the big bang essay

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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• Copy URL https://www.pbs.org/newshour/science/read-stephen-hawkings-final-theory-on-the-big-bang

Read Stephen Hawking’s final theory on the Big Bang

Before he passed away in March, theoretical physicist Stephen Hawking had published more than 230 articles on the birth of the universe, black holes and quantum mechanics. It turns out he had one more theory left in the locker.

On Wednesday, the Journal of High Energy Physics published the British scientist’s final thoughts on the Big Bang, the leading theory for how the universe began. The new report, co-authored by Belgian physicist Thomas Hertog, counters the longstanding idea that the universe will expand for eternity. Instead, the authors argue the Big Bang had a finite boundary, defined by string theory and holograms.

Wait, what?

If you asked an astrophysicist today to describe what happened after the Big Bang, he would likely start with the concept of “cosmic inflation.” Cosmic inflation argues that right after the Big Bang — we’re talking after a teeny fraction of a second — the universe expanded at breakneck speed like dough in an oven.

But this exponential expansion should create, due to quantum mechanics, regions where the universe continues to grow forever and regions where that growth stalls. The result would be a multiverse, a collection of bubblelike pockets, each defined by its own laws of physics.

Diagram of evolution of the (observable part) of the universe from the Big Bang (left) to the present. After the Big Bang and inflation, the expansion of the universe gradually slowed down for the next several billion years, as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image and caption by NASA

“The local laws of physics and chemistry can differ from one pocket universe to another, which together would form a multiverse,” Hertog said in a statement . “But I have never been a fan of the multiverse. If the scale of different universes in the multiverse is large or infinite the theory can’t be tested.”

Along with being difficult to support, the multiverse theory, which was co-developed by Hawking in 1983, doesn’t jibe with classical physics, namely the contributions of Einstein’s theory of general relativity as they relate to the structure and dynamics of the universe.

“As a consequence, Einstein’s theory breaks down in eternal inflation,” Hertog said.

Einstein’s theory of general relativity says space and time aren’t fixed, but bending to the forces of gravity. But the concept doesn’t fit with quantum mechanics, the quirky physics that deals with the smallest of things — subatomic particles.

Einstein spent his life searching for a unified theory, a way to reconcile the biggest and smallest of things, general relativity and quantum mechanics. He died never having achieved that goal, but leagues of physicists like Hawking followed in Einstein’s footsteps. One path led to holograms.

The new theory

Last July, Hawking and Hertog presented this new theory of the multiverse during a conference at the University of Cambridge to celebrate Hawking’s 75th birthday.

Their idea hinges on the so-called “holographic principle.” As its name suggests, the principle argues that the universe is a hologram. The principle hails from string theory, the branch of physics trying to make general relativity and quantum mechanics coexist.

String theory stipulates the world is made of strings, existing mathematically as nine dimensions of space and one dimension of time. You’re familiar with the first three dimensions — length, height and depth — and hopefully the fourth, time.

But one of those strings dictates the presence of gravity. Remove it, and gravity no longer exists.

Hawking and Hertog’s new theory plays off this idea, suggesting the world as we know it can be reduced mathematically into a simplified version of itself — that can be expressed without gravity. It’s almost like a 3-D projection being explained in 2-D — or a hologram. (There’s actually evidence that the world works this way).

Now, imagine you were to do the same thing for time: remove the string that governs it. This forms the basis of Hawking and Hertog’s new theory.

Why it matters

If time is a removable string, then there could be places or moments in the history of the universe that operate without it.

In Hawking and Hertog’s newest theory, time is the fly in the ointment, keeping cosmic inflation from aligning with general relativity. So they treated the moments after the Big Bang, when inflation occurred, as a timeless state. In essence, they’re saying this period of inflation operated outside the bounds of Einstein’s relativity.

That, in turn, would reduce the number of local laws governing the universe and bring scientists closer to a unified theory.

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This concept contradicts an idea that Hawking proposed decades ago: that the beginning of time had no boundaries. “Now we’re saying that there is a boundary in our past,” Hertog said. In other words, that boundary is the absence of time right after the Big Bang.

Their report said these calculations tame the multiverse, creating a simpler and more consistent structure of the universe.

