taking place in 2016. 

A major gravitational wave breakthrough was announced on June 28, 2023 when teams of scientists around the world reported the discovery of a "low-pitch hum" of these cosmic ripples flowing through the Milky Way. While astronomers don't definitively know what's causing the hum, the detected signal is "compelling evidence" and consistent with theoretical expectations of gravitational waves emerging from copious pairs of "the most massive black holes in the entire universe" weighing as much as billions of  suns , said Stephen Taylor, a gravitational wave astrophysicist at Vanderbilt University in Tennessee who co-led the research.

Read more: The gravitational wave background of the universe has been heard for the 1st time

Was the Big Bang an explosion?

Although the Big Bang is often described as an "explosion", that's a misleading image. In an explosion, fragments are flung out from a central point into a pre-existing space. If you were at the central point, you'd see all the fragments moving away from you at roughly the same speed. 

But the Big Bang wasn't like that. It was an expansion of space itself – a concept that comes out of Einstein's equations of general relativity but has no counterpart in the classical physics of everyday life. It means that all the distances in the universe are stretching out at the same rate. Any two galaxies separated by distance X are receding from each other at the same speed, while a galaxy at distance 2X recedes at twice that speed.

The universe is not only expanding, but expanding faster. This means that with time, nobody will be able to spot other galaxies from Earth or any other vantage point within our galaxy.

"We will see distant galaxies moving away from us, but their speed is increasing with time," Harvard University astronomer Avi Loeb said in a March 2014 Space.com article.

"So, if you wait long enough, eventually, a distant galaxy will reach the speed of light. What that means is that even light won't be able to bridge the gap that's being opened between that galaxy and us. There's no way for extraterrestrials on that galaxy to communicate with us, to send any signals that will reach us, once their galaxy is moving faster than light relative to us."

Related: 5 weird facts about seeing the universe's birth

— The 1st few seconds of the Big Bang: What we know and what we don't

— The universe: Big Bang to now in 10 easy steps

— Images: Black holes of the universe

Some physicists also suggest that the universe we experience is just one of many. In the " multiverse " model, different universes would coexist with each other like bubbles lying side by side. The theory suggests that in that first big push of inflation, different parts of space-time grew at different rates . This could have carved off different sections — different universes — with potentially different laws of physics.

Related: Best multiverse movies and TV shows: from Dr Strange to Dr Who

"It's hard to build models of inflation that don't lead to a multiverse," Alan Guth, a theoretical physicist at the Massachusetts Institute of Technology, said during a news conference in March 2014 concerning the gravitational waves discovery. (Guth is not affiliated with that study.)

"It's not impossible, so I think there's still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [the idea of a] multiverse seriously."

While we can understand how the universe we see came to be, it's possible that the Big Bang was not the first inflationary period the universe experienced. Some scientists believe we live in a cosmos that goes through regular cycles of inflation and deflation, and that we just happen to be living in one of these phases.

A telescope is almost like a time machine, allowing us to peer back into the distant past. With the aid of the Hubble space telescope, NASA has shown us galaxies as they were many billions of years ago — and Hubble's successor, the James Webb Space Telescope , has the ability to look even deeper into the past. 

NASA hopes it will see all the way back to when the first galaxies formed, nearly 13.6 billion years ago. And unlike Hubble, which sees mainly in the visible waveband , JWST is an infrared telescope — a big advantage when looking at very distant galaxies. The expansion of the universe means that waves emitted from them are stretched out, so light that was emitted at visible wavelengths actually reaches us in the infrared.

The Big Bang Theory: becoming a household name

The name "Big Bang Theory" has been a popular way to talk about the concept among astrophysicists for decades, but it hit the mainstream in 2007 when a comedy T.V. show with the same name premiered on CBS. 

Running for 279 episodes over 12 seasons, the show "The Big Bang Theory" followed the lives of a group of scientists, which included physicists, astrophysicists and aerospace engineers. The show explores the group's nerdy friendships, romances and squabbles. Its first season premiered on Sept. 24, 2007, and the show officially ended on May 16, 2019.

Although the show itself didn't dive too much into actual science, the showrunners did hire UCLA astrophysicist David Saltzberg as a science consultant for the entire run of the show, according to Science magazine . Science consultants are often hired for sci-fi and science-related shows and movies to help keep certain aspects realistic.

Thanks to Saltzberg, the characters' vocabulary included a host of science jargon and the whiteboards in the background of labs, offices and apartments throughout the show were filled with a variety of equations and information. 

Over the course of the show, Saltzberg said, those whiteboards became coveted space as researchers sent him new work that they hoped might be featured there. In one episode, Saltzberg recalled, new evidence of gravitational waves was scrawled across a whiteboard that ostensibly belonged to famed physicist Steven Hawking, who also approved the text.

The show took some liberties , as it was fictional. This included fabricating some new scientific concepts and fictionalizing the politics of Nobel prizes and academia, according to Fermilab physicist Don Lincoln.

Related: How 'The Big Bang Theory' sent Howard Wolowitz to space

Notably, several characters in the series take trips. One episode sees main characters Leonard, Sheldon, Raj and Howard set out on a research expedition to the Arctic — many physics experiments are best performed at or near the extreme environments of the poles. Another put aerospace engineer Howard on a Russian Soyuz spacecraft and, later, a model of the International Space Station along with real-life astronaut Mike Massimino .  

Discover more about CMB on NASA's webpage on putting the Big Bang theory to the test. NASA has also put together what the Big Bang might have looked it in this animation . Here are 5 quick facts about the Big Bang from How It Works magazine. 

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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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The Big Bang Theory: How the Universe Began

The Big Bang theory represents cosmologists ' best attempts to reconstruct the 14 billion year story of the universe based on the sliver of existence visible today.

Different people use the term "Big Bang" in different ways. Most generally, it illustrates the arc of the observable universe as it thinned out and cooled down from an initially dense, hot state. This description boils down to the idea that the cosmos is expanding, a broad principle analogous to survival of the fittest in biology that few would consider debatable.

More specifically, the Big Bang can also refer to the birth of the observable universe itself — the moment something changed, kickstarting the events that led to today. Cosmologists have argued for decades about the details of that fraction of a second, and the discussion continues today. [ From Big Bang to Present: Snapshots of Our Universe Through Time ]

The classic Big Bang theory

For most of human history, observers of the sky assumed it eternal and unchanging. Edwin Hubble dealt this story an experimental blow in the 1920s when his observations showed both that galaxies outside the Milky Way existed, and that their light appeared stretched — a sign that they were rushing away from Earth .

George Lemaître, a contemporary Belgian physicist, interpreted data from Hubble and others as evidence of an expanding universe, a possibility permitted by Einstein's recently published field equations of general relativity . Thinking backwards, Lemaître inferred that today's separating galaxies must have started out together in what he called the "primeval atom."

The first public use of the modern term for Lemaître's idea actually came from a critic — English astronomer Fred Hoyle. On March 28, 1949, Hoyle coined the phrase during a defense of his preferred theory of an eternal universe that created matter to cancel out the dilution of expansion. Hoyle said the notion that "all matter of the universe was created in one big bang at a particular time in the remote past," was irrational. In later interviews, Hoyle denied intentionally inventing a slanderous name , but the moniker stuck, much to the frustration of some.

"The Big Bang is a really bad term," said Paul Steinhardt, a cosmologist at Princeton. "The Big Stretch would capture the right idea." The mental image of an explosion causes all kinds of confusion, according to Steinhardt. It implies a central point, an expanding frontier, and a scene where light shrapnel flies faster than heavier chunks. But an expanding universe looks nothing like that, he said. There's no center, no edge, and galaxies large and small all slide apart in the same way (although more distant galaxies move away faster under the cosmologically recent influence of dark energy).

Regardless of its name, the Big Bang theory found widespread acceptance for its unparalleled ability to explain what we see. The balance of light with particles like protons and neutrons during the first 3 minutes, for instance, let early elements form at a rate predicting the current amounts of helium and other light atoms.

"There was a small window in time where it was possible for nuclei to form," said Glennys Farrar, a cosmologist at New York University. "After that, the universe kept expanding and they couldn't find each other, and before [the window] it was too hot."

A cloudy plasma filled the universe for the next 378,000 years, until further cooling let electrons and protons form neutral hydrogen atoms, and the fog cleared. The light emitted during this process, which has since stretched into microwaves, is the earliest known object researchers can study directly. Known as the cosmic microwave background (CMB) radiation, many researchers consider it the strongest evidence for the Big Bang.

An explosive update

But as cosmologists pushed farther back into the universe's first moments, the story unraveled. General relativity's equations suggested an initial speck of unlimited heat and density — a singularity. In addition to not making much physical sense, a singular origin didn't match the smooth, flat CMB. Fluctuations in the speck's formidable temperature and density would have produced swaths of sky with different properties , but the CMB's temperature varies by just a fraction of a degree. The curvature of space-time also looks quite flat, which implies an initially near-perfect balance of matter and curvature that most cosmologists find improbable.

Alan Guth proposed a new picture of the first fraction of a second in the 1980s, suggesting that the universe spent its earliest moments growing exponentially faster than it does today. At some point this process stopped, and putting on the brakes produced a dense and hot (but not infinitely so) mess of particles that takes the place of the singularity. "In my own mind I think of that as the Big Bang, when the universe got hot," Farrar said.

The inflation theory, as it's called, now has a plethora of competing models. Although no one knew much about what made the universe expand so rapidly, the theory has grown popular for its ability to explain the seemingly improbable featureless CMB: Inflation preserved minor fluctuations (which developed into today's galaxy clusters), while flattening the major ones. "It's a very sweet story," Steinhardt said, who helped develop the theory. "It's the one we tell our kids."

Beyond inflation

Recent research has introduced two wrinkles into the inflation theory's cosmic narrative. Work by Steinhardt and others suggests that inflation would have stopped in some regions (such as our observable universe) but continued in others, producing an array of separate territories with "every conceivable set of cosmological properties," as Steinhardt puts it. Many physicists find this " multiverse " picture distasteful, because it makes an infinite number of untestable predictions.

