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The Big Bang
1931· Theory
ProvenTested many times; part of working physics.
The theory that the universe was extremely hot and dense about 13.8 billion years ago and has been expanding and cooling ever since.
See it on the Science map →How it works
The name has two meanings. One is the instant t = 0, found by running the expansion backward, where relativity predicts infinite density and fails. The other is the hot, dense, expanding phase; in inflation theory it began at reheating, a split second later. The tested theory is about the second: not an explosion at a point, but all of space starting hot and stretching. Fred Hoyle, a rival, coined the name on BBC radio in 1949. Planck (2018) puts the age at 13.79 billion years.
Why it matters
It is the accepted history of everything, from the origin of hydrogen to why the night sky is dark, and it made cosmology a precise, testable science.
How we know
Pillars: receding galaxies (1929); the 2.7 K afterglow (1965); hydrogen-helium-deuterium ratios; galaxies maturing over time, now traced by JWST to about 300 million years. Distant gas clouds show the afterglow was hotter in the past, as predicted.
Still a mystery
What happened at t = 0, if anything; why matter beat antimatter; why the start had such low entropy; and why two methods disagree on today's expansion rate (about 67 versus 73 km/s/Mpc).
Connections (11)
▸The arrow of time traces back to the low-entropy start of The Big BangWell supported
The relationship
Time has a direction because entropy, roughly disorder, keeps rising: eggs break, heat spreads. That only works if entropy started very low. Tracing back, the low point is the Big Bang, which was remarkably smooth and orderly in the gravitational sense. Every arrow we experience, memory, aging, cause before effect, inherits that one starting condition.
How the link was found
Ludwig Boltzmann linked entropy to probability in the 1870s-90s. Roger Penrose emphasized from 1979 that the Big Bang's low entropy is the real puzzle. Philosopher David Albert later called it the 'Past Hypothesis'.
Is it proven?
The microwave background shows the early universe was smooth to 1 part in 100,000, a very low-entropy state for gravity. This link is well supported reasoning rather than a direct measurement.
Open questions and mysteries
Why did the universe start in such an improbable, low-entropy state? Inflation, a multiverse or a deeper law are candidate answers; none is established.
▸Georges Lemaître proposed The Big BangProven
The relationship
Having shown the universe expands, Lemaître ran the expansion backwards. In 1931 he proposed that everything began compressed into a single 'primeval atom' that broke apart, starting space, time and the expansion. That is the core of the Big Bang theory: a definite, hot, dense beginning, rather than a universe that has always existed.
How the link was found
Georges Lemaître, in a 1931 letter to the journal Nature, partly prompted by debates with Arthur Eddington about the universe's start.
Why it's proven
The 1965 discovery of the cosmic microwave background confirmed a hot, dense early phase. Lemaître learned of it shortly before his death in 1966.
Open questions and mysteries
His 'atom' was a guess at the first instant. Modern physics still cannot describe that instant; general relativity breaks down there.
▸The universe is expanding run backwards implies The Big BangProven
The relationship
If galaxies are moving apart now, they were closer in the past. Rewind far enough and all matter was packed together, extremely hot and dense. Dividing distances by speeds gives a rough age of about 14 billion years; full models refine it to 13.8 billion. Expansion alone does not prove a hot beginning, but it makes one the natural reading.
How the link was found
Lemaître made the argument in 1931. George Gamow and colleagues developed the hot version in the late 1940s.
Why it's proven
A rival 'Steady State' theory (Hoyle, Bondi, Gold, 1948) kept the universe unchanging by creating new matter. It failed once the microwave background was found in 1965.
Open questions and mysteries
Rewinding all the way gives infinite density, where the theory breaks down. What happened at that point is unknown.
▸The Big Bang predicted the afterglow now seen as Cosmic microwave background: the Big Bang's afterglowProven
The relationship
A hot, dense early universe would have been filled with glowing gas. Once it cooled enough for atoms to form, about 380,000 years in, that light was released and has traveled ever since. Expansion stretched it from near-infrared and visible light to microwaves. The theory therefore predicts a faint, uniform glow from every direction, at a few degrees above absolute zero.
How the link was found
Ralph Alpher and Robert Herman predicted it in 1948, estimating about 5 degrees. Penzias and Wilson found it by accident in 1964-65 while Princeton's Robert Dicke and Jim Peebles were building an antenna to look.
Why it's proven
COBE showed in 1990 its spectrum is a perfect thermal glow at about 2.7 degrees, matching the prediction.
Open questions and mysteries
Its tiny temperature variations were seeded earlier, probably during inflation, which is not yet confirmed.
▸The Big Bang cooked The first elements: hydrogen and heliumProven
The relationship
In the first minutes, the universe was a nuclear reactor filling all of space. Protons and neutrons fused, mostly into helium, until expansion cooled things too much to continue. The theory predicts the leftover mix precisely: about 75% hydrogen and 25% helium by mass, with traces of deuterium and lithium, depending mainly on how much ordinary matter exists.
How the link was found
Ralph Alpher, Hans Bethe and George Gamow in 1948 (the famous 'alpha-beta-gamma' paper); refined by Robert Wagoner, William Fowler and Fred Hoyle in 1967.
Why it's proven
Helium in pristine gas measures about 24-25%. Deuterium levels match the ordinary-matter amount independently read from the microwave background.