“We are not down to a single, unique universe, but our findings imply a significant reduction of the multiverse, to a much smaller range of possible universes,” Hawking said in an interview last autumn.

Some physicists point out that the Hawking-Hertog theory is preliminary and should be considered speculation until other mathematicians can replicate its equations.

You can judge for yourself. Hawking’s final paper is open-access and available for download at the Journal of High Energy Physics. Or you can read a preprint version of the piece down below, courtesy of arXiv :

Nsikan Akpan is the digital science producer for PBS NewsHour and co-creator of the award-winning, NewsHour digital series ScienceScope .

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• 25 March 2024

How did the Big Bang get its name? Here’s the real story

• Helge Kragh 0

Helge Kragh is a historian of science at the University of Copenhagen.

You can also search for this author in PubMed   Google Scholar

Today 'Big Bang' is a household phrase, used even by people who have no idea of how the Universe was born some 14 billion years ago. Credit: Henning Dalhoff/SPL

You have full access to this article via your institution.

“Words are like harpoons,” UK physicist and astronomer Fred Hoyle told an interviewer in 1995. “Once they go in, they are very hard to pull out.” Hoyle, then 80 years old, was referring to the term Big Bang, which he had coined on 28 March 1949 to describe the origin of the Universe. Today, it is a household phrase, known to and routinely used by people who have no idea of how the Universe was born some 14 billion years ago. Ironically, Hoyle deeply disliked the idea of a Big Bang and remained, until his death in 2001, a staunch critic of mainstream Big Bang cosmology.

Several misconceptions linger concerning the origin and impact of the popular term. One is whether Hoyle introduced the nickname to ridicule or denigrate the small community of cosmologists who thought that the Universe had a violent beginning — a hypothesis that then seemed irrational. Another is that this group adopted ‘Big Bang’ eagerly, and it then migrated to other sciences and to everyday language. In reality, for decades, scientists ignored the catchy phrase, even as it spread in more-popular contexts.

This new map of the Universe suggests dark matter shaped the cosmos

The first cosmological theory of the Big Bang type dates back to 1931, when Belgian physicist and Catholic priest Georges Lemaître proposed a model based on the radioactive explosion of what he called a “primeval atom” at a fixed time in the past. He conceived that this primordial object was highly radioactive and so dense that it comprised all the matter, space and energy of the entire Universe. From the original explosion caused by radioactive decay, stars and galaxies would eventually form, he reasoned. Lemaître spoke metaphorically of his model as a “fireworks theory” of the Universe, the fireworks consisting of the decay products of the initial explosion.

However, Big Bang cosmology in its modern meaning — that the Universe was created in a flash of energy and has expanded and cooled down since — took off only in the late 1940s, with a series of papers by the Soviet–US nuclear physicist George Gamow and his US associates Ralph Alpher and Robert Herman. Gamow hypothesized that the early Universe must have been so hot and dense that it was filled with a primordial soup of radiation and nuclear particles, namely neutrons and protons. Under such conditions, those particles would gradually come together to form atomic nuclei as the temperature cooled. By following the thermonuclear processes that would have taken place in this fiery young Universe, Gamow and his collaborators tried to calculate the present abundance of chemical elements in an influential 1948 paper 1 .

Competing ideas

The same year, a radically different picture of the Universe was announced by Hoyle and Austrian-born cosmologists Hermann Bondi and Thomas Gold. Their steady-state theory assumed that, on a large scale, the Universe had always looked the same and would always do so, for eternity. According to Gamow, the idea of an ‘early Universe’ and an ‘old Universe’ were meaningless in a steady-state cosmology that posited a Universe with no beginning or end.

Over the next two decades, an epic controversy between these two incompatible systems evolved. It is often portrayed as a fight between the Big Bang theory and the steady-state theory, or even personalized as a battle between Gamow and Hoyle. But this is a misrepresentation.

Soviet–US nuclear physicist George Gamow was an early proponent of Big Bang cosmology. Credit: Bettmann/Getty

Both parties, and most other physicists of the time, accepted that the Universe was expanding — as US astronomer Edwin Hubble demonstrated in the late 1920s by observing that most galaxies are rushing away from our own. But the idea that is so familiar today, of the Universe beginning at one point in time, was widely seen as irrational. After all, how could the cause of the original explosion be explained, given that time only came into existence with it? In fact, Gamow’s theory of the early Universe played almost no part in this debate.