On the experimental front, cosmologists expect that inflation should have produced galaxy-spanning gravitational waves in the CMB just as it produced slight temperature and density variations. Current experiments should be sensitive enough to find them, but the primordial space-time ripples haven't shown up (despite one false alarm in 2014 ).

Many researchers await more precise CMB measurements that could kill, or validate , the many inflation models that still stand. Other physicists, however, don't see the cosmos's smoothness as a problem at all — it started off uniform and needs no explanation .

While experimentalists strive for new levels of precision, some theorists have turned away from inflation to seek other ways to squash the universe flat. Steinhardt, for instance, is working on a "big bounce" model, which pushes the starting clock back even further, to an earlier period of contraction that smoothed space-time and set the stage for an explosive expansion. He hopes that before too long, new signatures, in addition to problems like the lack of primordial gravitational waves, will set cosmologists up with a new creation story to tell. "Are there any other observable features to look for?" Steinhardt said, "Ask me again in a few years and I hope to have an answer."

Additional resources :

• Fermilab's Don Lincoln explains exactly what the Big Bang theory does, and doesn't say.
• Read about why some think it's odd how flat the universe is .
• PBS's SpaceTime explains why inflation has proved such an attractive idea .

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Charlie Wood is a staff writer at Quanta Magazine, where he covers physics both on and off the planet. In addition to Live Science, his work has also appeared in Popular Science, Scientific American, The Christian Science Monitor, and other publications. Previously, he taught physics and English in Mozambique and Japan, and he holds an undergraduate degree in physics from Brown University. 

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December 18, 2015

What is the Big Bang Theory?

by Matt Williams, Universe Today

How was our Universe created? How did it come to be the seemingly infinite place we know of today? And what will become of it, ages from now? These are the questions that have been puzzling philosophers and scholars since the beginning the time, and led to some pretty wild and interesting theories. Today, the consensus among scientists, astronomers and cosmologists is that the Universe as we know it was created in a massive explosion that not only created the majority of matter, but the physical laws that govern our ever-expanding cosmos.

This is known as The Big Bang Theory. For almost a century, the term has been bandied about by scholars and non-scholars alike. This should come as no surprise, seeing as how it is the most accepted theory of our origins. But what exactly does it mean? How was our Universe conceived in a massive explosion, what proof is there of this, and what does the theory say about the long-term projections for our Universe?

The basics of the theory are fairly simple. In short, the Big Bang hypothesis states that all of the current and past matter in the Universe came into existence at the same time, roughly 13.8 billion years ago. At this time, all matter was compacted into a very small ball with infinite density and intense heat called a Singularity. Suddenly, the Singularity began expanding, and the universe as we know it began.

While this is not the only modern theory of how the Universe came into being – for example, there is the Steady State Theory or the Oscillating Universe Theory – it is the most widely accepted and popular. Not only does the model explain the origin of all known matter, the laws of physics, and the large scale structure of the Universe, it also accounts for the expansion of the Universe and a broad range of other phenomena.

Working backwards from the current state of the Universe, scientists have theorized that it must have originated at a single point of infinite density and finite time that began to expand. After the initial expansion, the theory maintains that Universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.

This all began roughly 13.8 billion years ago, and is thus considered to be the age of the universe. Through the testing of theoretical principles, experiments involving particle accelerators and high-energy states, and astronomical studies that have observed the deep universe, scientists have constructed a timeline of events that began with the Big Bang and has led to the current state of cosmic evolution.

However, the earliest times of the Universe – lasting from approximately 10 -43 to 10 -11 seconds after the Big Bang – are the subject of extensive speculation. Given that the laws of physics as we know them could not have existed at this time, it is difficult to fathom how the Universe could have been governed. What's more, experiments that can create the kinds of energies involved have not yet been conducted. Still, many theories prevail as to what took place in this initial instant in time, many of which are compatible.

Singularity:

Also known as the Planck Epoch (or Planck Era), this was the earliest known period of the Universe. At this time, all matter was condensed on a single point of infinite density and extreme heat. During this period, it is believed that the quantum effects of gravity dominated physical interactions and that no other physical forces were of equal strength to gravitation.

This Planck period of time extends from point 0 to approximately 10 -43 seconds, and is so named because it can only be measured in Planck time. Due to the extreme heat and density of matter, the state of the universe was highly unstable. It thus began to expand and cool, leading to the manifestation of the fundamental forces of physics.

From approximately 10 -43 second and 10 -36 , the universe began to cross transition temperatures. It is here that the fundamental forces that govern the Universe are believed to have began separating from each other. The first step in this was the force of gravitation separating from gauge forces, which account for strong and weak nuclear forces and electromagnetism.

Then, from 10 -36 to 10 -32 seconds after the Big Bang, the temperature of the universe was low enough (10 28 K) that the forces of electromagnetism (strong force) and weak nuclear forces (weak interaction) were able to separate as well, forming two distinct forces.

Inflation Epoch:

With the creation of the first fundamental forces of the universe, the Inflation Epoch began, lasting from 10 -32 seconds in Planck time to an unknown point. Most cosmological models suggest that the Universe at this point was filled homogeneously with a high-energy density, and that the incredibly high temperatures and pressure gave rise to rapid expansion and cooling.

This began at 10 -37 seconds, where the phase transition that caused for the separation of forces also led to a period where the universe grew exponentially. It was also at this point in time that baryogenesis occurred, which refers to a hypothetical event where temperatures were so high that the random motions of particles occurred at relativistic speeds.

As a result of this, particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions, which is believed to have led to the predominance of matter over antimatter in the present universe. After inflation stopped, the universe consisted of a quark–gluon plasma, as well as all other elementary particles. From this point onward, the Universe began to cool and matter coalesced and formed.

Cooling Epoch:

As the universe continued to decrease in density and temperature, the energy of each particle began to decrease and phase transitions continued until the fundamental forces of physics and elementary particles changed into their present form. Since particle energies would have dropped to values that can be obtained by particle physics experiments, this period onward is subject to less speculation.

For example, scientists believe that about 10 -11 seconds after the Big Bang, particle energies dropped considerably. At about 10-6 seconds, quarks and gluons combined to form baryons such as protons and neutrons, and a small excess of quarks over antiquarks led to a small excess of baryons over antibaryons.

Since temperatures were not high enough to create new proton-antiproton pairs (or neutron-anitneutron pairs), mass annihilation immediately followed, leaving just one in 10 10 of the original protons and neutrons and none of their antiparticles. A similar process happened at about 1 second after the Big Bang for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons – and to a lesser extent, neutrinos.

A few minutes into the expansion, the period known as Big Bang nucleosynthesis also began. Thanks to temperatures dropping to 1 billion kelvin and the energy densities dropping to about the equivalent of air, neutrons and protons began to combine to form the universe's first deuterium (a stable isotope of Hydrogen) and helium atoms. However, most of the Universe's protons remained uncombined as hydrogen nuclei.

After about 379,000 years, electrons combined with these nuclei to form atoms (again, mostly hydrogen), while the radiation decoupled from matter and continued to expand through space, largely unimpeded. This radiation is now known to be what constitutes the Cosmic Microwave Background (CMB), which today is the oldest light in the Universe.

As the CMB expanded, it gradually lost density and energy, and is currently estimated to have a temperature of 2.7260 ± 0.0013 K (-270.424 °C/ -454.763 °F ) and an energy density of 0.25 eV/cm 3 (or 4.005×10 -14 J/m 3 ; 400–500 photons/cm 3 ). The CMB can be seen in all directions at a distance of roughly 13.8 billion light years, but estimates of its actual distance place it at about 46 billion light years from the center of the Universe.

Structure Epoch:

Over the course of the several billion years that followed, the slightly denser regions of the almost uniformly distributed matter of the Universe began to become gravitationally attracted to each other. They therefore grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures that we regularly observe today.

This is what is known as the Structure Epoch, since it was during this time that the modern Universe began to take shape. This consists of visible matter distributed in structures of various sizes, ranging from stars and planets to galaxies, galaxy clusters, and super clusters – where matter is concentrated – that are separated by enormous gulfs containing few galaxies.

The details of this process depend on the amount and type of matter in the universe, with cold dark matter , warm dark matter, hot dark matter, and baryonic matter being the four suggested types. However, the Lambda-Cold Dark Matter model (Lambda-CDM), in which the dark matter particles moved slowly compared to the speed of light, is the considered to be the standard model of Big Bang cosmology, as it best fits the available data.

In this model, cold dark matter is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. The Lambda refers to the Cosmological Constant, a theory originally proposed by Albert Einstein that attempted to show that the balance of mass-energy in the universe was static. In this case, it is associated with Dark Energy, which served to accelerate the expansion of the universe and keep its large-scale structure largely uniform.

Long-term Predictions:

Hypothesizing that the Universe had a starting point naturally gives rise to questions about a possible end point. If the Universe began as a tiny point of infinite density that started to expand, does that mean it will continue to expand indefinitely? Or will it one day run out of expansive force, and begin retreating inward until all matter crunches back into a tiny ball?

Answering this question has been a major focus of cosmologists ever since the debate about which model of the Universe was the correct one began. With the acceptance of the Big Bang Theory, but prior to the observation of Dark Energy in the 1990s, cosmologists had come to agree on two scenarios as being the most likely outcomes for our Universe.

In the first, commonly known as the "Big Crunch" scenario, the universe will reach a maximum size and then begin to collapse in on itself. This will only be possible if the mass density of the Universe is greater than the critical density. In other words, as long as the density of matter remains at or above a certain value (1-3 ×10 -26 kg of matter per m 3 ), the Universe will eventually contract.

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. In this scenario, known as the "Big Freeze", the Universe would go on until star formation eventually ceased with the consumption of all the interstellar gas in each galaxy. Meanwhile, all existing stars would burn out and become white dwarfs, neutron stars, and black holes.

Very gradually, collisions between these black holes would result in mass accumulating into larger and larger black holes. The average temperature of the universe would approach absolute zero, and black holes would evaporate after emitting the last of their Hawking radiation. Finally, the entropy of the universe would increase to the point where no organized form of energy could be extracted from it (a scenarios known as "heat death").

