It’s near impossible to comprehend the size of our universe without busting a mental cog or spraining your sense of awe. However, the origins of our universe has exactly the opposite problem: it was once mind-bogglingly small — tinier than a single particle. Physicist Lawrence Krauss explains the principle of inflation, and how within the first billionth of a second of the Big Bang, our universe increased in size by a factor of 10 to the 30th—for comparison, that’s the size of a single atom, to the size of a basketball. How did it do this? It involves a ‘frozen’ Higgs field, some cooling, and then an enormous explosion. Krauss uses an analogy we’ve all been at the mercy of: putting a beer in the freezer and forgetting it for a few too many hours. Lawrence Krauss' most recent book is The Greatest Story Ever Told -- So Far: Why Are We Here?.
Lawrence Krauss: Our picture of the earliest moments of the universe has been evolving, and I'm happy to say, in some sense has more empirical support than it did before. The discovery of the Higgs field implies that you can get fields that freeze in empty space. And that's a central part of what we think happened in the very early universe.
And if we can detect gravitational waves from the Big Bang we'd have a window on the universe back to a time when it was a billionth of a billionth of a billionth of a billionth of a second old, answering questions about the origin of the universe as we know it—ideas that I speculated upon in my last book, for example—for which we have new evidence that I've described in my new book.
But because the temperature of the universe and the energies and particles were so extreme at that early time—when the entire visible universe was contained in a region that was smaller than the size of an atom—there's a wonderful symbiosis between large scales and small scales.
And if we can probe the early universe back to a time that I described we'll actually be probing physics on scales that are much smaller than we can see at the Large Hadron Collider, 12 orders of magnitude smaller in scale (or higher in energy) than we can probe with our highest-energy accelerator now.
To build an accelerator that would directly probe those energies, we would have to have an accelerator that's not just 26 km around, as the Large Hadron Collider is, but whose circumference is the earth-moon distance, and that's not going to be built in our lifetime (and probably ever). So we may have to rely on the universe to give us new information, and that's why we're looking for such signals.
When the universe was a billionth of a billionth of a billionth of a billionth of a second old our current picture suggests: A field very similar to the Higgs field froze in space, but it was in what is called a metastable state. Sort of like… if you have a beer party and you put beer in the freezer because you forgot to until the few minutes before the party, and then during the party you forget that it's in the freezer, and you take it out later. And it's there—liquid—and you open it up, and suddenly it turns to ice, and the bottle cracks: The beer is in a metastable state. At that temperature it would rather be frozen except it's under a high pressure. The minute you release the pressure it freezes instantaneously, releasing a lot of energy. As our universe cooled we think the same thing happened; basically a field got frozen but in the wrong configuration, and as the universe cooled, suddenly—boom!— like those beer bottles, it changed its state, releasing a huge amount of energy, creating the hot Big Bang.
Now the interesting thing is, while it was in that metastable state and storing energy, general relativity tells us that if you have a field in empty space that's storing energy it produces a gravitational effect that's repulsive, not attractive. So during that brief time gravity is repulsive, and the expansion of our universe started speeding up faster and faster and faster, and the size of our universe (we think) increased by a factor of 10 to the 30th in scale, or at least 10 to the 90th in volume, in a time interval of a billionth of a billionth of a billionth of a second. That means it went from the size of an atom to the size of a basketball in a short time, and that rapid expansion produced characteristics which pervaded the universe today: The fact that our observed universal looks flat, the fluctuations, and the cosmic microwave background radiation all came from quantum fluctuations that happened during inflation.
Inflation is the only First Principles idea that in principle explains why our universe looks the way it does. And what's wonderful about it is it doesn't require any exotic ideas of quantum gravity or theories we don't have, it's based on ideas that are central to our current understanding of the standard model of particle physics, just extrapolating them somewhat. So it's very well-motivated; even though it is hard to believe that it could have happened, we think it did.