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What came before the Big Bang?
· Open question
Open questionNobody knows the answer.
The open question of what, if anything, preceded or caused the hot, dense beginning of our universe.
See it on the Science map →How it works
The Big Bang theory describes the universe from a tiny fraction of a second onward. Rewind general relativity to the very start and it predicts infinite density, a sign the theory has broken down. A quantum theory of gravity is needed and does not exist. Proposals include: time itself begins there, so 'before' has no meaning (Hartle and Hawking, 1983); a previous universe collapsed and bounced; or endless inflation preceded our patch.
Why it matters
It is the ultimate origin question, and the place where physics, philosophy and theology meet.
How we know
No direct evidence for any proposal. Roger Penrose has claimed signs of a previous cycle in the microwave background; most analysts find the patterns consistent with chance.
Still a mystery
Whether the question even makes sense is itself unknown. Answering it probably requires uniting gravity with quantum physics.
Connections (11)
▸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.
▸Quantum gravity: the missing link is needed to answer What came before the Big Bang?Open question
The relationship
Relativity predicts the universe began from a singularity, where the theory itself fails. To ask what came before, or whether "before" means anything, requires a theory that works in that extreme. Proposals built on quantum gravity give different answers: a bounce from an earlier universe, time emerging from space, or a quantum birth from nothing.
How the link was found
Discussed since the 1960s; concrete proposals include Hartle and Hawking's no-boundary idea (1983), loop quantum cosmology's bounce (2000s), and string-based cyclic models.
Is it proven?
None of the proposals is confirmed. Some predict faint patterns in the cosmic microwave background or primordial gravitational waves that future telescopes might look for.
Open questions and mysteries
Is the question even meaningful? Would quantum gravity allow any information about "before" to reach us?
▸Why is there something rather than nothing? runs deeper than What came before the Big Bang?Open question
The relationship
Knowing what preceded the Big Bang, whether a bounce, cycles or eternal inflation, would only extend the history. The 'why anything' question applies to that earlier stage too. Leibniz made the point with a geometry book copied from an older copy, back forever: the endless chain still does not explain why the book exists at all.
How the link was found
Leibniz, 'On the Ultimate Origination of Things' (1697). The split between 'how did it begin' and 'why does it exist' runs through later philosophy of cosmology.
Is it proven?
A conceptual distinction, not an empirical claim. It holds even for a universe with no beginning: an eternal universe still faces the question of why it exists.
Open questions and mysteries
If physics found the universe had no beginning, would the deeper question lose its force, as some philosophers argue, or stay untouched?
▸The first-cause argument offers an uncaused cause in answer to What came before the Big Bang?Philosophy
The relationship
Kalam defenders read the Big Bang as the universe 'beginning to exist', which by their first premise needs a cause beyond space and time. Aquinas argued differently: his first cause sustains the universe at every moment and would be needed even if it had always existed. So the argument does not strictly require a 'before' in time.
How the link was found
William Lane Craig tied kalam to Big Bang cosmology in 1979 and later cited the 2003 Borde–Guth–Vilenkin theorem. Aquinas wrote, about 1271, that reason alone cannot prove the world had a beginning.
Is it proven?
Philosophy, not physics. Physicists object that the Big Bang may not be an absolute beginning, and that 'cause' may mean nothing without time; Sean Carroll argued this against Craig in a 2014 debate.
Open questions and mysteries
If time began at the Big Bang, can anything come 'before' it or cause it? Might quantum events need no cause at all?
▸The universe as a vacuum fluctuation offers an answer to What came before the Big Bang?Speculative
The relationship
Tryon's answer: before our universe there was a larger, empty quantum vacuum, and our universe appeared in it as a spontaneous fluctuation. The Big Bang would be one event in a bigger, possibly eternal space, and others could happen elsewhere. This differs from later 'from nothing' ideas, which remove the pre-existing space entirely.
How the link was found
Edward Tryon, 1973. Later physicists, including Vilenkin in 1982, kept the spirit of the idea but dropped the parent space.
Is it proven?
Untested. Nothing observed points to a parent vacuum, and there is no known way to look outside our universe.
Open questions and mysteries
Where did the parent vacuum and its laws come from? How often should such fluctuations occur, and why is ours so large and smooth?
▸A universe tunneling from nothing offers an answer to What came before the Big Bang?Speculative
The relationship
Its answer: before the universe there was no before. Time starts with the tunneling event, so there is no earlier moment to ask about. The universe appears at a tiny size, inflates, and then reheats into the hot Big Bang. It describes a birth event, where Hartle and Hawking describe a smooth rounded-off past, though the math is similar.
How the link was found
Vilenkin, 1982. The Borde–Guth–Vilenkin theorem (2003), showing inflation alone cannot extend forever into the past, strengthened the case that some beginning is needed.