Modern observations, which include the existence of Dark Energy and its influence on cosmic expansion, have led to the conclusion that more and more of the currently visible universe will pass beyond our event horizon (i.e. the CMB, the edge of what we can see) and become invisible to us. The eventual result of this is not currently known, but "heat death" is considered a likely end point in this scenario too.

Other explanations of dark energy, called phantom energy theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion. This scenario is known as the "Big Rip", in which the expansion of the Universe itself will eventually be its undoing.

History of the Big Bang Theory:

The earliest indications of the Big Bang occurred as a result of deep-space observations conducted in the early 20th century. In 1912, American astronomer Vesto Slipher conducted a series of observations of spiral galaxies (which were believed to be nebulae) and measured their Doppler Redshift. In almost all cases, the spiral galaxies were observed to be moving away from our own.

In 1922, Russian cosmologist Alexander Friedmann developed what are known as the Friedmann equations, which were derived from Einstein's equations for general relativity. Contrary to Einstein's was advocating at the time with his a Cosmological Constant, Friedmann's work showed that the universe was likely in a state of expansion.

In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebula showed that these systems were indeed other galaxies. At the same time, Hubble began developing a series of distance indicators using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory. And by 1929, Hubble discovered a correlation between distance and recession velocity – which is now known as Hubble's law.

And then in 1927, Georges Lemaitre, a Belgian physicist and Roman Catholic priest, independently derived the same results as Friedmann's equations and proposed that the inferred recession of the galaxies was due to the expansion of the universe. In 1931, he took this further, suggesting that the current expansion of the Universe meant that the father back in time one went, the smaller the Universe would be. At some point in the past, he argued, the entire mass of the universe would have been concentrated into a single point from which the very fabric of space and time originated.

These discoveries triggered a debate between physicists throughout the 1920s and 30s, with the majority advocating that the universe was in a steady state. In this model, new matter is continuously created as the universe expands, thus preserving the uniformity and density of matter over time. Among these scientists, the idea of a Big Bang seemed more theological than scientific, and accusations of bias were made against Lemaitre based on his religious background.

Other theories were advocated during this time as well, such as the Milne Model and the Oscillary Universe model. Both of these theories were based on Einstein's theory of general relativity (the latter being endorsed by Einstein himself), and held that the universe follows infinite, or indefinite, self-sustaining cycles.

After World War II, the debate came to a head between proponents of the Steady State Model (which had come to be formalized by astronomer Fred Hoyle) and proponents of the Big Bang Theory – which was growing in popularity. Ironically, it was Hoyle who coined the phrase "Big Bang" during a BBC Radio broadcast in March 1949, which was believed by some to be a pejorative dismissal (which Hoyle denied).

Eventually, the observational evidence began to favor Big Bang over Steady State. The discovery and confirmation of the cosmic microwave background radiation in 1965 secured the Big Bang as the best theory of the origin and evolution of the universe. From the late 60s to the 1990s, astronomers and cosmologist made an even better case for the Big Bang by resolving theoretical problems it raised.

These included papers submitted by Stephen Hawking and other physicists that showed that singularities were an inevitable initial condition of general relativity and a Big Bang model of cosmology. In 1981, physicist Alan Guth theorized of a period of rapid cosmic expansion (aka. the "Inflation" Epoch) that resolved other theoretical problems.

The 1990s also saw the rise of Dark Energy as an attempt to resolve outstanding issues in cosmology. In addition to providing an explanation as to the universe's missing mass (along with Dark Matter, originally proposed in 1932 by Jan Oort), it also provided an explanation as to why the universe is still accelerating, as well as offering a resolution to Einstein's Cosmological Constant.

Significant progress was made thanks to advances in telescopes, satellites, and computer simulations, which have allowed astronomers and cosmologists to see more of the universe and gain a better understanding of its true age. The introduction of space telescopes – such as the Cosmic Background Explorer (COBE), the Hubble Space Telescope, Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck Observatory – have also been of immeasurable value.

Today, cosmologists have fairly precise and accurate measurements of many of the parameters of the Big Bang model, not to mention the age of the Universe itself. And it all began with the noted observation that massive stellar objects, many light years distant, were slowly moving away from us. And while we still are not sure how it will all end, we do know that on a cosmological scale, that won't be for a long, LONG time!

Source: Universe Today

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CERN Accelerating science

The early universe

All matter in the universe was formed in one explosive event 13.7 billion years ago – the Big Bang

The Big Bang

In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts. Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum. Hubble’s observation implied that distant galaxies were moving away from us, as the furthest galaxies had the fastest apparent velocities. If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together.

Hubble’s discovery was the first observational support for Georges Lemaître’s Big Bang theory of the universe, proposed in 1927. Lemaître proposed that the universe expanded explosively from an extremely dense and hot state, and continues to expand today. Subsequent calculations have dated this Big Bang to approximately 13.7 billion years ago. In 1998 two teams of astronomers working independently at Berkeley, California observed that supernovae – exploding stars – were moving away from Earth at an accelerating rate. This earned them the Nobel prize in physics in 2011 . Physicists had assumed that matter in the universe would slow its rate of expansion; gravity would eventually cause the universe to fall back on its centre. Though the Big Bang theory cannot describe what the conditions were at the very beginning of the universe, it can help physicists describe the earliest moments after the start of the expansion.

In the first moments after the Big Bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. A few millionths of a second later, quarks aggregated to produce protons and neutrons. Within minutes, these protons and neutrons combined into nuclei. As the universe continued to expand and cool, things began to happen more slowly. It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang. Heavier atoms such as carbon, oxygen and iron, have since been continuously produced in the hearts of stars and catapulted throughout the universe in spectacular stellar explosions called supernovae.

But stars and galaxies do not tell the whole story. Astronomical and physical calculations suggest that the visible universe is only a tiny amount (4%) of what the universe is actually made of. A very large fraction of the universe, in fact 26%, is made of an unknown type of matter called " dark matter ". Unlike stars and galaxies, dark matter does not emit any light or electromagnetic radiation of any kind, so that we can detect it only through its gravitational effects. 

An even more mysterious form of energy called “dark energy” accounts for about 70% of the mass-energy content of the universe. Even less is known about it than dark matter. This idea stems from the observation that all galaxies seems to be receding from each other at an accelerating pace, implying that some invisible extra energy is at work.

• Science News
• The origin of 'Big Bang': Unraveling the true story

The origin of 'Big Bang': Unraveling the true story

About the Author

At TOI World Desk, our dedicated team of seasoned journalists and passionate writers tirelessly sifts through the vast tapestry of global events to bring you the latest news and diverse perspectives round the clock. With an unwavering commitment to accuracy, depth, and timeliness, we strive to keep you informed about the ever-evolving world, delivering a nuanced understanding of international affairs to our readers. Join us on a journey across continents as we unravel the stories that shape our interconnected world. Read More

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Claudia Dreifus

On a bright spring morning 50 years ago, two young astronomers at Bell Laboratories were tuning a 20-foot, horn-shaped antenna pointed toward the sky over New Jersey. Their goal was to measure the Milky Way galaxy, home to planet Earth.

To their puzzlement, Robert W. Wilson and Arno A. Penzias heard the insistent hiss of radio signals coming from every direction—and from beyond the Milky Way. It took a full year of testing, experimenting and calculating for them and another group of researchers at Princeton to explain the phenomenon: It was cosmic microwave background radiation, a residue of the primordial explosion of energy and matter that suddenly gave rise to the universe some 13.8 billion years ago. The scientists had found evidence that would confirm the Big Bang theory, first proposed by Georges Lemaître in 1931.

“Until then, some cosmologists believed that the universe was in a steady state without a singular beginning,” says Wilson, now 78 and a senior scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “The finding helped rule that out.”

That assessment seems a bit modest for a discovery that received the Nobel Prize in Physics in 1978 and is now, on its semicentennial, celebrated as the Rosetta stone of modern cosmology, the key that has allowed generations of scientists to parse the origins of the universe.

Avi Loeb was a toddler on a farm in Israel when Wilson and Penzias began investigating those mysterious signals. Today, he’s a colleague of Wilson’s at the Center for Astrophysics and chair of Harvard’s astronomy department, and one of the world’s foremost researchers on what has been called the “cosmic dawn.” The theoretical physicist, now 52, has published more than 450 papers on aspects of the early universe, including the formation of stars and galaxies and the origins of the first black holes. He has done pioneering work on the three-dimensional mapping of the universe, and he has explored the implications of the impending collision between the Milky Way and the Andromeda galaxy (which will not happen, he adds, for several billion years).

Loeb recently made headlines with a paper submitted to the journal Astrobiology suggesting that just 15 million years after the Big Bang, the temperature from the cosmic background microwave radiation was 0 to 30 degrees Celsius—warm enough, he says, to allow “liquid water to exist on the surface of planets, if any existed,” without the warmth of a star. “So life in the universe could have started then.” By contrast, the earliest evidence of life on Earth is only 3.5 billion years old. Loeb’s proposition would add about ten billion years to the timeline of life in the universe.

“I’ve been trying to understand the beginning of the process before the Milky Way and its stars were formed,” he says. “It turns out that the first stars were more massive than the Sun and the first galaxies were smaller than the Milky Way.” This period is compelling, he says, because “it is the scientific version of the story of Genesis. I don’t want to offend religious people, but the first chapter of the Bible needs revising—the sequence of events needs to be modified. It is true that there was a beginning in time. As in the biblical story, ‘Let there be light.’ This light can be thought of as the cosmic microwave background.”

Loeb’s cherubic demeanor and puckish sense of humor play well on his YouTube videos , and Time and Popular Mechanics have cited his influence among space scientists. The title of his paper “How to Nurture Scientific Discoveries Despite Their Unpredictable Nature” reflects his appreciation of the accidental, such as the story behind the Wilson-Penzias discovery.

Recently, Wilson and Loeb have been working together on efforts to map the black hole at the center of the Milky Way. “I think Avi is a theorist who is very good at picking problems to work on that have testable results,” Wilson says.

As for the rigors of exploring deep time and places where no humans are likely ever to tread, Loeb says, “It’s kind of thrilling, like finding a trail in the woods that nobody has thought about. There’s a lot of loneliness. You have to get used to thinking about ideas.”