Rather, a bigger question at the time was whether the Universe was evolving in accordance with German physicist Albert Einstein’s general theory of relativity, which predicted that it was either expanding or contracting, not steady. Although Einstein’s theory doesn’t require a Big Bang, it does imply that the Universe looked different in the past than it does now. And an ever-expanding Universe does not necessarily entail the beginning of time. An expanding Universe could have blown up from a smaller precursor, Lemaître suggested in 1927.

An apt but innocent phrase

On 28 March 1949, Hoyle — a well-known popularizer of science — gave a radio talk to the BBC Third Programme, in which he contrasted these two views of the Universe. He referred to “the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past”. This lecture was indeed the origin of the cosmological term ‘Big Bang’. A transcript of the talk was reproduced in full in the BBC’s The Listener magazine, and Hoyle mentioned it in his 1950 book The Nature of the Universe , which was based on a series of BBC broadcasts he made earlier the same year.

How dwarf galaxies lit up the Universe after the Big Bang

Although Hoyle resolutely dismissed the idea of a sudden origin of the Universe as unacceptable on both scientific and philosophical grounds, he later said that he did not mean it in ridiculing or mocking terms, such as was often stated. None of the few cosmologists in favour of the exploding Universe, such as Lemaître and Gamow, was offended by the term. Hoyle later explained that he needed visual metaphors in his broadcast to get across technical points to the public, and the casual coining of ‘Big Bang’ was one of them. He did not mean it to be derogatory or, for that matter, of any importance.

Hoyle’s ‘Big Bang’ was a new term as far as cosmology was concerned, but it was not in general contexts. The word ‘bang’ often refers to an ordinary explosion, say, of gunpowder, and a big bang might simply mean a very large and noisy explosion, something similar to Lemaître’s fireworks. And indeed, before March 1949, there were examples in the scientific literature of meteorologists and geophysicists using the term in their publications. Whereas they referred to real explosions, Hoyle’s Big Bang was purely metaphorical, in that he did not actually think that the Universe originated in a blast.

The Big Bang was not a big deal

For the next two decades, the catchy term that Hoyle had coined was largely ignored by physicists and astronomers. Lemaître never used ‘Big Bang’ and Gamow used it only once in his numerous publications on cosmology. One might think that at least Hoyle took it seriously and promoted his coinage, but he returned to it only in 1965, after a silence of 16 years. It took until 1957 before ‘Big Bang’ appeared in a research publication 2 , namely in a paper on the formation of elements in stars in Scientific Monthly by the US nuclear physicist William Fowler, a close collaborator of Hoyle and a future Nobel laureate.

How Einstein built on the past to make his breakthroughs

Before 1965, the cosmological Big Bang seems to have been referenced just a few dozen times, mostly in popular-science literature. I have counted 34 sources that mentioned the name and, of these, 23 are of a popular or general nature, 7 are scientific papers and 4 are philosophical studies. The authors include 16 people from the United States, 7 from the United Kingdom, one from Germany and one from Australia. None of the scientific papers appeared in astronomy journals.

Among those that used the term for the origin of the Universe was the US philosopher Norwood Russell Hanson, who in 1963 coined his own word for advocates of what he called the ‘Disneyoid picture’ of the cosmic explosion. He called them ‘big bangers’, a term which still can be found in the popular literature — in which the ultimate big banger is sometimes identified as God.

A popular misnomer

A watershed moment in the history of modern cosmology soon followed. In 1965, US physicists Arno Penzias and Robert Wilson’s report of the discovery of the cosmic microwave background — a faint bath of radio waves coming from all over the sky — was understood as a fossil remnant of radiation from the hot cosmic past. “Signals Imply a ‘Big Bang’ Universe” announced the New York Times on 21 May 1965. The Universe did indeed have a baby phase, as was suggested by Gamow and Lemaître. The cosmological battle had effectively come to an end, with the steady-state theory as the loser and the Big Bang theory emerging as a paradigm in cosmological research. Yet, for a while, physicists and astronomers hesitated to embrace Hoyle’s term.

