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.