On Thursday, February 20 at 7:30, Wilson and Loeb will be joined in a panel discussion by cosmologist Alan Guth and astronomer Robert Kirshner at the Harvard-Smithsonian Center for Astrophysics , in celebration of the 50th anniversary of the confirmation of the Big Bang Theory. Watch the discussion live on YouTube .

Get the latest on what's happening At the Smithsonian in your inbox.

Claudia Dreifus | | READ MORE

Author and educator Claudia Dreifus produces the feature "Conversation With. . ." in the New York Times . @claudiadreifus

Origins of the universe, explained

The most popular theory of our universe's origin centers on a cosmic cataclysm unmatched in all of history—the big bang.

The best-supported theory of our universe's origin centers on an event known as the big bang. This theory was born of the observation that other galaxies are moving away from our own at great speed in all directions, as if they had all been propelled by an ancient explosive force.

A Belgian priest named Georges Lemaître first suggested the big bang theory in the 1920s, when he theorized that the universe began from a single primordial atom. The idea received major boosts from Edwin Hubble's observations that galaxies are speeding away from us in all directions, as well as from the 1960s discovery of cosmic microwave radiation—interpreted as echoes of the big bang—by Arno Penzias and Robert Wilson.

Further work has helped clarify the big bang's tempo. Here’s the theory: In the first 10^-43 seconds of its existence, the universe was very compact, less than a million billion billionth the size of a single atom. It's thought that at such an incomprehensibly dense, energetic state, the four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were forged into a single force, but our current theories haven't yet figured out how a single, unified force would work. To pull this off, we'd need to know how gravity works on the subatomic scale, but we currently don't.

It's also thought that the extremely close quarters allowed the universe's very first particles to mix, mingle, and settle into roughly the same temperature. Then, in an unimaginably small fraction of a second, all that matter and energy expanded outward more or less evenly, with tiny variations provided by fluctuations on the quantum scale. That model of breakneck expansion, called inflation, may explain why the universe has such an even temperature and distribution of matter.

After inflation, the universe continued to expand but at a much slower rate. It's still unclear what exactly powered inflation.

Aftermath of cosmic inflation

As time passed and matter cooled, more diverse kinds of particles began to form, and they eventually condensed into the stars and galaxies of our present universe.

By the time the universe was a billionth of a second old, the universe had cooled down enough for the four fundamental forces to separate from one another. The universe's fundamental particles also formed. It was still so hot, though, that these particles hadn't yet assembled into many of the subatomic particles we have today, such as the proton. As the universe kept expanding, this piping-hot primordial soup—called the quark-gluon plasma—continued to cool. Some particle colliders, such as CERN's Large Hadron Collider , are powerful enough to re-create the quark-gluon plasma.

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Radiation in the early universe was so intense that colliding photons could form pairs of particles made of matter and antimatter, which is like regular matter in every way except with the opposite electrical charge. It's thought that the early universe contained equal amounts of matter and antimatter. But as the universe cooled, photons no longer packed enough punch to make matter-antimatter pairs. So like an extreme game of musical chairs, many particles of matter and antimatter paired off and annihilated one another.

Somehow, some excess matter survived—and it's now the stuff that people, planets, and galaxies are made of. Our existence is a clear sign that the laws of nature treat matter and antimatter slightly differently. Researchers have experimentally observed this rule imbalance, called CP violation , in action. Physicists are still trying to figure out exactly how matter won out in the early universe.

Building atoms

Within the universe's first second, it was cool enough for the remaining matter to coalesce into protons and neutrons, the familiar particles that make up atoms' nuclei. And after the first three minutes, the protons and neutrons had assembled into hydrogen and helium nuclei. By mass, hydrogen was 75 percent of the early universe's matter, and helium was 25 percent. The abundance of helium is a key prediction of big bang theory, and it's been confirmed by scientific observations.

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Despite having atomic nuclei, the young universe was still too hot for electrons to settle in around them to form stable atoms. The universe's matter remained an electrically charged fog that was so dense, light had a hard time bouncing its way through. It would take another 380,000 years or so for the universe to cool down enough for neutral atoms to form—a pivotal moment called recombination. The cooler universe made it transparent for the first time, which let the photons rattling around within it finally zip through unimpeded.

We still see this primordial afterglow today as cosmic microwave background radiation , which is found throughout the universe. The radiation is similar to that used to transmit TV signals via antennae. But it is the oldest radiation known and may hold many secrets about the universe's earliest moments.

From the first stars to today

There wasn't a single star in the universe until about 180 million years after the big bang. It took that long for gravity to gather clouds of hydrogen and forge them into stars. Many physicists think that vast clouds of dark matter , a still-unknown material that outweighs visible matter by more than five to one, provided a gravitational scaffold for the first galaxies and stars.

Once the universe's first stars ignited , the light they unleashed packed enough punch to once again strip electrons from neutral atoms, a key chapter of the universe called reionization. In February 2018, an Australian team announced that they may have detected signs of this “cosmic dawn.” By 400 million years after the big bang , the first galaxies were born. In the billions of years since, stars, galaxies, and clusters of galaxies have formed and re-formed—eventually yielding our home galaxy, the Milky Way, and our cosmic home, the solar system.

Even now the universe is expanding , and to astronomers' surprise, the pace of expansion is accelerating. It's thought that this acceleration is driven by a force that repels gravity called dark energy . We still don't know what dark energy is, but it’s thought that it makes up 68 percent of the universe's total matter and energy. Dark matter makes up another 27 percent. In essence, all the matter you've ever seen—from your first love to the stars overhead—makes up less than five percent of the universe.

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Explore Cosmic History

Study how the universe evolved, learn about the fundamental forces , and discover what the cosmos is made of.

The origin, evolution, and nature of the universe have fascinated and confounded humankind for centuries. New ideas and major discoveries made during the 20th century transformed cosmology – the term for the way we conceptualize and study the universe – although much remains unknown.

Big Bang Stories

What is Dark Energy? Inside our accelerating, expanding Universe

NASA’s Webb Identifies the Earliest Strands of the Cosmic Web

Lunar Crater Radio Telescope: Illuminating the Cosmic Dark Ages

Roman Space Telescope Could Image 100 Hubble Ultra Deep Fields at Once

NASA’s Roman Space Telescope to Uncover Echoes of the Universe’s Creation

Discover More Topics From NASA

Dark Matter & Dark Energy

Black Holes

What Is the Big Bang?

What's this big bang all about.

In 1927, an astronomer named Georges Lemaître had a big idea. He said that a very long time ago, the universe started as just a single point . He said the universe stretched and expanded to get as big as it is now, and that it could keep on stretching.

What an Idea!

The universe is a very big place, and it’s been around for a very long time. Thinking about how it all started is hard to imagine.

Some More Information

Just two years later, an astronomer named Edwin Hubble noticed that other galaxies were moving away from us. And that’s not all. The farthest galaxies were moving faster than the ones close to us.

This meant that the universe was still expanding , just like Lemaître thought. If things were moving apart, it meant that long ago, everything had been close together.

Everything we can see in our universe today—stars, planets, comets, asteroids—they weren't there at the beginning. Where did they come from?

A Tiny, Hot Beginning

When the universe began, it was just hot, tiny particles mixed with light and energy . It was nothing like what we see now. As everything expanded and took up more space, it cooled down.

The tiny particles grouped together. They formed atoms . Then those atoms grouped together. Over lots of time, atoms came together to form stars and galaxies .

The first stars created bigger atoms and groups of atoms. That led to more stars being born. At the same time, galaxies were crashing and grouping together. As new stars were being born and dying, then things like asteroids, comets, planets, and black holes formed!

A Super Long Time

How long did all of this take? Well, we now know that the universe is 13,800,000,000 years old—that’s 13.8 billion . That is a very long time .

What's in a Name?

That’s pretty much how the universe began. Because it got so big and led to such great things, some people call it the " Big Bang ." But maybe a better name would be the " Everywhere Stretch ." What do you think?

Explore some more!

Have you heard the name Hubble before? He has a spacecraft named after him: the Hubble Space Telescope . It orbits Earth and takes amazing pictures of our universe.

If you liked this, you may like:

What is the Big Bang theory?

The Big Bang is the singular cataclysmic event that started it all: the instant 13.8 billion years ago that marked the fiery birth of the universe. No one was there to bear witness, of course, but physicists and astronomers are confident that the Big Bang theory — or model, as they often call it — provides an accurate picture of how the cosmos came into existence.

What happened?

Physicists sometimes liken space and time to a “soup” of virtual particles continually popping into and out of existence — and theorize that a random fluctuation in this “ quantum foam ” triggered the Big Bang.

What happened at the precise instant things got started is still a bit hazy, but we know what came next. The baby universe expanded rapidly, cooling as it did. One second after the Big Bang, the temperature had fallen to about a trillion degrees Fahrenheit, and protons and neutrons appeared on the scene. Then came several minutes of nucleosynthesis, when these subatomic particles fused to form the nuclei of the first atoms — mostly hydrogen and helium.

Space What happened before the Big Bang?

By 375,000 years after the Big Bang, the temperature had fallen to 5,400 degrees Fahrenheit and the universe became transparent, freeing visible light and other forms of electromagnetic radiation to travel across the cosmos. Much later, some 300 to 500 million years after the Big Bang, the first stars and galaxies formed — and the rest is history.

Who came up with the Big Bang theory?

The term “Big Bang” was coined by the British astronomer Fred Hoyle in 1949. But the key ideas go back decades earlier.

After Einstein published his general theory of relativity in 1916, physicists began to think of the universe as dynamic and evolving (before the 20th century, most scientists thought of it as constant and unchanging).

In the 1920s, astronomers observed that distant galaxies are rapidly receding from Earth and from one another. That observation was consistent with the idea of an ever-expanding universe, and it suggested that the universe had been smaller in the past. In the 1920s, a Belgian physicist and Roman Catholic priest named Georges Lemaître speculated that if the history of the universe could be “run backward,” the universe would become denser and hotter (as well as smaller) until all the matter was concentrated in what he called a “primeval atom” — what we now call the Big Bang.

