Our universe have some peculiar properties-- properties
that couldn't be explained by conventional Big Bang theory.
For example, it's flat, meaning it's at just the right
mass density that it will neither expand forever
nor collapse back on itself.
Why should it be flat?
And completely opposite sides of the universe
that haven't had time to interact
are at the same temperature.
How can this possibly be?
These were two of the biggest questions in cosmology.
It wasn't until the 1980s, with the theory
of cosmic inflation proposed by Alan Guth,
that we found some answers.
Inflation took us back to the beginning of the universe
and, with exotic physics like repulsive gravity
and false vacuums, answered the why and what of the Big Bang.
The first problem introduced by the old Big Bang theory
was called the horizon problem.
Which is basically the problem of trying
to understand how the universe got to be so uniform.
Why does the universe look the same over there
as it does if you look that way?
And why was this a problem?
Well, think of a cup of tea.
If you pour milk into your tea, it'll
take some time for the molecules to interact
and eventually come to about the same temperature,
but it won't happen instantly.
The same is true on a larger scale.
The fastest any two objects can interact
is as soon as light has had time to travel between them.
Well, according to conventional Big Bang theory,
light hasn't had time to travel from one
side of the observable universe to the other,
so why should they be at the same temperature?
So inflation gets around that in really a very simple way,
is that if we trace back the universe that we're looking
at now to what it looked like before inflation,
it was vastly smaller than anybody
would have thought without the inflationary theory.
Vastly smaller is not an exaggeration.
Before inflation, everything in our observable universe
fit in a volume a billionth the size of a proton.
Then the universe went through two expansions-- inflation
and after.
Both expanded space by a factor of 10 to the 28,
but the second expansion took 13.8 billion years.
That first expansion, inflation, took 10
to the minus 38 seconds.
It's just an unfathomable rate.
Back to you, Guth.
And it was during the time before inflation,
when the universe was incredibly tiny,
that there was plenty of time for every piece of the universe
to communicate with every other piece and plenty of time
for it to come to essentially a uniform density of energy
and uniform temperature.
So now we know the universe was super tiny.
Well, the true genius of Guth's theory
was not the incredibly tiny universe,
but how it could have expanded so fast.
Inflation provided the mechanism for expansion-- repulsive
gravity.
In Newton's theory of gravity, gravity was always attractive.
There just was no other option, but it turned out
that the more complicated theory of general relativity
actually allows for the possibility
of a repulsive form of gravity.
Yes, in very specific circumstances,
gravity can provide a push, not a pull.
It comes from something called the false vacuum,
a state of matter in the early universe
that allows expansion of space while the mass density stays
constant, and that understanding for the mechanism of inflation
brought a solution to the horizon people.
The second problem was called the flatness problem.
Why is the universe so flat, and what do I mean by flat?
Well, the curvature of the universe
is defined by the mass density of space,
or the amount of energy and mass per volume.
If there is a lot of matter, the universe
is closed and collapses back in on itself.
If there's not much matter, the universe is open,
and it will expand forever.
If, however, the universe is in perfect balance
and the density is exactly critical density,
the universe will be flat, and it
will continue to expand forever, but at an increasingly
slower rate.
So it will eventually stop, but when time reaches infinity.
The mass density we have measured so far
appears to be exactly critical.
Why is that?
In fact, if it had started even slightly open or closed,
it would have been pushed even further away
from critical density over time by the expansion
of the universe, just like the longer an arrow has
to travel toward a target, the straighter you
had to have initially shot the arrow.
But we're so close to hitting a bullseye.
We're so close to critical density.
Why?
Inflation forces the universe toward critical density.
How?
Well, in the conventional Big Bang theory,
the universe gets larger, but as it gets larger,
it gets much, much less dense.
During inflation, the universe is getting larger and larger,
and it's flatter and flatter at a fixed mass density.
General relativity implies a direct relationship
between the mass density and the geometry
of space or the flatness, so as space expands,
the geometry gets flatter.
Imagine space like the surface of a balloon.
As you blow the balloon up, the surface
gets flatter and flatter.
Space does this in three dimensions.
And as space gets flatter, we go back
to our relationship between geometry and mass density,
and we find that the density is pushed toward critical density.
In fact, it's pushed very quickly toward critical density
since the expansion of space during inflation
is exponential, which explains why we're so
close to critical mass density.
And, boom, flatness problem solved.
So you may be wondering, if inflation theory was
so revolutionary and imaginative, why
haven't we heard more about it?
Well, inflation is still a hot topic of debate.
As we discussed, it solves many of the problems
of conventional Big Bang theory and is widely
accepted throughout the scientific community,
but like all good theories, it has
to make more predictions which we then observe.
We're hoping for more observations that
support inflationary theory, or whatever theory that
might improve upon it.
That's what we're working towards.
Thanks for watching.
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