All civilisations have an origin myth. We are the first to have got it right.
Our origin myth goes by the name of the Big Bang theory. It is a wonderfully evocative name, but one that seeds confusion from the off. The Big Bang theory does not say that the universe started with a bang. In fact, the Big Bang theory has nothing at all to say about the birth of the universe. There is a very simple answer to the question “how did the universe begin?” which is “we don’t know”.
Instead our origin myth is more modest in scope. It tells us only what the universe was like when it was very much younger. Our story starts from a simple observation: the universe is expanding. This means, of course, that in earlier times everything was closer together. We take this observation and push it to the extreme. As objects are forced closer together, they get hotter. The Big Bang theory postulates that there was a time, in the distant past, when the Universe was so hot that matter, atoms and even nuclei melted and all of space was filled with a fireball. The Big Bang theory is a collection of ideas, calculations and predictions that explain what happened in this fireball, and how it subsequently evolved into the universe we see around us today.
The word “theory” in the Big Bang theory might suggest an element of doubt. This is misleading. The Big Bang theory is a “theory” in the same way that evolution is a “theory”. In other words, it happened. We know that the universe was filled with a fireball for a very simple reason: we’ve seen it. In fact, not only have we seen it, we have taken a photograph of it. Of course, this being science we don’t like to brag about these things, so rather than jumping up and down and shouting “we’ve taken a fucking photograph of the fucking Big Bang”, we instead wrap it up in dull technical words. We call it the cosmic microwave background radiation. We may, as a community, have underplayed our hand a little here. The photograph is shown in Figure 1 and contains a wealth of information about what the universe was like when it was much younger.
As we inch further back towards the moment, known colloquially but inaccurately as “the Big Bang”, the universe gets hotter and energies involved get higher. One of the goals of cosmology is to push back in time as far as possible to get closer to that mysterious moment. Progress here has been nothing short of astonishing. As we will learn, we have a very good idea of what was happening a minute or so after the Big Bang, with detailed calculations of the way different elements are forged in the early universe in perfect agreeement with observations. As we go back further, the observational evidence is harder to come by, but our theories of particle physics give us a reasonable level of confidence back to seconds after the Big Bang. As we will see, there are also good reasons to think that, at still earlier times, there was a period of very rapid expansion in the universe known as inflation.
It feels strange to talk with any level of seriousness about the universe when it was a few minutes old, let alone at time seconds. Nonetheless, there are a number of clues surviving in the universe to tell us about these early times, all of which can be explained with impressive accuracy by applying some simple and well tested physical ideas to this most extreme of environments.
The purpose of these lectures is to tell the story above in some detail, to describe 13.8 billion years of history, starting when the Universe was just a fraction of a second old, and extending to the present day.