Is it proven?
Not proven. The 2003 theorem is accepted math, but it assumes classical spacetime, which quantum gravity could evade. The tunneling event itself leaves no known observable trace.
Open questions and mysteries
Is 'no before' a real answer or a way of stopping the question? Would a working quantum gravity theory keep or remove this beginning?
▸The no-boundary proposal (Hartle–Hawking) says "there is no before" to What came before the Big Bang?Speculative
The relationship
The past is finite but has no first instant and no singularity. Shrink the universe backward and, near the start, time smoothly becomes space, so the history closes off instead of ending at an edge. With no edge, nothing has to be set from outside; the laws alone fix the starting state. 'Before' simply has nothing to refer to.
How the link was found
Hartle and Hawking, 1983, a year after Vilenkin's rival tunneling proposal. Both aimed to make the beginning itself part of physics rather than an unexplained input.
Is it proven?
No observation yet tests it. Its predictions for inflation and the microwave-background ripples depend on contested calculations, so it can be neither confirmed nor ruled out.
Open questions and mysteries
Does 'no before' answer the question or dissolve it? And does it produce a smooth start like ours, or a different universe?
▸Cosmic inflation pushes back the mystery of What came before the Big Bang?Speculative
The relationship
If inflation happened, the hot Big Bang did not start at t = 0 but at reheating, when inflation ended. The question 'what came before the hot Big Bang?' gets an answer, inflation, and is replaced by a harder one: what came before inflation? Inflation also erases clues, since anything earlier was stretched far beyond view.
How the link was found
Arvind Borde, Alan Guth and Alexander Vilenkin proved in 2003 that inflating spacetimes cannot extend forever into the past, so inflation alone cannot remove the need for a beginning.
Is it proven?
The 2003 theorem is a mathematical result that does not even depend on Einstein's equations, only on space expanding on average. What it implies, a true beginning or just the edge of our description, is debated.
Open questions and mysteries
What started inflation: a quantum birth from nothing, the no-boundary state, a bounce from an earlier universe, or something else? Inflation hides the answer behind its own stretching.
▸The Big Bounce answers "a shrinking universe" to What came before the Big Bang?Speculative
The relationship
Its answer is an earlier universe much like ours run in reverse: contracting, growing denser and hotter, until quantum effects halted the collapse and turned it into expansion. On this view the Big Bang was the moment of maximum density, not the start of time, and our past continues back through it.
How the link was found
Alexander Friedmann noted a possible 'periodic' universe in 1922, and Richard Tolman explored oscillating universes in the 1930s. Bojowald argued in 2007 that a bounce would erase some details of the earlier universe.
Is it proven?
No observational evidence yet. Any message from before would have to survive extreme density and then inflation, which stretches and dilutes it.
Open questions and mysteries
Did the earlier universe have its own big bang, making the past infinite? Why was it so orderly before the bounce?
▸Cyclic universes (ekpyrotic and Penrose's CCC) answer "an earlier cycle" to What came before the Big Bang?Speculative
The relationship
Each version says our Big Bang had a predecessor. In the ekpyrotic cycle, a previous phase of slow contraction smoothed and flattened space, doing inflation's job, before the branes collided. In Penrose's scheme, the previous aeon expanded for an immense time until only radiation remained, then became our Big Bang. Either way, 'before' was another cosmic era.
How the link was found
Khoury, Ovrut, Steinhardt and Turok (2001); Steinhardt and Turok (2002); Penrose from about 2005, set out in his 2010 book 'Cycles of Time'.
Is it proven?
Not established. Both fit current data only with adjustments, and neither has passed a test that inflation fails. Coming searches for primordial gravitational waves in the microwave background could rule out the ekpyrotic version.
Open questions and mysteries
Did the cycles themselves have a beginning, as the Borde–Guth–Vilenkin theorem suggests for expanding phases? How does structure pass from one cycle to the next?
▸A universe born inside a black hole offers an answer to What came before the Big Bang?Speculative
The relationship
Its answer: before our Big Bang was a collapsing star or other massive object in a parent universe. Its matter, crushed to extreme density, rebounded into our expanding space. In Popławski's version, spinning particles twist spacetime, supplying the repulsion. Our universe might then inherit a preferred direction from the parent black hole's spin.
How the link was found
Pathria (1972); Smolin (1992); Nikodem Popławski, in papers from 2010, using the Einstein–Cartan extension of relativity, which includes this twisting, called torsion.
Is it proven?
No evidence. A 2025 study of early galaxies seen by JWST reported more spinning one way than the other, which some linked to a parent's spin; it is unconfirmed and may be an observational bias.
Open questions and mysteries
Where did the first parent universe come from? The idea moves the origin question back a generation without answering it.