Work by US physicists Arno Penzias and Robert Wilson vindicated the Big Bang theory. Credit: Bettmann/Getty

It took until March 1966 for the name to turn up in a Nature research article 3 . The Web of Science database lists only 11 scientific papers in the period 1965–69 with the name in their titles, followed by 30 papers in 1970–74 and 42 in 1975–79. Cosmology textbooks published in the early 1970s showed no unity with regard to the nomenclature. Some authors included the term Big Bang, some mentioned it only in passing and others avoided it altogether. They preferred to speak of the ‘standard model’ or the ‘theory of the hot universe’, instead of the undignified and admittedly misleading Big Bang metaphor.

Nonetheless, by the 1980s, the misnomer had become firmly entrenched in the literature and in common speech. The phrase has been adopted in many languages other than English, including French ( théorie du Big Bang ), Italian ( teoria del Big Bang ) and Swedish ( Big Bang teorin ). Germans have constructed their own version, namely Urknall , meaning ‘the original bang’, a word that is close to the Dutch oerknal . Later attempts to replace Hoyle’s term with alternative and more-appropriate names have failed miserably.

The many faces of the metaphor

By the 1990s, ‘Big Bang’ had migrated to commercial, political and artistic uses. During the 1950s and 1960s, the term frequently alluded to the danger of nuclear warfare as it did in UK playwright John Osborne’s play Look Back in Anger, first performed in 1956. The association of nuclear weapons and the explosive origin of the Universe can be found as early as 1948, before Hoyle coined his term. As its popularity increased, ‘Big Bang’ began being used to express a forceful beginning or radical change of almost any kind — such as the Bristol Sessions, a series of recording sessions in 1927, being referred to as the ‘Big Bang’ of modern country music.

In the United Kingdom, the term was widely used for a major transformation of the London Stock Exchange in 1986. “After the Big Bang tomorrow, the City will never be the same again,” wrote Sunday Express Magazine on 26 October that year. That use spread to the United States. In 1987, the linguistic journal American Speech included ‘Big Bang’ in its list of new words and defined ‘big banger’ as “one involved with the Big Bang on the London Stock Exchange”.

Today, searching online for the ‘Big Bang theory’ directs you first not to cosmology, but to a popular US sitcom. Seventy-five years on, the name that Hoyle so casually coined has indeed metamorphosed into a harpoon-like word: very hard to pull out once in.

Nature 627 , 726-728 (2024)

doi: https://doi.org/10.1038/d41586-024-00894-z

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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 happened in the early universe?

About 13.8 billion years ago, the Big Bang gave rise to everything, everywhere, and everywhen—the entire known Universe. What caused the Big Bang? What happened that first moment at the beginning of the Big Bang? When did the first stars form?

Our knowledge of the events and forces that shaped the early Universe is dependent on our ability to understand the most extreme conditions.   On one hand, the Universe’s origin was incomprehensibly small, on dimensions much tinier than the smallest known subatomic particles, and it was completely transformed over an immeasurably brief period, much shorter than any observable time scale. On the other, the densities and temperatures were extraordinarily large, far exceeding anything existing in the present-day Universe.

To study the birth of the Universe, Scientists at the Center for Astrophysics | Harvard & Smithsonian travel to the most remote observing site on the planet: the South Pole. Due to the lack of water vapor in the air, this is one of the best sites to observe the CMB. The Amundsen-Scott South Pole Station includes BICEP3, the Keck Array, and the South Pole Telescope, all specifically designed to look for signatures of Inflation in the CMB. 

Large clouds of hydrogen give off radio waves at a particular frequency.  Astronomers study the signal to weigh “nearby” galaxies and measure their motion through space.  To study the distant cosmic dark age, LEDA works with a custom radio telescope to identify corresponding signals from hydrogen generated at the end of the dark age, less than 100 million years after the Big Bang or less than  1% of the age of the universe. The signal will be very, very faint, but study will lay out how the first large-scale structures in the universe and the first small-scale structures–stars and black holes, formed.

To complement LEDA’s cosmic dark age research, scientists at the CFA Institute for Theory and Computation run simulations of the early Universe and how the first stars formed. After dark matter first clumped together, it attracted large clouds of hydrogen. When the clouds grew large enough, the heat and pressure from gravity started fusing the hydrogen, igniting the first star.

