© Matt Strassler, [March 15, 2014]
Here and in the linked articles below, you will find the history of the universe — or more precisely, the part of the history that we know, or at least strongly suspect — and even more precisely, not of the whole universe but rather just the part of the universe (called, on this website, “the observable patch of the universe“) that we can observe today, with many different types of telescopes. We don’t know anything about what lies outside the observable patch; the universe may extend on and on, and many parts of it, perhaps the vast majority of it, may be very different from the observable patch.
[So far] I have written articles on three periods of the universe.
- The most recent period is called the Hot Big Bang.
- Previous to this may have been a period called Inflation.
- Here’s what we know about what happened before inflation.
We got more evidence in favor of inflation on March 18th, 2014. This evidence did not stand up under heavy scrutiny and is now discredited.
Here on this page are some general, overall comments on the history.
Despite the fact that almost every popular description of the universe’s history says that “The Universe began with a Big Bang”, it has been known for decades that this statement is highly ambiguous, misleading, and in some ways wrong.
First, the Big Bang is not an explosion (despite the name, and despite hordes of books and videos and TV programs that say or imply otherwise.) It involves an expansion of space itself. See here for more details.
Second, the Big Bang theory does not tell you about the beginning of the universe; it never did. It simply says that at some moment long ago, the part of the universe in which we now live was extremely hot, dense, and almost uniform, full of a dense soup of particles, and in the process of expanding and cooling. Here’s more about that time. (We know it was hot enough to destroy atomic nuclei, and probably quite a bit hotter than that; but we don’t know the maximum temperature. [
After BICEP2’s new measurement, we have evidence for a maximum possible temperature. This measurement did not hold up under scrutiny.]) With these assumptions you can solve Einstein’s equations for gravity and figure out what happened after that time. Any guesses as to what happened before that time are just that: guesses. Many people have made scientifically-sound guesses, which is perfectly respectable activity for scientists. But unfortunately many articles/videos/images made for the public do not distinguish what scientists can calculate, predict and measure about the forward progress about the Big Bang — robust knowledge — from the guesswork that at some point sets in when we try to figure out what might have happened in the earlier stages of the universe, by trying to run the Big Bang backward. In my view, knowledge and guesswork should always be carefully distinguished in describing science to the public; unfortunately, in my experience, they rarely are. (Even Stephen Hawking’s books and videos fail to do this; the same is true of those by Michio Kaku and others. I would really like to encourage my colleagues to change this pattern.)
Third, the modern Big Bang theory inserts a period of inflation before the time when the region of the universe in which we live became hot. Inflation is explained here. Whether you say inflation is part of the Big Bang, or you say the Big Bang begins after inflation, is simply a matter of definition (see the end of the Inflation article). The really important thing about inflation is that during inflation the universe’s expansion was vastly more rapid than it was in the subsequent periods, and also, the universe was extremely cold during that period. Let me say that again: it was expanding extremely rapidly, and it was deathly cold. Bar from your brain any notion that the expansion of the universe inevitably has something to do with it being hot. The Big Bang is not like an explosion, no matter how you define it.
[Note added: I have to say that I regret that I wrote the previous paragraph this way. There’s a quantum subtlety with the previous remark, and I’ve obscured it. During inflation the universe is maximally empty, cold and dark — as empty, cold and dark as an inflating universe can become. But “extremely” is not a good term here… it’s too definitive. The problem is that there’s a complicated and very subtle effect from quantum fluctuations, one that deserves an article on its own, which I’ll try to write soon.]
Fourth, although people have made scientific guesses about what might have preceded inflation — or whether inflation has a beginning at all — no one knows. That’s where our knowledge completely disappears. Indeed, please remember we’re not even yet sure inflation occurred, though evidence has been growing in recent years.
Today, we live roughly 13.7 billion (13,700,000,000) years after the start of the Hot Big Bang. Notice I don’t say that “the universe is 13.7 billion years old” or that “the beginning of the universe was 13.7 billion years ago”… we don’t know that. What we do know is just that the Hot Big Bang began 13.7 billion years ago — but we don’t know if that moment was close to the beginning of the universe as a whole, or anything about what that beginning might have been like, if there even was a beginning.