Despite Lemaître’s insight, the idea of an unchanging “steady state” universe persisted for decades. But the clincher came with the 1964 discovery of the “ cosmic microwave background ” (CMB) radiation, a sort of echo of the Big Bang that persists to this day.

What proof is there that the Big Bang actually happened?

Several lines of evidence that strongly support the Big Bang model. One involves the relative abundance of the chemical elements that make up the stars and galaxies; the amount of hydrogen, helium, and heavier elements that we observe is just what the theory predicts.

The observed speeding away of distant galaxies is another clue, as is the existence of the CMB. More recently, scientists using orbiting telescopes like the Wilkinson Microwave Anisotropy Probe and the Planck observatory have measured the CMB in fine detail — and its characteristics are consistent with the theory.

But can we really be sure that the Big Bang happened? Sean Carroll, a Caltech physicist and the author of several popular science books, says that the idea that the universe “started in a hot, dense, rapidly expanding state, and then cooled off” to form stars and galaxies “is 100 percent super-duper true. There is just no chance that that’s not right, in broad outline.”

We have a detailed picture of everything that happened from about the one-second mark onward, Carroll says, and theoretical models offer a “plausible” story all the way back to the so-called “Planck time.” That’s when the universe was a mere 10^-43 seconds old (that’s a decimal point followed by 42 zeros and then a 1).

What came before the Big Bang?

No one is sure, and some physicists contend that the word “before” has no clear meaning in this context. The universe “could have been eternal, or it could have had a beginning — we just don’t know,” Carroll says.

In recent years, scientists have developed several theoretical models that attempt to describe epochs that preceded the Big Bang . These include the “ cyclic model ” developed by physicists Paul Steinhardt and Neil Turok, in which “branes” (membranes that exist in a higher-dimensional space) collide periodically to create the conditions associated with the Big Bang, and “ conformal cyclic cosmology ,” an idea put forward by mathematical physicist Roger Penrose, in which Big Bangs happen repeatedly. But these remains highly speculative.

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Science News Explores

It all started with the big bang — and then what happened.

Scientists explain what really puzzles them about how the universe became what it is

The Big Bang is the name we use to describe the instant our universe exploded into existence.

gremlin/E+/Getty Images Plus; adapted by L. Steenblik Hwang

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By Trisha Muro

October 20, 2022 at 6:30 am

Our universe started with a bang. The Big Bang ! Energy, mass and space flashed into existence — all within a fleeting instant. But what exactly happened during this event remains one of the toughest puzzles facing science.

This question was sparked nearly a century ago by a discovery made by astronomer Edwin Hubble. In 1929, Hubble found that distant galaxies were moving away from Earth. Importantly, galaxies farther away were moving away faster. This was true no matter in which direction he looked.

That pattern came to be known as Hubble’s law . Since then, images taken by telescopes gazing across the cosmos have confirmed it. And it seems to point to one mind-boggling conclusion: The universe is expanding.

This expansion is a primary piece of evidence for the Big Bang. After all, if everything in the universe is expanding away from everything else, it’s easy to imagine “rewinding” that motion. That rewind video might show everything coming closer and closer together as time runs backward to the beginning — until the entire cosmos squishes into a single point.

The term Big Bang is cosmologists’ nickname for the almost unimaginable process by which the entire universe expanded from a single point. It marks the beginning of everything that we now see, feel and know. It describes how all matter was created and how our most fundamental laws of nature evolved. It may even mark the beginning of time itself. And it’s thought to have started when the early universe was infinitely dense.

To many scientists who are trying to understand the Big Bang, the first hint of trouble is that phrase: “infinitely dense.”

“Any time you get infinity as an answer, you know that something is wrong,” says Marc Kamionkowski. He’s a physicist at Johns Hopkins University in Baltimore, Md. Coming to infinity “means that we either did something wrong, or we don’t understand something well enough,” he says. “Or our theory is wrong.”

Scientific theories can describe with incredible accuracy how the universe evolved over time after the Big Bang. Telescope observations have confirmed those theories. But every one of those theories breaks apart at a certain point. That point is within a tiny fraction of the first second after the Big Bang.

Most scientists believe that our laws of physics are leading us in the right direction to understand the universe’s first moments. We’re just not there yet. Cosmologists are still struggling to understand the early infancy — and perhaps conception of — our universe and everything in it.

Evidence for the Big Bang

One of the strongest pieces of evidence for the Big Bang also presents one of its biggest challenges: cosmic background radiation . This faint glow fills the cosmos. It is leftover heat from the explosive Big Bang.

Everywhere astronomers gaze, they can measure the temperature of that background radiation. And everywhere, it’s almost exactly the same. This condition is known as a homogeneity (Hoh-moh-jeh-NAY-ih-tee). The universe does, of course, have big differences in temperature here and there. Those places are where stars, planets and other celestial objects exist. But between them, the background temperature in all directions appears the same: a very frigid 2.7 kelvins (–455 degrees Fahrenheit).

The big question is why, says Eva Silverstein. This physicist works at the Stanford Institute for Theoretical Physics in California. There, she investigates how certain structures appear to have formed after the Big Bang. Summarizing the sense of mystery she sees in current theories, she says, “Nobody promised us that we would understand everything.”

The seemingly even spread of cosmic background heat suggests that everything that burst out of the Big Bang should have cooled off the same way. But when we look across the universe now, Silverstein says, we see distinct structures everywhere. We see stars and planets and galaxies . How did they start to form if everything had originally started out as one uniform thing?

“Think about mixing liquids, and how they will come to the same temperature,” Silverstein says. “If you pour cold water into hot water, it will just become warm water.” It won’t become beads of cold water that persist within a pot of otherwise hot water. Likewise, one would expect the universe today to look like a fairly even spread of matter and energy. But instead, there are cold stretches of space dotted with hot stars and galaxies.

Over the last few decades, astronomers think they may have found an answer to this question. They have measured tiny differences in the cosmic background’s temperature. These differences are on the scale of one hundred-thousandth of a degree kelvin (0.00001 K). But if such tiny variations existed right after the Big Bang, they might have grown over time into what we see now as structures.

It’s like blowing up a balloon. Draw a tiny dot onto an empty balloon. Now inflate it. That dot will end up looking a lot bigger once the balloon is full.

Scientists have named this period after the Big Bang inflation . It’s when the newborn universe expanded so tremendously that it’s truly hard to comprehend.

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Explosively fast inflation

Inflation appears to have been fast — far faster than any expansion before or since. It also took place during a stretch of time so tiny it’s hard to imagine. The idea of inflation is well-supported by telescope observations. Scientists have not, however, fully proven it. Inflation is also extremely difficult to physically describe.

“The Big Bang was not an explosion of matter into space. It is an explosion of space,” explains astronomer Adrienne Erickcek. Her work at the University of North Carolina at Chapel Hill focuses on how the universe expanded within the first few seconds and minutes after the Big Bang.

Lots of astronomers use the idea of raisin bread to illustrate this. If you leave a ball of fresh raisin-bread dough on a countertop, that dough will rise. The raisins will spread apart from each other as the dough expands. In this analogy, raisins represent stars, galaxies and everything else in space. Dough represents space itself.

Erickcek offers a more mathematical way to think about the expansion of the universe. “It’s like laying down an image of a grid across all of space, with galaxies at all of the points where the lines meet.” Now imagine that the expansion of the cosmos is like the gridlines themselves expanding. “Everything stays at their places on the grid,” she says. “But the spacing between the gridlines is expanding.”

This part of the Big Bang theory is extremely well-proven. But when we imagine a grid, it’s hard not to wonder about the edges of that grid.

“There’s no edge,” Erickcek points out. “The grid goes infinitely in all directions. So, every point seems like the center of the expansion.”

She emphasizes this because people so often ask if the universe has an edge. Or a center. In fact, she says, there is neither. On that imaginary grid, “every point is getting farther away from all of the others,” she notes. “And the farther away two points are, the faster they seem to be moving away from each other.”

This may be hard to wrap your head around, she admits. But this is what we see in the data. Space itself is what’s expanding. “That grid,” she reminds us, “is infinite. It’s not expanding into anything. There’s no empty space we’re expanding into.”

So where did the Big Bang happen? “Everywhere,” says Erickcek. “By definition, the Big Bang is that moment when the infinite number of gridlines were infinitely close together. The Big Bang was dense — and hot. But there was still no edge. And everywhere was the center.”

Erickcek works to bring theories together with observations. There’s a lot of evidence to support the universe’s inflation. But what caused that inflation? (To go back to the raisin bread analogy, what is the yeast of the universe?) To answer that, a new source of data may be needed.

Hints of the Big Bang in dark matter and gravity waves

To learn what spurred inflation, we may need to look in unexpected places. The invisible, unidentified substance known as dark matter , for instance. Or ripples in spacetime called gravity waves . Or weird new particle physics . Any of these scientific curiosities may hold the secrets to inflation.

Let’s start with dark matter. In the late 1970s, astronomer Vera Rubin discovered that galaxies were rotating far faster than their mass should allow. She proposed the existence of unseen matter — dark matter — as the missing mass. Since then, dark matter has become an important part of cosmology.

Physicists estimate that more than one-fourth of the universe is composed of dark matter. (Only 4 to 5 percent is the “regular” matter that fills our everyday lives and also includes all stars, planets and galaxies. The rest of the universe — almost two-thirds of it — is made of dark energy .) Alas, we still don’t know what dark matter is.

Historically, scientists have looked for clues about the Big Bang among the regular matter we can see. But dark matter is a huge blind spot in the universe. If scientists understood it better, maybe they’d uncover how it — and ordinary matter — came to be.

Until we know for sure how the universe works, it’s good to ask lots of questions and come up with new ideas, says Katelin Schutz. This astronomer works at McGill University in Montreal, Canada. There, she studies dark matter and gra v itational waves .  Her specialty is studying how these things might have interacted in the early universe to form stars and the other structures we see today.

“Right now, we’re thinking about dark matter as if it’s only one kind of particle,” Schutz says. In fact, dark matter could be as complex as visible matter.