Simulations predict that these first stars were enormous, perhaps hundreds of times larger than our sun. Stars this size burn their fuel extremely quickly and die spectacularly in a supernova, sometimes leaving behind a black hole. These black holes may be the seeds of the supermassive black holes we now find in the center of large galaxies, including the Milky Way.   Studies into the Early Universe can provide meaningful insight into our origins, and scientists at the Center for Astrophysics are leading the way.

Looking Into The Distant Past

In a moment so fleetingly, immeasurably small, scientists theorize that the Big Bang was preceded by an “Inflationary Period.” In a billionth of a trillionth of a trillionth of a second, the Universe grew by a factor of 10 26 , comparable to a single bacterium expanding to the size of the Milky Way.

Inflation projected infinitesimal quantum fluctuations in the young Universe into cosmic scales, leaving some patches with a little more or a little less matter. These variations became the scaffolding for the structure of the Universe.

As the Universe expanded, the seething plasma of subatomic particles cooled to form hydrogen, the first atoms. Light was able to travel unimpeded through the Universe for the first time, a faint glow of radiation that permeates the entire Universe. This Cosmic Microwave Background (CMB) is the oldest observable source of light, a relic left over from when the Universe was only 380,000 years old.

Scientists believe the CMB still holds traces of Inflation, and with it, a window into the earliest moments of our Universe. Center for Astrophysics | Harvard & Smithsonian scientists are hard at work building and operating telescopes, like BICEP3, to observe the intricate features of this radiation, providing clues into the structure and history of the Universe. 

While the theory of inflation is the most popular proposal, the Big Bang might in fact have a different origin. For example, in some other theories the Big Bang was a result of a bounce from a Big Crunch of the entire universe. Scientists at the Institute for Theory and Computation work to propose new methods of deciding which theory is correct through astrophysical observations and discovering the exact origin of the Big Bang from experiments.

A depiction of the universe's 13.8 billion-year history, with the Big Bang at the left and the present day at the right. Researchers seek indirect ways to study the first instants of cosmic history, which are hidden from us.

Cosmic Dark Ages

After the Universe cooled sufficiently to allow atoms to form, what was once an incredibly hot and bright place turned cold and dark. Gravity slowly amplified tiny inhomogeneities in the distribution of gas, forming empty voids and massive clouds of hydrogen.  As gravity drove clouds to collapse further, they became peppered with something new – stars.  How long was it between the Big Bang and "cosmic dawn"?

Scientists hypothesize that unlike stars today, the earliest ones were massive and short-lived.  As generation after generation died, were black holes left behind in droves? Were these seeds for the supermassive singularities known to exist in the centers of galaxies today?  Scientists have no direct observations of this era with which to test hypotheses.  It is a literal and figurative “dark age.”

Center for Astrophysics | Harvard & Smithsonian scientists formulated the Large Aperture Experiment to Detect the Dark Ages (LEDA) in an effort to zero in on when the first stars and black holes formed, and to test cosmologists' hypotheses about conditions in the universe before stars.

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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.

For Hungry Minds

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.

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.

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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.

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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What caused the Big Bang?

The origin of the Big Bang can be answered using quantum physics, but that doesn't mean you're going to like the answer.

Dr Alastair Gunn

Asked by: Federico Tak, Oxfordshire

The Big Bang is the moment that space and time (or ‘space-time’) came into existence. Before the Big Bang there was no space or time. So, it is actually meaningless to ask what caused the Big Bang to happen – there was no Universe in which that cause could have existed.

This might seem like a bit of a cheat, but there are other good reasons to suppose a cause for the Big Bang might not exist. Quantum physics has shown us that some events have no cause at all. Things can happen randomly, spontaneously, and for no particular reason. This unpredictable and ‘causeless’ nature of the Universe is experimentally verified but has nothing to do with our inability to observe correctly – it is a fundamental property of the Universe. So, although there may have been a cause for the Big Bang that we are unaware of, modern cosmology neither defines nor requires one.

• Who really discovered the heat from the Big Bang?
• Did the Big Bang go bang?

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Home — Essay Samples — Science — Big Bang Theory — The Concept of the Big Bang Theory

The Concept of The Big Bang Theory

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Evidence for the big bang.

Astronomers think the Big Bang is the best way to explain how the Universe started and how it changed over time. All science is based on evidence. So what is the evidence for the Big Bang?

