Quirks and Quarks Guide to Space

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 wordsbeginning,hot, anddensehave 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 wasLebans, Jim is the author of 'Quirks and Quarks Guide to Space', published 2008 under ISBN 9780771050039 and ISBN 0771050038.