“It would be weird if we only have complexity on our side — with normal matter, which is what allows us to have people and ice cream and planets,” Schutz says. But “maybe dark matter is similar, in the sense that it is multiple particles.” Teasing out those details could help reveal how the Big Bang created ordinary and dark matter. 

Schutz’s other research focus, gravitational waves, could also offer clues about the Big Bang’s aftermath. As more sensitive telescopes look farther out into space — and therefore further back in time — scientists hope to spot gravitational waves created shortly after the Big Bang.

Such wrinkles in spacetime could have formed while the evolving universe changed quickly, like a growth spurt — as would have happened during inflation. Gravitational waves aren’t a form of light, so they might offer scientists an unfiltered glimpse of the Big Bang. These gravitational waves could offer “a really interesting window on that time, when we don’t have a lot of other data,” Schutz points out.

Dealing with uncertainties to our origins

So how did stars, galaxies and other cosmic structures come into being? Cosmologists have some idea, but the precise processes remain blurry.

“Honestly, we may never know,” says Schutz. “And I’m okay with that.” She remains excited about the vast frontiers of questions she can investigate. “My favorite theory is one that I know how to test.” And there’s no way to test ideas about the Big Bang in the lab without starting another universe.

“It’s kind of remarkable to me how successful physics has managed to be,” with this huge gap in knowledge about the beginning of time, says Adrienne Erickcek at UNC. New theories and observations are helping shrink that gap. But unanswered questions still abound. And that’s okay. In our search for the answers to fundamental questions, many cosmologists, like Schutz, are comfortable concluding, “I don’t know — at least not yet.”

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Big Bang theory

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Half of the 2019 Nobel Prize in Physics was awarded to James Peebles “for theoretical discoveries in physical cosmology” and the other half is shared by Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star”.

When we look at the night sky, we see not only the Moon but hundreds or thousands of stars — maybe even one of the brighter planets in our Solar System. It’s hard to grasp that this visible matter makes up a mere five per cent of our Universe. With the remainder thought to be dark matter and dark energy, we seem to know little about the Universe. However, to honour the knowledge that we have, this year’s Nobel Prize in Physics was awarded to James Peebles, Michel Mayor and Didier Queloz “for [their] contributions to our understanding of the evolution of the Universe and Earth’s place in the cosmos”.

An early indication for the origin of our Universe was the suggestion by Robert Dicke, James Peebles, Peter Roll and David Wilkinson that the cosmic microwave background could originate from a hot Big Bang 1 . But this would not remain Peebles’s only important contribution to modern cosmology. Throughout his career, many more publications ranging from the cosmic microwave background to galaxy formation followed.

One of his most notable works, carried out jointly with Jeremiah Ostriker, was the discovery that large amounts of dark matter must be present in the halo of spiral galaxies such as the Milky Way as otherwise the flat galactic disk would be unstable 2 . In 1982, Peebles’s studies of non-relativistic or cold dark matter (CDM; ref. 3 ) laid the foundations for the standard model of cosmology — the ΛCDM model. Apart from standard baryonic matter, this model incorporates CDM and dark energy associated with the cosmological constant Λ , which Peebles put back on the map after its famous dismissal by Albert Einstein.

Whereas Peebles shaped our current understanding of how galaxies and galaxy clusters form, Mayor’s and Queloz’s discovery influenced our knowledge of planet formation. When the two started their monitoring campaign, planets outside our Solar System (exoplanets) orbiting a pulsar had been discovered, but not in orbit around a solar-type star. Periodic variations in the radial velocity of the star 51 Pegasi revealed such a planetary companion, 51 Pegasi b (ref. 4 ). The exoplanet’s mass was estimated to be at least half of Jupiter’s — a puzzling observation in light of the short orbital period of around four days.

The discovery of 51 Pegasi b posed a riddle to planet formation as its separation from 51 Pegasi was too small for the planet to be Jupiter-like. The authors speculated that the exoplanet might have been formed from a stripped brown dwarf. Since Mayor’s and Queloz’s observations, thousands of exoplanets have been discovered and continue to inspire advances in planetary formation models.

Despite this huge leap in understanding of our Universe, plenty of discoveries are still waiting to be made — from the exact process of planet formation to figuring out what dark matter is made of. Whatever we’ll find along the way, we know that it all started with the Big Bang.

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The Big Bang Theory's Scientific Accuracy May Surprise Fans (And Anger Critics)

The success of "The Big Bang Theory" proves that plenty of people enjoy the nerd-centric sitcom, but despite being a ratings smash for CBS, it's arguably one of the most maligned comedy series of the 21st century, with many critics noting that it portrays geeks, nerds, and scientists in a negative light. Those naysayers might have to eat their words, however, as the show is more positive toward science than people realize.

According to Physics Today , "The Big Bang Theory" is a scientifically accurate sitcom. During its 12-season run, David Saltzberg, a professor of physics and astronomy at UCLA, worked as a consultant on the show, ensuring each script was factual when it came to his area of expertise. The esteemed professor also made suggestions regarding research topics to cover in storylines, as well as what laboratory equipment to decorate the sets with. This attention to detail adds to the sitcom's scientific authenticity, thus dispelling the belief that it treats the profession as a joke.

What's more, Saltzberg isn't the only genius mind who's synonymous with "The Big Bang Theory." One of the show's key cast members is also a scientist, and she knows people like Sheldon Cooper ( Jim Parsons ), Leonard Hofstadter (Johnny Galecki), Howard Wolowitz (Simon Helberg), and Rajesh Koothrappali (Kunal Nayyar) in real life.

Mayim Bialik has a neuroscience background

On "The Big Bang Theory," Mayim Bialik plays Amy Farrah Fowler , a neuroscientist and the eccentric love interest of Sheldon Cooper. While the character is totally different from the performer who plays her, they're similar in the sense that they're both scientific geniuses. Bialik got her doctorate in neuroscience from UCLA in 2007, which set her up for joining the CBS sitcom in Season 3.

According to Bialik, she knew she'd be right at home on "The Big Bang Theory" after discovering the series. "So I googled Sheldon Cooper, and I saw ... this character Jim Parsons created, and I watched maybe 10 or 15 seconds, and I was like, 'Oh, I know people like this. I can do this. I've been in grad school. We're all like this,' Bialik told a crowd at the University of Tennessee in 2019 (per  Knox News ).

Naturally, her expertise also came in handy on the set. In an interview with  Wired , David Saltzberg revealed that Bialik backed him up when it came to fact-checking all mentions of biology in "The Big Bang Theory." At the end of the day, this series had enough scientific minds on its team to make sure it was true to life, and that's a reality the show's critics just have to accept.

May 24, 2023

The Universe Began with a Bang, Not a Bounce, New Studies Find

New research pokes holes in the idea that the cosmos expanded and then contracted before beginning again

By James Riordon

Science History Images/Alamy Stock Photo

How did the universe start? Did we begin with a big bang, or was there a bounce? Might the cosmos evolve in a cycle of expansion and collapse , over and over for all eternity? Now, in two papers, researchers have poked holes in different models of a so-called bouncing universe , suggesting the universe we see around us is probably a one-and-done proposition.

Bouncing universe proponents argue that our cosmos didn’t emerge on its own out of nothing. Instead, advocates claim, a prior universe shrunk in on itself and then regrew into the one we live in. This may have happened once or, according to some theories, an infinite number of times .

So which scenario is correct? The most widely accepted explanation for the history of the universe has it beginning with a big bang, followed by a period of rapid expansion known as cosmic inflation. According to that model, the glow left over from when the universe was hot and young, called the cosmic microwave background (CMB), should look pretty much the same no matter which direction you face. But data from the Planck space observatory, which mapped the CMB from 2009 to 2013, showed unexpected variations in the microwave radiation. They could be meaningless statistical fluctuations in the temperature of the universe, or they might be signs of something interesting going on.

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One possibility is that the CMB anomalies imply that the universe didn’t emerge out of nothing. Instead it came about after a prior universe collapsed and bounced back to create the space and time we live in today.

Bouncing universe models can explain these CMB patterns as well as account for lingering quibbles about the standard description of the universe’s origin and evolution. In particular, the big bang model of the universe begins with a singularity—a point that appeared out of nothing and contained the precursors of everything in the universe in a region so small that it had essentially no size at all. The idea is that the universe grew from the singularity and, after inflation, settled into the more gradually expanding universe we see today. But singularities are problematic because physics, and math itself, doesn’t make sense when everything is packed into a point that’s infinitely small. Many physicists prefer to avoid singularities.

One bouncing model that averts singularities and makes the CMB anomalies a little less anomalous is known as loop quantum cosmology (LQC). It relies on a bridge between classical physics and quantum mechanics known as loop quantum gravity, which posits that the force of gravity peters out at very small distances rather than increasing to infinity. “Cosmological models inspired by loop quantum gravity can solve some problems,” says University of Geneva cosmologist Ruth Durrer, “especially the singularity problem.” Durrer co-authored one of the two new studies on bouncing universes. In it, she and her colleagues looked for astronomical signs of such models .

In an LQC model, a precursor to our universe might have contracted under the force of gravity until it became extremely compact. Eventually quantum mechanics would have taken over. Instead of collapsing to a singularity, the universe would have started to expand again and may even have gone through an inflationary phase, as many cosmologists believe ours did.

If that happened, says physicist Ivan Agullo of Louisiana State University, it should have left a mark on the universe. Agullo, who was not affiliated with either of the recent analyses, has proposed that the mark would turn up in a feature in the CMB data known as the “bispectrum,” a measure of how different portions of the universe would have interacted in a bouncing scenario. The bispectrum would not be apparent in an image of the CMB, but it would show up in analyses of the frequencies in the ancient CMB microwaves.

“If observed,” Agullo says, the bispectrum “would serve as a smoking gun for the existence of a bounce instead of a bang.” Agullo’s group previously calculated the bispectrum as it would have appeared shortly after a cosmic bounce. Durrer and her colleagues took the calculation further, but when they compared it with the present-day Planck CMB data, there was no significant sign of a bispectrum imprint.

