There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.
There is another theory which states that this has already happened. -Douglas Adams
We started off our series on The Greatest Story Ever Told by talking about inflation, or a period when the Universe was expanding exponentially. We see plenty of evidence that inflation happened from looking at the geometry of the Universe (it looks like it was stretched flat),
at the fact that the physical properties of the Universe start off with the same initial conditions (implying they once were in contact with one another despite being billions of light years apart), and at the spectrum of primordial fluctuations, which are exactly the kind that we expect to arise from inflation.
Well, what comes next?
If you guessed “the Big Bang,” you’re on the right track. Inflation took care of three important things for us, and made the Universe flat, nearly-perfectly uniform, and gave us slight imperfections from that uniformity. But beyond that, we need to wind up with a Universe full of matter, anti-matter, and energy. We need that Universe to be very hot, extremely dense, and expanding very rapidly — but not exponentially — with the knowledge that it’s going to cool.
So how do you do it? How do you go from an inflating Universe:
to one that’s set to give us the seeds of the one we live in today? The problem with the inflationary Universe is that you take a teeny-tiny region of space that has a whole bunch of random stuff in it, and you stretch it out, exponentially, over many billions (and possibly many factors of 10 more that mere billions) of light years. No matter how much matter and energy was in that little region, you’ve taken that amount and stretched it across a region that’s bigger than the observable Universe, dropping the density down to something like one proton per Universe.
Whatever the temperature of your Universe was at the onset of inflation, it becomes awfully cold once inflation gets going. Like, absolute zero-ish cold. And yet, we want to create a big bang, and make all the matter and energy in the Universe today. How do we do this?
The same way this hydroelectric dam works. The water at the top of the falls has a whole bunch of potential energy that is stored in it. As the water falls lower and lower in elevation, that potential energy gets converted into kinetic energy, or the energy associated with motion. But it can also be used to turn turbines and generate electricity. This remarkably simple and useful phenomenon — that energy can be turned from one form into another — is the foundation of much of our modern technology.
Well, during inflation, something is driving the Universe to expand exponentially. Our best theoretical work tells us that this is some sort of incredibly large vacuum energy, which is a fancy way of saying,
during this time (inflation), space itself — this exponentially expanding space — has an incredibly large amount of potential energy stored within itself.
So this magic word, reheating, is the process where all of this potential energy is turned into another form of energy, known as mass.
How does this happen, physically? In our hydroelectric dam, gravity provides the potential energy, and when you slide down to a lower gravitational energy (at a lower elevation), the water gains speed and momentum, which can be used for turning turbines and generating energy.
In the inflating Universe, the thing causing inflation itself holds the potential energy. Do we know exactly what that thing causing inflation is? No. We can extrapolate the physics we know now and invent a new class of particle that could cause inflation — the inflaton — but we don’t know for certain that is a reasonable extrapolation. If any other particle, even just one of them, such as a quark, an electron, or a photon, has any interaction with this inflaton, then it will pull the potential energy out of the field the same way the gravitational potential energy of a ball gets pulled out and converted into other forms as it rolls down a hill.
For one take, see Mark Trodden’s post involving reheating. (But do bear in mind that I’m a little more conservative than Mark is when stating whether the existence of the inflaton is a definite or not.)
This question, the one of how we went from an inflating Universe to the one dominated by matter and energy that we live in today, is one of the great open questions of physics and astronomy. It’s great ground for theorists, because we have many good ideas and some observational constraints, but there is plenty of room for imagination. On this front, personally? I’m content to say “I don’t know” for the time being. We know inflation happened, but we don’t know what caused it. We know that it ended at some point and gave rise to the big bang, but we don’t know what the physical mechanism was that caused that.
I’ll say that again. So far — to recap — we don’t know what the physical mechanism was that either caused inflation to happen in the first place or caused it to end and give rise to our matter-and-energy-filled Universe. This will be a recurring theme in the very early and very late Universe: having evidence that a phenomenon happened, but not knowing what the correct mechanism for that phenomenon is. I’ll see you again for part 3 (spoiler alert), where we finally will get to talking about the big bang!