OriginsExplainer
What Happened in the First Second After the Big Bang?
From the first atomic nuclei back to a possible burst of inflation, here is what scientists have tested about the first second, and where knowledge runs out.
THE QUESTION LIBRARY ORIGINS
Did time begin at the Big Bang?
Scientists have measured the hot early universe in detail, but whether time itself had a first moment is still an open question.
THE SHORT ANSWER
Nobody knows yet. About 13.8 billion years ago the universe was extremely hot and dense, and it has been expanding and cooling ever since; that part is well measured. But Einstein’s theory of gravity breaks down at the very start, so it cannot tell us whether time began there. Some ideas give time a true first moment. Others say the Big Bang was a bounce, or a new chapter of something older. No test has settled it.
- About 13.8 billion years ago the universe was hot and dense, and it has expanded and cooled ever since; ESA’s Planck mission and NASA both support this.
- The Big Bang was not a blast at one spot in empty space; it happened everywhere at once, as space itself began to stretch.
- Run Einstein’s equations backward and the math breaks down, which marks the edge of the theory, not a proven first moment of time.
- Bounce, cycle and “no-boundary” ideas give different answers about time, and none has been confirmed by observation.
About this picture
NASA’s timeline of the universe runs from the earliest moment scientists can probe, at left, to today. Its labels use earlier WMAP-era figures; ESA’s Planck later gave about 13.8 billion years. The glow at the far left edge is where this question lives.
WHERE THE EVIDENCE STANDS
How sure are we?
- The universe was extremely hot and dense about 13.8 billion years ago and has been expanding and cooling since.ESA’s Planck map of the cosmic microwave background implies an age of 13.82 billion years; NASA’s cosmic history and the light-element abundances agree.
- The Big Bang was an explosion of matter flying out from one center into empty space.NASA astrophysicists explain that it happened everywhere at once, with no location you could point to, as the start of an expansion of space.
- Time itself began at the Big Bang.Singularity theorems in Einstein’s theory point to a finite past, but quantum effects may change that; NASA says there is no definitive answer yet.
- Something came before the hot Big Bang, such as a bounce, an earlier cycle or a collision of branes.Peer-reviewed models exist, but none has been confirmed; a 2010 claim of rings from an earlier era failed independent checks.
- HD-Blast: our hot beginning may be the ripple left on our universe by a brief blast in a larger, higher-dimensional space.Ricardo Maldonado’s proposal in GOD PLAYS DICE, Chapter 2; the book names a possible gravitational-wave test, and nothing has been confirmed.
WHAT WE DON’T KNOW YET
Nobody knows whether time had a first moment, or whether our hot beginning grew out of something older. Finding out will probably take a tested theory that joins gravity with quantum physics, and no such theory is finished yet.
WHAT WOULD CHANGE THIS ANSWER
A clear signal from before the hot Big Bang would change this answer, especially a pattern in gravitational waves (ripples in space and time) or ancient light that one model predicted in advance and the others did not. A tested theory of quantum gravity that says what happens at “time zero” would change it too.
Nobody knows yet. That is the honest answer, and it is more interesting than it sounds.
Scientists know a great deal about what came after the start. About 13.8 billion years ago, the universe was extremely hot and packed tight. Since then it has been expanding and cooling. A faint glow left over from that early time still fills the whole sky, and ESA’s Planck spacecraft has mapped it in fine detail. That part of the story is Established.
The trouble comes when you try to rewind to the very first instant. Einstein’s theory of gravity, called general relativity, works beautifully for planets, stars and galaxies. Run its equations backward, though, and they reach a point where density and temperature climb to infinity. Physicists usually read that as a sign that the theory has run out of road. It is not a clear picture of a first moment. To go further, we need a theory that joins gravity with quantum physics, the rules of the very small. Several candidates exist, but none is finished or confirmed.
People ask this question in three ways. Did time begin at the Big Bang? What was there before it? Was the Big Bang an explosion somewhere in space? The last one has a clearer answer: no. It was not a blast at one spot in an empty room. It happened everywhere at once, as space itself began to stretch.
For the first two questions, serious ideas compete. In some, time truly starts. In others, our universe bounced back from an earlier shrinking phase, or is one chapter in a long cycle. Each idea points to different clues we could look for, and none has been confirmed.