1. Redshift of Galaxies

The light we observe from galaxies has been stretched by the time it reaches us. It looks redder than it should. This redshift is the result of galaxies moving away from us. Observations show that pretty much everything in the Universe is moving apart. The redshift of distant galaxies tells us the Universe is expanding .

If you could wind time backwards, you would see galaxies getting closer together. If you could go back far enough, everything in the Universe would have been in one place. 

2. Microwave Background

A long, long time ago , the whole Universe was very hot. As it grew in the size, the heat left a "glow" which fills the entire Universe. The Big Bang theory predicts this glow should still exist. It also predicts that we should be able to detect this glow as microwave light . 

Scientist have found this Cosmic Microwave Background. They have accurately measured it using orbiting detectors. It is very good evidence that the Big Bang theory is correct.

3. Mixture of Elements

Some chemical elements were created soon after the Big Bang. Elements like hydrogen and helium. The Big Bang theory predicts how much of each element was made in the early universe. When astronomers look at very old galaxies and stars , the amount of each chemical they see agrees with the Big Bang theory.

You cannot look for this evidence in new stars, like the Sun . This is because newer stars contain chemical elements made by older stars. So the chemical make-up of new stars is very different from stars which existed soon after the Big Bang.

4. Looking back in time

There is an alternative to the Big Bang theory. It is called the Steady State theory . In this theory, the Universe does not change much over time.

Remember - light takes a long time to travel across the Universe. So when we look at galaxies which are very far away, we are also looking back in time.

This means we can see that very old galaxies are very different from newer galaxies. This shows the Universe has changed. This evidence fits better with the Big Bang theory than the Steady State theory.

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Physics: Big Bang Theory Essay

The origin of the universe, on the premises of the superstring theory, was from nothing. In a way, this supports and opposes the creationist theory. It supports it because it implies that matter was created from nothing. Conversely, it contradicts it because the “physical” proof of nothing in the scientific understanding precludes the creator, who creationists claim transcends temporal and spatial dimensions.

The fundamental idea on which the string theory is based is that different particles in the standard model are manifestations of the primary object, which in this case, are represented by a hypothetical string. This begs the question, “how is that plausible?” For it to make sense, one must reconsider the traditional assumptions that underline the idea of electrons.

Contrary to the popular assumption that it is a point if the string theory is true, then it means that it is a point. The string theory makes sense because a string is capable of other actions apart from motion that electrons in the universe cannot achieve. A string can oscillate in various ways. However, when it is oscillating, one cannot distinguish the movement from a distance.

Thus, only a point is visible, which gives evidence to the idea of electrons. Retrospectively, scientists have been able to perceive these strings, and they assume that they are photons. Hypothetically, the string is so small compared to an electron. It is like a computer mouse compared with the solar system. However, following the logic of the string theory to its conclusion ultimately leads one into concluding that the universe is made up of strings as opposed to the conventional electrons (points).

From the information about the superstring theory, one can deduce the solution to other problems that have dominated theoretical physics for years. That is the explanation of why gravity appears to be so weak as compared to the other fundamental forces. Assuming that the strings are too small to measure, or incorporate into other dimensions, scientists have postulated that gravity in its entirety can only be felt at higher dimensions that human beings are incapable of perceiving.

Furthermore, given that the strings are too small to be seen through the application of the current technologies, some have proposed that the string theory is more of a philosophical than a scientific approach. Mathematically though, the string theory remains coherent and logical. However, to the frustration of many scientists, it remains stubbornly abstract and theoretically rendering any attempts to apply it or even experiment with it for empirical results.

While admitting that the theory indeed has a singular elegance in its mathematical profundity, one is nevertheless forced to contend with the fact that there are over five schools of thought dedicated to the superstring theory and, irrespective of the level of elegance, none of them can be deemed conclusive.

However, in this respect, there is a glimmer of hope since it is postulated that if an eleventh dimension was included, these theories might be reconciled as one different way of looking at the same thing rather than five different explanations of the same concept. This reasoning can ultimately lead to connecting the superstring theory with the supergravity one, which was postulated in the early 1980s, although it has been relegated to the periphery by contemporary theories about the universe.

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Interpersonal Communication Issues in "The Big Bang Theory" Show

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