[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]