If you have lain awake wondering about this, you are in old company. More than 1,500 years ago, the Christian thinker Augustine of Hippo argued that there was no “then” before time began. Modern physics is still working on its own answer.
THE LONG ANSWER
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Keep asking
Why is it so hard to test ideas about what happened before the hot Big Bang?
We infer earlier history from later traces.
See this path: Where did everything come from?
New here?
God Plays Dice answers big questions in plain words, with sources and one of six honest labels.
Words worth knowing (6)
- Big Bang
- The hot, dense early state of the universe about 13.8 billion years ago, and the expansion and cooling that followed. It was not an explosion at one place in space.
- Cosmic microwave background
- Faint leftover light from when the universe was about 380,000 years old; the oldest light we can observe.
- General relativity
- Einstein’s theory of gravity, which describes gravity as the bending of space and time.
- Singularity
- The point where the equations of general relativity give infinite values, a sign that the theory has reached its limit.
- Quantum gravity
- A hoped-for theory joining gravity with quantum physics, the rules of the very small. Several candidates exist; none is finished or confirmed.
- Brane
- In some theories, a membrane-like world, such as our universe, sitting in a space with extra dimensions.
Sources & further reading
(10) · Checked Sep 28, 2026
- Planck reveals an almost perfect Universe ↗European Space Agency (ESA)21 March 2013 release: Planck’s cosmic microwave background map, then the most detailed ever made; the data imply an age of 13.82 billion years; the light was imprinted when the universe was 380,000 years old.
- Overview (The Universe) ↗NASA ScienceInflation around 13.8 billion years ago; scientists aren’t sure what came before inflation or what powered it; first elements in the first minutes; recombination and the microwave background about 380,000 years after the big bang.
- Ask an Astrophysicist: Cosmology ↗NASA Goddard Space Flight Center, Imagine the Universe!The Big Bang occurred everywhere all at once and was the start of an expansion; raisin-bread analogy has no crust in the real universe; no definitive answer on time before the Big Bang; a higher-dimensional embedding space might be unmeasurable.
- Philosophy of Cosmology (Christopher Smeenk and George Ellis) ↗Stanford Encyclopedia of PhilosophySections 3.1–3.5 and 4.3: singularity theorems (1960s) and a finite past; no first moment because time breaks down as t approaches 0; GR ends at the singularity; quantum bounce; puzzles of the initial state; Penrose’s cyclic idea; multiverse branch point and the testability critique; physical origin theories do not answer why the universe exists.
- The Beginning of Time (lecture, 1996) ↗Stephen Hawking EstateHawking on cutting pre-Big Bang events out of the theory, the no-boundary proposal with James Hartle, the North Pole picture, and his statement that the proposal is a hypothesis to be tested.
- Inflationary spacetimes are incomplete in past directions (Borde, Guth and Vilenkin, 2003) ↗Physical Review Letters 90, 151301An inflating, or sufficiently fast expanding, universe must be past-incomplete; inflation requires other physics at its past boundary.
- Quantum Nature of the Big Bang (Ashtekar, Pawlowski and Singh, 2006) ↗Physical Review Letters 96, 141301In simplified loop quantum cosmology models, the big bang is replaced by a big bounce.
- A Cyclic Model of the Universe (Steinhardt and Turok, 2002) ↗Science 296, 1436–1439Endless sequence of epochs, each beginning with a bang and ending in a crunch, with finite temperature and density at each transition.
- Confessions, Book XI (Augustine; trans. J. G. Pilkington, 1887) ↗New Advent (from Nicene and Post-Nicene Fathers, ed. Philip Schaff)Chapters 12–13: Augustine refuses to mock the question “What was God doing before He made heaven and earth?” and writes “there was no then when time was not.”
- GOD PLAYS DICE, Volume OneRicardo MaldonadoThe pond picture of the higher-dimensional blast idea; the obligations any such model must meet; the proposed gravitational-wave target. Author’s hypothesis.
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CONTINUE THE THREAD
Where this question leads.
OriginsAnswer
Why is it so hard to test ideas about what happened before the hot Big Bang?
An earlier history needs a calculable path to a measurement that separates it from competing explanations.
OriginsAnswer
Could there be other universes beyond ours?
Several serious ideas in physics allow other universes, so here is what each one says, how scientists have tried to test them, and how a scientific braneworld differs from the multiverse in the movies.