How could the Big Bang arise from nothing?

"The last star will slowly cool and fade away. With its passing, the universe will become once more a void, without light or life or meaning." So warned the physicist Brian Cox in the recent BBC series Universe. The fading of that last star will only be the beginning of an infinitely long, dark epoch. All matter will eventually be consumed by monstrous black holes, which in their turn will evaporate away into the dimmest glimmers of light. Space will expand ever outwards until even that dim light becomes too spread out to interact. Activity will cease. Or will it?

Strangely enough, some cosmologists believe a previous, cold dark empty universe like the one which lies in our far future could have been the source of our very own Big Bang.

The first matter

But before we get to that, let's take a look at how "material" – physical matter – first came about. If we are aiming to explain the origins of stable matter made of atoms or molecules, there was certainly none of that around at the Big Bang – nor for hundreds of thousands of years afterwards. We do in fact have a pretty detailed understanding of how the first atoms formed out of simpler particles once conditions cooled down enough for complex matter to be stable, and how these atoms were later fused into heavier elements inside stars. But that understanding doesn't address the question of whether something came from nothing.

So, let's think further back. The first long-lived matter particles of any kind were protons and neutrons, which together make up the atomic nucleus. These came into existence around one ten-thousandth of a second after the Big Bang. Before that point, there was really no material in any familiar sense of the word. But physics lets us keep on tracing the timeline backwards – to physical processes which predate any stable matter. This takes us to the so-called "grand unified epoch".

By now, we are well into the realm of speculative physics, as we can't produce enough energy in our experiments to probe the sort of processes that were going on at the time. But a plausible hypothesis is that the physical world was made up of a soup of short-lived elementary particles – including quarks, the building blocks of protons and neutrons. There was both matter and "antimatter" in roughly equal quantities: each type of matter particle, such as the quark, has an antimatter "mirror image" companion, which is near identical to itself, differing only in one aspect.

However, matter and antimatter annihilate in a flash of energy when they meet, meaning these particles were constantly created and destroyed. But how did these particles come to exist in the first place? Quantum field theory tells us that even a vacuum, supposedly corresponding to empty spacetime, is full of physical activity in the form of energy fluctuations. These fluctuations can give rise to particles popping out, only to be disappear shortly after.

This may sound like a mathematical quirk rather than real physics, but such particles have been spotted in countless experiments. The spacetime vacuum state is seething with particles constantly being created and destroyed, apparently "out of nothing". But perhaps all this really tells us is that the quantum vacuum is (despite its name) a something rather than a nothing. The philosopher David Albert has memorably criticised accounts of the Big Bang which promise to get something from nothing in this way.

Suppose we ask: where did spacetime itself arise from? Then we can go on turning the clock yet further back, into the truly ancient "Planck epoch" – a period so early in the universe's history that our best theories of physics break down. This era occurred only one ten-millionth of a trillionth of a trillionth of a trillionth of a second after the Big Bang.

At this point, space and time themselves became subject to quantum fluctuations. Physicists ordinarily work separately with quantum mechanics, which rules the microworld of particles, and with general relativity, which applies on large, cosmic scales. But to truly understand the Planck epoch, we need a complete theory of quantum gravity, merging the two. We still don't have a perfect theory of quantum gravity, but there are attempts – like string theory and loop quantum gravity. In these attempts, ordinary space and time are typically seen as emergent, like the waves on the surface of a deep ocean.

What we experience as space and time are the product of quantum processes operating at a deeper, microscopic level – processes that don't make much sense to us as creatures rooted in the macroscopic world. In the Planck epoch, our ordinary understanding of space and time breaks down, so we can't any longer rely on our ordinary understanding of cause and effect either.

Despite this, all candidate theories of quantum gravity describe something physical that was going on in the Planck epoch – some quantum precursor of ordinary space and time. But where did that come from? Even if causality no longer applies in any ordinary fashion, it might still be possible to explain one component of the Planck-epoch universe in terms of another.

Unfortunately, by now even our best physics fails completely to provide answers. Until we make further progress towards a "theory of everything", we won't be able to give any definitive answer. The most we can say with confidence at this stage is that physics has so far found no confirmed instances of something arising from nothing.

(The Conversation)