What came before the Big Bang?

Nobody knows what came before the Big Bang, and it may not be a meaningful question. Our best theory of gravity breaks down at the very beginning, so it cannot see past it. Physicists have five serious proposals: time began there, eternal inflation, a bounce, cycles of universes, or colliding branes. None is confirmed yet.

This is one of the few places in cosmology where the honest answer is “we do not know”, and it is worth saying clearly, because many popular articles blur the line between established science and speculation.

Why is the question so hard?

The Big Bang theory describes how the universe evolved from a hot, dense state. It does not describe a moment of creation. Run Einstein’s equations backward and the density and temperature climb without limit until they become infinite at a “singularity”. That is not a physical answer; it is the equations announcing they no longer apply.

Roger Penrose proved in 1965 that, under general conditions, general relativity makes singularities unavoidable inside collapsing stars (Penrose’s singularity theorem). He and Stephen Hawking then extended the argument to the expanding universe: classical Big Bang models must begin at a singularity. To get past it, physics needs a quantum theory of gravity, and there is no complete, tested one.

There is also a practical wall. The oldest light we can see, the cosmic microwave background, was released about 380,000 years after the Big Bang. Earlier than that, the universe was opaque.

Does cosmic inflation answer it?

Partly. Cosmic inflation, proposed by Alan Guth in 1981 (Guth’s inflationary universe), says space expanded exponentially for a tiny fraction of a second, and the hot Big Bang began when that expansion ended. In this picture the hot Big Bang was not the very first event: inflation came “before” it.

Inflation fits the data well. It predicts nearly uniform ripples with a slight tilt, and Planck measured that tilt, the spectral index, at 0.965 ± 0.004 (Planck 2018 cosmological parameters).

But inflation only moves the question back. In 2003 Arvind Borde, Alan Guth and Alexander Vilenkin showed that an inflating spacetime cannot extend infinitely into the past (“Inflationary spacetimes are incomplete in past directions”, Physical Review Letters). Inflation needs a beginning of its own, or something else before it.

The five main ideas

Here are the serious proposals, what each says came “before”, and whether it can be tested:

Idea	Key authors	What came before	Testable?
No-boundary universe	James Hartle and Stephen Hawking, 1983	Nothing: time has no sharp beginning	Indirectly, through predictions for inflation
Eternal inflation	Andrei Linde, Alexander Vilenkin and others	An inflating space that never stops making new “bubble” universes	Hard; possible bubble-collision marks in the microwave background
Big Bounce (loop quantum cosmology)	Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh, 2006	A contracting universe that rebounded	Possibly, via subtle features in the microwave background
Ekpyrotic and cyclic universe	Justin Khoury, Burt Ovrut, Paul Steinhardt, Neil Turok, 2001 to 2002	Collisions of higher-dimensional “branes”, repeating in cycles	Yes: predicts essentially no primordial gravitational waves
Conformal cyclic cosmology	Roger Penrose	The far future of a previous “aeon”	Claimed circles in the microwave background; not confirmed

1. Time began with the universe

In 1983 James Hartle and Stephen Hawking published “Wave function of the Universe” in Physical Review D. In their quantum description, near the beginning time behaves like a direction in space, so the universe has no boundary in time. Hawking liked to compare the question “what came before?” to asking what lies south of the South Pole. The question is not unanswered; it has no answer.

2. Eternal inflation

In many versions of inflation, the inflating state ends in some regions but continues in others, which keep expanding and spawning more regions. Our Big Bang would be the end of inflation in one patch, inside a much larger, perhaps endless, inflating space. This is one route to the idea of a “multiverse”. It is popular among theorists and very hard to test.

3. A Big Bounce

Loop quantum cosmology, an approach to quantum gravity, suggests that when matter is squeezed to near the Planck density, gravity turns repulsive. In 2006 Abhay Ashtekar, Tomasz Pawlowski and Parampreet Singh showed in a simplified model that the singularity is replaced by a bounce: a previous contracting universe rebounded into ours.

4. Colliding branes and cyclic universes

In 2001 Justin Khoury, Burt Ovrut, Paul Steinhardt and Neil Turok proposed the “ekpyrotic” universe, in which the hot Big Bang is the collision of two membrane-like worlds in a higher-dimensional space. Steinhardt and Turok extended it in 2002 into a cyclic model, published in Science, with endless rounds of expansion and collision. It makes a sharp prediction: almost no primordial gravitational waves.

5. Conformal cyclic cosmology

Roger Penrose proposes that the far future of one universe, when only radiation remains and scale loses meaning, becomes the Big Bang of the next. In 2010 he and Vahe Gurzadyan reported concentric circles in WMAP microwave maps as possible traces of the previous aeon. Independent teams searched the same data that year and found no significant signal, so the claim is not accepted.

How could we test any of this?

The most promising test is the faint twist that primordial gravitational waves would leave in the polarization of the cosmic microwave background:

• Most inflation models predict gravitational waves at some level. BICEP/Keck set the tightest limit in 2021: a tensor-to-scalar ratio below 0.036.
• Ekpyrotic and cyclic models predict essentially none.
• A detection would strongly favor inflation; continued silence would squeeze many simple inflation models and keep the alternatives alive.

Experiments at the South Pole and in Chile are pushing these limits down, and gravitational-wave detectors in space such as LISA may one day probe the early universe directly.

The bottom line

The Big Bang, as a hot and dense early state, is one of the best-tested ideas in science. What came before it is not. Treat any confident answer you read, including “there was nothing”, as an opinion about physics we do not have yet. The early chapters of the universe’s story, and its long future, are laid out in our Deep Time explorer.