Although lots of other bouncing cosmos models may still be viable, the failure to find a significant bispectrum means that models that rely on LQC to deal with the anomalies in the CMB can be ruled out. It’s a sad result for Agullo, who had high hopes of finding concrete evidence of a bouncing universe. He still considers many bouncing universe models viable, however. And Paola Delgado, a cosmology Ph.D. candidate at Jagiellonian University in Poland, who worked on the new analysis that was co-authored by Durrer, says there’s one potential upside. “I heard for a long time that [attempts to merge quantum physics and cosmology] cannot be tested,” Delgado says. “I think it was really nice to see that for some classes of models, you still have some contact with observations.”

Ruling out signs of an LQC-driven cosmic bounce in Planck data means the CMB anomalies remain unexplained. But an even larger cosmic issue lingers: Did the universe have a beginning at all? As far as advocates of the big bang are concerned, it did. But that leaves us with the inscrutable singularity that started everything off.

Alternatively, according to theories of so-called cyclic cosmologies, the universe is immortal and is going through endless bounces. Although a bouncing universe may experience one or more cycles, a truly cyclic universe has no beginning and no end. It consists of a series of bounces that go back for an infinite number of cycles and will continue for an infinite number more. And because such a universe doesn’t have a beginning, there’s no big bang and no singularity.

The study that Durrer and Delgado co-authored doesn’t rule out immortal cyclic cosmologies. Plenty of theories describe such a bouncing universe in ways that would be difficult or impossible to distinguish from the “big bang plus inflation” model by looking at Planck CMB data.

But a critical flaw lurks in the idea of an eternally cycling universe, according to physicist William Kinney of the University at Buffalo, who co-authored the second recent analysis. That flaw is entropy, which builds up as a universe bounces. Often thought of as the amount of disorder in a system, entropy is related to the system’s amount of useful energy: the higher the entropy, the less energy available. If the universe increases in entropy and disorder with each bounce, the amount of usable energy available decreases each time. In that case, the cosmos would have had larger amounts of useful energy in earlier epochs. If you extrapolate back far enough, that implies a big bang–like beginning with an infinitely small amount of entropy, even for a universe that subsequently goes through cyclic bounces. (If you’re wondering how this scenario doesn’t violate the law of conservation of energy, we’re talking about available energy. Although the total amount of energy in the cosmos remains static, the amount that can do useful work decreases with increasing entropy.)

New cyclic models get around the problem, Kinney says, by requiring that the universe expands by a lot with each cycle. The expansion allows the universe to smooth out, dissipating the entropy before collapsing again. Although this explanation solves the entropy problem, Kinney and his University at Buffalo co-author Nina Stein calculated in their recent paper that the solution itself ensures that the universe is not immortal . “I feel like we’ve demonstrated something fundamental about the universe,” Kinney says, “which is that it probably had a beginning.” That implies a big bang occurred at some point, even if that event happened many bouncing universes ago, which in turn suggests that it took a singularity to get everything going in the first place.

Kinney’s paper is the latest in the debate over cyclic universes, but proponents of a universe without beginning or end have yet to respond in the scientific literature. Two leading proponents of a cyclic universe, astrophysicists Paul Steinhardt of Princeton University and Anna Ijjas of New York University, declined to comment for this article. If the history of the debate is any indication , though, we may soon hear of a work-around to counter Kinney’s analysis.

Some physicists say the Planck data only rule out a bounce under a loop quantum cosmology model that can explain away the CMB anomalies through the bispectrum, not other LQC bounce models that address anomalies using different mechanisms. Cosmologist Nelson Pinto-Neto of the Brazilian Center for Physics Research, who has studied bouncing and other cyclic models, agrees that LQC bounces that account for the CMB anomalies are likely off the table now, but he’s more sanguine on the question of a cyclic universe. “Existence is a fact. We are all here and now. Nonexistence is an abstraction of the human mind,” Nelson says. “This is the reason I think that a [cyclic universe], which has always existed, is simpler than one that has been created. However, as a scientist, I must be open to both possibilities.”

Editor’s Note (6/29/23): This article was edited after posting to better clarify scientits’ views on loop quantum cosmology bouncing universe models that account for the cosmic microwave background. The text was previously amended on June 1 to better clarify Ivan Agullo’s position on bouncing universe models.

Big Bang Theory

Big Bang Theory - The Premise The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment. According to the standard theory, our universe sprang into existence as "singularity" around 13.7 billion years ago. What is a "singularity" and where does it come from? Well, to be honest, we don't know for sure. Singularities are zones which defy our current understanding of physics. They are thought to exist at the core of "black holes." Black holes are areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squished into infinite density (a mathematical concept which truly boggles the mind). These zones of infinite density are called "singularities." Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense, something - a singularity. Where did it come from? We don't know. Why did it appear? We don't know. After its initial appearance, it apparently inflated (the "Big Bang"), expanded and cooled, going from very, very small and very, very hot, to the size and temperature of our current universe. It continues to expand and cool to this day and we are inside of it: incredible creatures living on a unique planet, circling a beautiful star clustered together with several hundred billion other stars in a galaxy soaring through the cosmos, all of which is inside of an expanding universe that began as an infinitesimal singularity which appeared out of nowhere for reasons unknown. This is the Big Bang theory.

Big Bang Theory - Common Misconceptions There are many misconceptions surrounding the Big Bang theory. For example, we tend to imagine a giant explosion. Experts however say that there was no explosion; there was (and continues to be) an expansion. Rather than imagining a balloon popping and releasing its contents, imagine a balloon expanding: an infinitesimally small balloon expanding to the size of our current universe. Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space. According to the many experts however, space didn't exist prior to the Big Bang. Back in the late '60s and early '70s, when men first walked upon the moon, "three British astrophysicists, Steven Hawking, George Ellis, and Roger Penrose turned their attention to the Theory of Relativity and its implications regarding our notions of time. In 1968 and 1970, they published papers in which they extended Einstein's Theory of General Relativity to include measurements of time and space. 1, 2 According to their calculations, time and space had a finite beginning that corresponded to the origin of matter and energy." 3 The singularity didn't appear in space; rather, space began inside of the singularity. Prior to the singularity, nothing existed, not space, time, matter, or energy - nothing. So where and in what did the singularity appear if not in space? We don't know. We don't know where it came from, why it's here, or even where it is. All we really know is that we are inside of it and at one time it didn't exist and neither did we.

Big Bang Theory - Evidence for the Theory What are the major evidences which support the Big Bang theory?

• First of all, we are reasonably certain that the universe had a beginning.
• Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.
• Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.
• Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.

Big Bang Theory - The Only Plausible Theory? Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renown Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that." 4 In 2003, Physicist Robert Gentry proposed an attractive alternative to the standard theory, an alternative which also accounts for the evidences listed above. 5 Dr. Gentry claims that the standard Big Bang model is founded upon a faulty paradigm (the Friedmann-lemaitre expanding-spacetime paradigm) which he claims is inconsistent with the empirical data. He chooses instead to base his model on Einstein's static-spacetime paradigm which he claims is the "genuine cosmic Rosetta." Gentry has published several papers outlining what he considers to be serious flaws in the standard Big Bang model. 6 Other high-profile dissenters include Nobel laureate Dr. Hannes Alfvén, Professor Geoffrey Burbidge, Dr. Halton Arp, and the renowned British astronomer Sir Fred Hoyle, who is accredited with first coining the term "the Big Bang" during a BBC radio broadcast in 1950.

Big Bang Theory - What About God? Any discussion of the Big Bang theory would be incomplete without asking the question, what about God? This is because cosmogony (the study of the origin of the universe) is an area where science and theology meet. Creation was a supernatural event. That is, it took place outside of the natural realm. This fact raises the question: is there anything else which exists outside of the natural realm? Specifically, is there a master Architect out there? We know that this universe had a beginning. Was God the "First Cause"? We won't attempt to answer that question in this short article. We just ask the question:

Does God Exist?

Footnotes: Steven W. Hawking, George F.R. Ellis, "The Cosmic Black-Body Radiation and the Existence of Singularities in our Universe," Astrophysical Journal, 152, (1968) pp. 25-36. Steven W. Hawking, Roger Penrose, "The Singularities of Gravitational Collapse and Cosmology," Proceedings of the Royal Society of London, series A, 314 (1970) pp. 529-548. Mark Eastman, Chuck Missler, The Creator: Beyond Time and Space, (1996) p. 11. W. Wayt Gibbs, "Profile: George F. R. Ellis," Scientific American, October 1995, Vol. 273, No.4, p. 55. See http://www.halos.com/reports/ext-2003-022.pdf See http://www.halos.com/reports/arxiv-1998-rosetta.pdf and http://www.halos.com/reports/ext-2003-021.pdf; see also http://www.halos.com/reports/arxiv-1998-redshift.pdf and http://www.halos.com/reports/arxiv-1998-affirmed.pdf

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Big Bang Theory: Assumptions & Scientific Evidence Essay

Introduction, assumptions and evidence for the big bang theory.

Since time immemorial, humans have questioned the origin of the universe. In a bid to establish the origin of the universe, scientists formulated some theories. The Big Bang Theory is one of the theories that explain the origin, development, and the nature of the universe. The theory uses physical laws and cosmic forces in elucidating the origin, development, and the nature of the universe. The premise of the Big Bang Theory is that the universe originated from a massive explosion of a mass of atoms. The massive explosion generated an explosive force that propelled matter at incomprehensible speed leading to the expansion of the matter into galaxies. Georges Lemaitre and Edwin Hubble are two discoverers of the Big Bang Theory, who made notable observations that the universe emanated from cosmic expansion because galaxies are drifting apart from the center of the universe (Bortz, 2014). The Big Bang Theory is very plausible in the scientific world because physical laws and cosmic forces support the expansion of matter and drifting of galaxies in the universe. This essay, therefore, examines the assumptions and scientific evidence that support the Big Bang Theory.

The Assumptions

Cosmological singularity.

