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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