[MUSIC PLAYING]
Cosmic inflation describes a period
of insane exponential expansion right
after the instant of the Big Bang.
It calls into question our very understanding
of what the beginning of the universe even means.
[MUSIC PLAYING]
The Big Bang theory describes the earliest epochs
of our universe amazingly well.
It has made predictions that have been verified
beyond reasonable doubt.
But two significant problems with the simple model
tell us that something strange must
have happened in early times, an insane growth spurt
that we call inflation.
Today, I want to explain what inflation is, why we need it,
and why essentially all cosmologists believe
that it really happened.
The observable universe is impossibly huge.
And I'm not exaggerating the situation here.
It's so huge, 93 billion light years
from one edge to the other, that those most distant points
should never have had time to communicate with each other.
And yet, at some point in the distant past,
they must have been in contact.
The cosmic microwave background tells us
that they were once close enough together to become perfectly,
smoothly mixed.
This smoothness of the CMB is called the horizon problem.
And we talk about it in the last video.
The horizon problem isn't the only troubling feature
of the CMB.
We can use the apparent size of the very subtle fluctuations
in the CMB to measure the flatness of the fabric
of the universe, of spacetime.
And the answer we get is very strange.
Let me explain.
A flat sheet of paper is flat.
Duh.
Draw any triangle on one and add up the angles.
It's always 180 degrees.
Draw the same triangle on the surface of a sphere, which
has what we call positive curvature,
and the angles add up to more than 180.
On a negative curvature hyperbolic plane--
a saddle-like structure-- they add up to less.
Triangles in 3D space obey exactly the same rules as
on 2D surfaces, and their geometry
measures the curvature of space.
So now, think of those blobs in the CMB
as the ends of very, very long triangles.
We know the size of the brightest of those blobs.
They're defined by how fast sound waves could
have traveled by the time the CMB was created.
And we know how far away they are.
They're really, really far.
Using basic trigonometry, those distances
tell us what the little angle at this end should be.
It should be 1 degree, assuming the universe is flat.
It should be larger if the universe is positively curved,
smaller if negatively curved.
And yes, it's pretty much exactly one degree.
Based on the precision of our measurements so far,
we know that the curvature is within 0.4
of 1% of perfect flatness.
OK.
So what?
The universe is flat.
No, actually, it's extremely weird.
An expanding universe doesn't tend to stay flat,
even if it starts that way.
Analogy-- one way to bowl a strike
is to keep the ball near the center of the alley all
the way to the pins.
If the ball isn't moving fast enough,
then any initial deviation from dead center
will send it towards the gutter.
Same with the universe.
If the center of the alley represents a flat universe,
then the gutters represent extreme curvature
in the positive or negative directions.
If the universe starts out even a little bit not flat,
then that not-flatness will amplify quickly.
So if our universe is flat to within 0.4 of a percent now,
then in the first instant, the universe
had to be flat to one part in 10 to the power of 62.
That's like rolling your ball really, really slowly
and having it stay within 0.4 of a percent
of the center of the alley, and the alley is a light year long.
Nice bowling, universe.
This flatness problem is just as much
of an issue as the horizon problem.
Both seem strange if we assume that regular gravity was always
the only force affecting the rate of expansion
after the initial kick of the Big Bang.
We need to throw out that assumption because it
is giving us the wrong answer.
That's a science thing, questioning your assumptions.
Try it.
It's fun.
So it turns out that we can fix both of these problems
with a single, elegant idea called inflation.
It goes like this-- start with a universe
so crunched down that the entire currently observable part of it
was all causally connected.
Then, for a very short period of time,
blow it up much faster than the speed of light
so that most of it appears causally disconnected,
at which point inflation stops and regular expansion
takes over.
This works because even a very blotchy, curvy universe
is going to be much smoother and flatter on its smallest scales.
Inflation takes a very tiny, smooth, flat speck
of that blotchy, curvy greater universe
and blows it up to a macroscopic volume really, really fast.
That inflated speck subsequently grows into the universe
that we know, but retains its once subatomic smoothness
and flatness.
That's right.
According to inflation, the universe that we see
is a tiny part of a vastly larger universe
that itself may well be curved.
The neatness with which this inflation
solves both the horizon and flatness problems
really has most cosmologists thinking that something
like this must have happened.
For this to work, that inflationary expansion
had to throw neighboring regions of space apart at many times
faster than the speed of light.
It needed to increase the size of the universe
by a factor of at least 10 to the power of 26 in less than 10
to the minus 32 seconds, ending when
the universe was just macroscopic,
something you could hold in your hand.
In the subsequent 13.7 billion years
since, the universe has expanded by about the same amount
that it did during inflation.
So what sort of mad physics could do something like that?
