What was the Big Bang (and what came before it)?

… In the beginning (you know we had to start like this) the Universe seems to have been an infinitely hot, infinitely dense concentration of energy, not that we are entirely sure that the words beginning, hot, and dense have any meaning in this context, as we don’t have a working theory of physics to describe how anything behaves in these conditions. Once the Big Bang was underway, however we’re on slightly more familiar territory. Space started to exist, and the clock began to tick. Then something unusual and important happened: the Universe blew up.

It didn’t blow up in the sense of an explosion that blasts energy and matter outward, but in the sense of a balloon inflating. The Universe expanded exponentially, and it did so very quickly — faster than the speed of light. This idea is known as inflation, and it’s become the dominant theory to explain this time in the Universe because it neatly deals with several problems physicists have struggled with. One is how the Universe can be as big as it is (which it couldn’t be without this sudden early inflation) and another is how it later evolved, developing the concentrations of mass and energy that became galaxies and stars. These questions are complicated, to say the least, but the theory of inflation solves them, so physicists have become quite fond of it. In any case, the Universe experienced a brief burst of incredible growth in a very short time — far less than a billionth of a second.

Then it ends. The Universe was tiny at this point, only tens of centimetres across. For the next few billionths of a second it grew at a fantastic rate, more slowly than in the burst of inflation, but faster than the speed of light. This might seem a bit confusing: as nothing can travel faster than the speed of light, how could the Universe have expanded faster than the speed of light? The explanation is that expansion is not the same as travel. Aclumsy
analogy is two airplanes flying in opposite directions at their maximum speed — say 500 kilometres an hour. They’re flying apart at 1,000 kilometres an hour, but neither is travelling faster than 500 kilometres an hour. The analogy isn’t exactly correct, as there were no objects in the Universe at this point, but space itself was getting larger. We did say this was confusing.

During all of this early expansion there was no ordinary matter in the Universe. It was simply too hot for anything like an atom to exist. Things cooled off, though, as space expanded as there was less pressure constraining the energy of the Big Bang, and less pressure means less heat. As the Universe cooled, the building blocks of ordinary matter began to form. First subatomic particles like quarks took shape and then as they cooled they combined into protons and neutrons. At this point, the Universe was only one second old, and things were still pretty hot — about a trillion degrees. It took about four more minutes for things to cool enough that atomic nuclei could form as protons and neutrons come together to form deuterium (heavy hydrogen) and helium. The Universe now is filled with plasma — a hot soup of atomic nuclei and electrons. It stayed like this for a very long time.

Around 400,000 years after the Big Bang, the plasma had cooled enough for electrons to settle into co-existence with
protons. What existed was a fog of hydrogen, deuterium, and helium. This was the only normal matter in the Universe. It was plenty hot, at a temperature of about 3,000 degrees. Because these electrons were no longer free, they were not intercepting photons any more, and the Universe became transparent. Before this stage it was impossible for light to travel through the plasma — plasma is opaque. There was, however, not much to see. This was the Universe’s dark age. There were no stars to illuminate the Universe.

Eventually, the hot fog condensed into discrete clouds, and these clouds collapsed to form the first stars and galaxies, and when the stars lit up, the Universe was something like what we see in the skies today. These first stars burned fast and hot, and in only a few million years they exploded in massive supernovae. Their ashes formed the next generations of stars, which populated the Universe we live in.

This succinct explanation omits a wealth of detail about the Universe’s dark ages and how the first stars formed, one of the hottest areas of astronomy today.

That takes care of the time after the Big Bang. The natural question that comes next is where did the Big Bang come from. This is a different question from what was before the Big Bang. When you think about it, that question doesn’t make sense, as time as we understand it came into existence with the Big Bang. So where did the Big Bang come from?

Unfortunately that’s not a question for which science has a good answer. Observational astronomy, along with particle physics, theoretical physics, and mathematics, has developed the picture of what happened when the Big Bang banged and time started. We’re still reaching for understanding about the infinite energy and density that had to exist as a precondition for the Big Bang. At best, this is the realm of (educated) speculation. So little is known that this section of the book might as well be labelled like medieval maps of unknown territories with a large illustrated inscription: “Here be Dragons.”

Theoretical physicists aren’t scared of dragons, however. Physicists working on String Theory and Loop Quantum Gravity and Quantum Cosmology and the Grand Unified Theory are attempting to develop physical and mathematical models that would describe the conditions that created the Big Bang. These theories are very difficult for laypeople to understand as in essence they are mathematical, and the metaphors used to describe them — like strings — aren’t always helpful.

So for now, at least, the question of what caused the Big Bang is best answered with a shrug — or maybe a polite change of subject. After all, what fun would it be if we knew everything?