Open questions and mysteries
Predicted lithium is about three times higher than observed in old stars. Measurement error, star physics, or new physics?
▸The Big Bang raises the question What came before the Big Bang?Open question
The relationship
The Big Bang theory describes how a hot, dense universe expanded and cooled, not how it began. Run general relativity back to the start and it predicts infinite density, a signal the theory has hit its limit. So the theory's success forces the question: what set up those starting conditions, and was there a 'before'?
How the link was found
Lemaître raised the question in 1931. Stephen Hawking and Roger Penrose proved in the 1960s and 1970 that general relativity necessarily breaks down at such a beginning.
Is it proven?
The breakdown is a mathematical result within general relativity. No observation yet reaches before the end of inflation.
Open questions and mysteries
Was there a prior universe, an eternal inflating sea, or no 'before' at all? A theory of quantum gravity is likely needed.
▸Singularities: where physics breaks sits at the start of (in relativity) The Big BangOpen question
The relationship
Run cosmic expansion backward and everything gets closer and denser. Relativity, applied without limit, says density reaches infinity at a single starting moment: a singularity, like a black hole's center run in reverse. The Hawking–Penrose theorems showed this is unavoidable given relativity and ordinary matter. So in pure relativity, the Big Bang begins with a singularity.
How the link was found
Hawking, building on Penrose, proved the cosmological singularity theorems in 1965–1970, ending hopes that the Big Bang singularity was an artifact of oversimplified models.
Is it proven?
The theorems are mathematical proofs. But they assume relativity holds all the way back, which almost no physicist believes; quantum effects near the Planck time should change the picture.
Open questions and mysteries
Did the universe begin at a true singularity, a bounce, a quantum fuzz, or a period of inflation that blurs the start? Unknown.
▸You are made of stardust traces your hydrogen back to The Big BangProven
The relationship
Not all of you came from stars. Hydrogen, in your water, fat and proteins, formed in the Big Bang's first moments about 13.8 billion years ago. Stars mostly destroy hydrogen by fusing it, so the hydrogen that survives is primordial. Roughly 60% of the atoms in your body are hydrogen, though they make up only about 10% of your mass.
How the link was found
Follows from the Big Bang element-building calculations of Alpher, Bethe and Gamow in 1948, combined with stellar element formation from 1957.
Why it's proven
Big Bang calculations match measured hydrogen and helium, and no known process makes large amounts of new hydrogen.
Open questions and mysteries
Where the matter itself came from, why there was more matter than antimatter, is unexplained.
▸The Big Bang was hailed by Pope Pius XII as echoing "In the beginning was the Word": creation by God's wordFaith & belief
The relationship
In an address to the Pontifical Academy of Sciences on 22 November 1951, Pope Pius XII said modern science, by tracing the universe back to a dense beginning, seemed to bear witness to the primordial 'Fiat lux', 'Let there be light' (Genesis 1:3), where God creates by speaking. He presented expansion and age estimates as support for a moment of creation.
How the link was found
Pius XII, in the address known as 'Un'Ora'. Lemaître, a priest and the theory's founder, objected that it blurred science and faith; the pope's later remarks to astronomers, in 1952, were more cautious.
Is it proven?
This is interpretation, not physics. Science cannot test whether the universe was created by God. And the Big Bang theory describes the hot phase, not t = 0 itself, so it does not prove an absolute beginning either.
Open questions and mysteries
Thomas Aquinas held that reason alone cannot prove the world had a beginning; he accepted one on faith, but taught that creation means, above all, that everything depends on God. Whether the Big Bang was a true beginning remains open.
▸Reheating: inflation's energy becomes hot matter starts the hot phase of The Big BangWell supported
The relationship
'Big Bang' has two meanings: the extrapolated moment t = 0, and the hot, dense, expanding state that made the light elements and the microwave background. In inflation theory, the second begins at reheating. All the classic Big Bang evidence describes what happened after reheating; the infinitely dense start may never have been part of our past.
How the link was found
Became the standard reading after the 1982 slow-roll models; cosmology textbooks since the 1990s describe inflation as coming before the hot Big Bang.
Is it proven?
The hot-phase evidence (microwave background, light elements, neutrinos) is strong. That this phase began at reheating rests on inflation, which is well supported but not proven.
Open questions and mysteries
Did anything precede inflation, and so was there a true t = 0? Many cosmologists now use 'Big Bang' for the hot phase only, which changes what 'before the Big Bang' means.
▸Alexander Friedmann described a universe with a beginning, foreshadowing The Big BangProven
The relationship
Solving Einstein's equations in 1922, Friedmann found universes whose size changes with time. In some, everything grows from zero size at a definite moment, and he called the interval since then 'the time since the creation of the world'. He treated this as mathematics, not a claim about our actual universe, but it was the Big Bang's first outline.
How the link was found
Alexander Friedmann, 'On the curvature of space', Zeitschrift für Physik, 1922. Einstein first claimed it contained an error, then withdrew the objection in 1923. Lemaître, unaware of it, rediscovered expanding solutions in 1927.
Why it's proven
His solutions are exact mathematics within general relativity. Hubble's 1929 expansion data and the 1965 afterglow showed our universe follows one of them, back to a hot, dense start.
Open questions and mysteries
His equations assume evenly spread matter, and run back to zero size they give infinite density. Whether that start was real, or an artifact of the math, is still open.