Actually, Einstein came up with the exact mathematical
description that we need-- an antigravity term called
the cosmological constant.
In the field equations of his general theory of relativity,
he added this as a way to allow his theory
to describe a static space time, a universe that's neither
expanding nor contracting.
When it was later discovered that the universe is indeed
expanding, Einstein retracted his constant.
But this bit of math gives us exactly the type of expansion
that we need for inflation.
Incidentally, it also describes the effect of dark energy,
and that may not be a coincidence.
The cosmological constant represents something that
can happen to our spacetime.
Einstein is right, even when he's wrong.
The cosmological constant adds some energetic stuff
to empty space.
It doesn't tell us what this stuff is, just
that it's a property of space itself
and that it acts to drive expansion.
The more space, the more of this stuff.
And so the more space, the more expansion.
We'll delve pretty deep into how this
works in terms of general relativity
on a future episode on dark energy.
And we'll explore exactly what could
cause such a weird sort of energetic vacuum real
soon-- inflatons, scalar fields, forced vacuums, all of that.
For now, let's just go with the fact
that empty space can propel its own expansion
and will do so if the vacuum contains
a ubiquitous constant energy density.
Another really important thing about the driving mechanism
of inflation is that it stopped.
The universe slowed down from exponential
to the regular old expansion that we see today,
what we call Hubble expansion.
And while we know the minimum amount of inflation needed
before that stopping point, we don't really know when it began
or even if it had a beginning.
It may have, and there are some ideas
about what got it started.
But it's also possible that inflationary expansion
is the default state of the greater universe-- I should
say multiverse at this point.
This is the idea of eternal inflation.
I'll get to all of these wild ideas very soon.
With inflation, the Big Bang theory takes on new meaning.
When first conceived, the inflationary period
was thought to have started at a particular point
after the instant of the Big Bang.
But once you accept inflation, there isn't necessarily
a good reason to think that there was a normal expansion
period before, if there was even was a "before inflation."
In fact, the instant that inflation ended
can perhaps be thought of as the moment that our universe as we
know it came into being.
In that sense, inflation is the initial kick of the Big Bang.
We don't need to talk about an exploding singularity at all.
Time may not have begun with the Big Bang.
And so we'll rewind to before the beginning of the universe
very soon on "Space Time."
In a recent episode, we told you why space things
are the shape they are.
And you guys brought it with the big questions.
Frank Schneider asks, "Why does dark matter in a galaxy
seem to form a sphere?"
Well, this is because dark matter doesn't really
interact with itself except gravitationally.
The Milky Way and our solar system
were originally made of gas, giant clouds of the stuff.
And gas does interact with itself.
It drags on itself.
So even though the gas originally
had motion in many directions, over time,
it sweeps into a single bog flow.
Dark matter doesn't sweep itself.
It just passes by with a tiny gravitational tug.
So the orbits of any bit of dark matter
can be in any orientation or direction.
And random orientation orbits give you a spheroid.
This is also true of the stars in a eliptical galaxies.
Stars a small enough, compared to the distances between them,
that they can be in these random orbits.
The reason spiral galaxies are discy
is that those discs formed before the stars actually
formed, back when the material was mostly gas.
DBlanding and a few others questioned
my use of the concept of a centrifugal force.
And DBlanding, I definitely chose yours because
of your particular phrasing.
It's true that this force is, in a sense, fictitious.
It emerges in a rotating reference
frame as a force-like term that resists inward motion.
It doesn't exist in the inertial reference frames for which
Newton's laws are valid.
For example, in a reference frame
of an unmoving center of mass, you
have a center-pointing or centripetal force--
in this case, that's gravity-- acting on the orbiting object.
And the force is resisted by nothing.
The orbiting object is subject to the full acceleration caused
by gravity, which is what causes it to move in an orbit.
In that reference frame, it's the velocity perpendicular
to the centripetal acceleration that resists radial infall.
But centrifugal force is easier to say,
and it's still accurate.
A couple of you asked about the shape
of a black hole and its accretion disk
based on the spin of the black hole.
OK.
So first, the accretion disk that
forms around the black hole can either
be very flat or fatter, but probably more toroidal
than spherical.
Yet that doesn't have much to do with the spin of the black hole
itself.
It depends more on things like the rate of accretion,
the viscosity of the material, the way angular momentum
is lost by that material, stuff like that.
Although the magnetic field of a spinning black hole
can also play a part here.
However, the shape of the event horizon of the black hole
itself does depend on spin.
And indeed, a rapidly-rotating Kerr black hole
is flattened-- it's an oblate spheroid--
while a non-rotating Schwarzschild black hole is
a perfect sphere.
Omicron Vegra asks if I can please do the Blue Steel
face from "Zoolander."
I'm sorry.
I only do Magnum.
[MUSIC PLAYING]