One of the assumptions underlying the Big Bang Theory is the assumption of cosmological singularity. This assumption is very important in the explanation of the origin and development of the universe according to the principles of the Big Bang Theory. In this assumption of cosmological singularity, the Big Bang Theory holds that matter, cosmic forces , and physical laws originated from a finite time in the past prior to the existence of explosion and expansion of the universe (Tillery, Enger, & Ross, 2012). Cosmological singularity forms the basis of the Big Bang Theory because it assumes that matter originated from a finite point in time. The evolution of matter from a finite time resulted in cosmological singularity where the massive explosion took place and triggered the emergence of cosmic forces and physical laws. Scientists have formulated physical laws and identified cosmic forces , which could predict cosmological singularity. Hence, the major assumption of the Big Bang Theory is that matter existed prior to the emergence of the massive explosion.

Homogenous expansion space

In explaining the expansion of the universe, the Big Bang Theory assumes that expansion occurs in a definite space that is homogenous. In essence, the explosion of matter created a space that expands with time. The expanding matter that emanated from the explosion does not expand to fill a space, but it expands from within the expanding space while letting galaxies, objects, and other bodies to move proportionally (Tillery, Enger, & Ross, 2012). The law of relativity elucidates the expansion of space in terms of metric units of spacetime . Metric defines the distances between neighboring galaxies, objects, or other bodies that are in space. The coordinate chart can illustrate the distances of galaxies, objects, and other bodies in a manner that their relative positions indicate spacetime. Given that the expanding space is homogenous, the rate of expansion maintains the coordinates of the comoving points, and thus, coordinate distance remains proportionate to the expanding space. Hence, the assumption of the expanding space is central to the Big Bang Theory because it describes the nature of space in which explosion occurred and expanded into the universe.

Since the Big Bang Theory elucidates the occurrence of explosion and expansion of the matter in spacetime , it assumes that there are horizons, which mark the past and future limits. The assumption of the horizons holds that events of the Big Bang Theory have finite attributes as speed, light, temperature, pressure, and density amongst others. The assumption of horizons implies that some events in the universe are not comprehensible because they might have reached their finite spacetime before humans perceived their existence. This assumption, therefore, gives Big Bang Theory the leeway of predicting the existence of galaxies and objects that have exhausted their lifetime in the universe (Bortz, 2014). The past horizon illustrates the existence of the furthest objects that are observable. In contrast, the future horizon illustrates the existence of the furthest objects that are predictable using physical laws and cosmic forces. Thus, the past and future horizons remain mere assumptions because they are not verifiable.

Scientific Evidence

Redshift of light.

In elucidating the Big Bang Theory, Edwin Hubble used redshift of light as compelling evidence. The evidence is compelling because astronomers can determine the existence of redshift in the lights that galaxies emit (Rhee, 2013). Fundamentally, redshift means that observers on the earth surface perceive different frequencies of lights that galaxies emit in the universe owing to the difference in distances. During the explosion, the explosive force made different galaxies, objects, and other bodies to move at different speed. As the moving bodies migrate at different speeds, the lights that they emit experience the Doppler Effect. Essentially, Doppler Effect is the alteration of the frequency of the wave relative to the distance of the observer. In this case, the frequency of lights emitted by galaxies alters relative to distances of their emission from the earth surface. Observation of galaxies in space indicates that distant galaxies emit lights that have a longer wavelength than close galaxies. This observation is evident in the redshift of light emitted from distant galaxies.

Figure 1 and Figure 2 above illustrates redshift of light emitted from a distant galaxy relative to the sun (Rhee, 2013).

Edwin Hubble formulated the law that allows determination of redshift . The Hubble’s law recessional velocity of a galaxy is directly proportional to their comoving distances (Rhee, 2013). Essentially, the formula is v = Hd, where v is recessional velocity, d is comoving distance, and H is Hubble’s constant. Recessional velocities and distances of galaxies have a linear relationship in which the gradient is the Hubble’s constant. Since the distance from the sun to the earth is known, the luminosity of galaxies relative to the sun provides a way of estimating distances (Rhee, 2013). Parallax aids in the estimation of distances based on the relative luminosity of galaxies in space. In the determination of velocities, wavelengths and frequencies of lights are applicable for they indicate the extent of redshift relative to the velocity of known bodies in the universe. Hence, Hubble’s law is integral in elucidating the Big Bang Theory because it also offers compelling evidence. The existence of the mathematical formula that determines the existence of redshift supports the occurrence of the Big Bang Theory.

Cosmic background radiation

The discovery of cosmic background radiation by Robert Wilson and Arno Penzias in 1965 boosted the scientific stance of the Big Bang Theory. The premise of the cosmic background radiation is that the density of the universe has been decreasing with time owing to the expansion of space after the occurrence of the massive explosion. The interaction of matter and light in the universe creates cosmic radiation, which is the landmark of the Big Bang Theory. Rhee (2013) states that the Big Bang, which is the massive explosion that happed in the past horizon of spacetime , left the background radiation in the form of galaxies and stars. In essence, the background radiation offers a primitive view of space immediately after the occurrence of the Big Bang. Rhee (2013) asserts that the spectrum of the cosmic background radiation has undergone redshift owing to the expansion of space, according to the Big Bang Theory. The National Aeronautics and Space Administration ( NASA ) launched a satellite, which detects the existence and pattern of cosmic background radiation across the globe. Hence, the existence and the variation of the cosmic background radiation across the globe effectively support the Big Bang Theory.

The abundance of primordial elements

The primordial elements, namely, hydrogen, helium, deuterium, and lithium are the most abundant elements in the universe owing to the explanation of the Big Bang Theory. These elements are primordial elements because they formed matter that existed during the Big Bang and consequently expanded to fill the universe. Canetti, Drewes, and Shaposhnikov (2012) assert when the primitive atmosphere cooled and allowed the existence of lighter atoms, the Bing Bang triggered nucleosynthesis of helium, deuterium, and lithium. Thus, the abundances of helium and deuterium in the universe formed the basis of the emission of light by the bodies in the universe. Rhee (2013) argues that helium and deuterium are abundant in the sun because they burn and emit enormous heat and light that reach the earth surface. Hence, the discovery of these primordial elements in the sun led to their identification in the universe and the stars. Rhee (2013) gives a precise proportion of primordial elements by stating that “for every 10,000 atoms of hydrogen in the universe, there are 975 atoms of helium, six atoms of oxygen, and an atom of carbon” (p. 48). In this perspective, the nucleosynthesis of the primordial elements supports the Big Bang Theory for the massive explosion causes existence of the current abundances in the universe.

Structure and distribution of galaxies

The structure and distribution of galaxies in space are in line with the tenets of the Big Bang Theory. The structure of the universe shows that planets and other bodies have oval structure owing to the centrifugal and centripetal forces. Centrifugal force causes bulging at the equatorial planes while centripetal force causes flattening at the polar planes (Anderson, 2015). Moreover, the structure of bodies in space follows some organization because there is a pattern of density in the structure of galaxies, galaxy clusters, and huge bodies. The arrangement of cosmic background radiation varies according to the temperature and density of their sources. Regarding the distribution of bodies in space, observations made on the universe show that galaxies emanated from the Big Bang. The attributes of the redshift, which are evident when observed from the earth, clearly depict the distribution of bodies. The distribution confirms that the Big Bag provided an explosive force that accelerated galaxies and other bodies in the universe. Anderson (2015) argues that the formation of stars and galaxies follows a hierarchical pattern in which the smaller structures form first, and then, they later coalesce into large structures. Therefore, the Big Bang theory provides a plausible explanation the structure and distribution of bodies in space

Existence of Pristine Clouds

Recent discovery revealed that primordial clouds exist in the universe. Analysis of the spectra that emanate from the distant quasars and galaxies indicate that primordial gas forms pristine clouds in the universe. Prior to the discovery, astronomers had established that galaxies and stars contain atoms that are usually lighter than the primordial elements such as lithium, deuterium, and hydrogen. In this view, Canetti, Drewes, and Shaposhnikov (2012) affirm that the primordial clouds formed when lighter atoms combined during the Big Bang. The Big Bang nucleosynthesis accurately predicted the existence of such clouds in the primitive space prior to the expansion of the universe and emergence of bodies. Anderson (2015) reveals that primordial clouds because the primordial matter existed as neutral hydrogen. Hence, the existence of primordial clouds offers compelling evidence that indeed Big Bang took place in the past horizon.

The Big Bang Theory has stood the test of time because it offers compelling evidence regarding the origin, development, and the nature of the universe. The theory relies on three assumptions, namely, cosmological singularity, homogenous expansion space, and infinite horizons. These assumptions set the background of the compelling evidence that support the occurrence of the Big Bang. The r edshift of light, cosmic background radiation, the abundance of primordial elements, the structure and distribution of galaxies, and the existence of pristine clouds are five pillars that support the Big Bang Theory. Analysis of these evidence shows that the Big Bang Theory is still relevant to the modern world for astronomers continue to reveal additional findings.

Aunderson, R. (2015). The Cosmic Compendium: The Big Bang and the Early Universe. London: Lulu Publisher.

Bortz, F. (2014). The big bang theory: Edwin Hubble and the origins of the universe . New York: The Rosen Publishing Group.

Canetti, L., Drewes, M., & Shaposhnikov, M. (2012). Matter and anti-matter in the universe. New Journal of Physics, 14 (1), 1-20.

National Aeronautics Space and Administration: The Big Bang . (2015). Web.

Rhee, G. (2013). Cosmic dawn: The search for the first stars and galaxies . New York: Springer Science.

Tillery, B., Enger, E., & Ross, F. (2012). Integrated science. London: McGraw-Hill.

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IvyPanda. (2024, March 20). Big Bang Theory: Assumptions & Scientific Evidence. https://ivypanda.com/essays/big-bang-theory-assumptions-amp-scientific-evidence/

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Bibliography

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Check Out These Wild Facts About 'The Big Bang Theory'!

L et's start with a fact that may shock some of the biggest fans of the show: Jim Parsons has never seen Star Trek, even though "Sheldon" is obsessed with the series. His convincing and passionate rants about the show could have certainly fooled us, and it's difficult to imagine Jim Parsons not being a Trekkie.

Here's one that is maybe not all that surprising: The series has a real scientific consultant named David Saltzberg to assure that all the formulas and everything they talk about in the series is real. That would explain why the scientific lingo tends to go way above most of our heads...but the comedy sure doesn't!

Watch the video above to learn more!