[MUSIC PLAYING] When we look out
at distant galaxies, we see that they are
all racing away from us. We see that the
universe is expanding. The Big Bang Theory suggests
that once the entire universe was compacted into an
infinitely small speck at the beginning of time. But was this really the case? What parts of this theory are
still under serious question? [MUSIC PLAYING] In the last episode,
I showed you why the Big Bang Theory
is right, or at least, what parts of it pretty
much definitely happened. Direct and convincing
evidence tells us that the universe was once much
smaller, hotter, and denser than it is now. We're actually
pretty sure we know what happened all the way
down to approximately 10 to the power of minus
32 of a single second after the hypothetical
beginning of time. And at that point, the
entire observable universe was around the size
of a grain of sand. Now, we got down to that size by
rewinding the laws of physics, and in particular
running the math of Einstein's general theory
of relativity backwards. But how much further
can we rewind until we run into trouble? Well, things get pretty
weird before that 10 to the minus 32 seconds. Now, remember, when the universe
was younger than 400,000 years, it was too hot for
atoms to exist. Well, before 10 to
the minus 32 seconds, it was too hot for the
fundamental forces of nature as we know them to exist. In a previous episode, we
talked about how the Higgs field gives particles mass. Well, at temperatures
above 10 to the 15, or a quadrillion Kelvin,
it stops doing that. It turns out that when you
take this Higgs mass away from the particles that
carry the weak nuclear force, they become just like the
photon, which itself carries the electromagnetic force. This means that the weak and
electromagnetic forces sort of merge into the one
electroweak force. For a very brief period soon
after the beginning of time, these forces are combined. It's called the electroweak era. Sounds weird. But perhaps weirder is that it's
not really a mystery at all. We pretty much
know this happened because we can actually make
bits of the universe do this. We can create energies needed to
produce the electroweak states in the Large Hadron Collider. Yep, we can simulate the
instant just after the Big Bang. Our theories look really
good to that point. But once you get to a
crazy 10 to the power of 29 Kelvin at an age of around
10 to the minus 38 seconds, it's expected that
this electroweak force and the strong
nuclear force-- that's the force that holds
atomic nuclei together-- also become unified
into one force. There are a lot of ideas
of how this might happen. And we call these
grand unified theories, except they aren't theories in
the same sense as relativity or evolution because
we don't know which, if any, are actually correct. The problem here is that
we can't test them yet. We need to produce energies
a trillion times larger than is possible with the
Large Hadron Collider. Nothing we could build on
the surface of the Earth could do this. Perhaps that's for the best. So this is still a huge unknown. We do think that we
can describe gravity and the shape of space
time at these densities and temperatures. But we can't confidently
describe this stuff that the universe contained,
the weird state of matter that far back. OK, rewind a bit further
to 10 to the power of minus 42 seconds of age. And pack all of the galaxies in
the entire observable universe into a space 10 to the
power of minus 20th of the width of a proton. That's the Planck length. And here, physics kind
of goes out the window. See, at this point,
general relativity comes into serious conflict
with quantum mechanics. And we need a theory
of quantum gravity, a so-called theory of
everything, to go further. We can talk about why these
theories don't play nice together and what some
of the resolutions might be-- [CLEARS THROAT]
string theory-- another time. We're leaving it alone today
because we don't actually know whether the universe
was really ever this small. Remember, we've been
rewinding the universe using basic general relativity. Is that valid? Well, we don't yet
have direct evidence from those very early stages. But there are some clues
still visible at later times. And those clues tell us
we've missed something huge. Let's undo our rewind a
bit, fast forward again to 400,000 years after
that crazy first fraction of a second. The universe is space sized. But it's still
1,000 times smaller than the modern universe. It's full of this hot
glowing hydrogen plasma. But at 400,000 years, it's
cooled down just enough to form the very first
atoms, and in the process, release the cosmic
background radiation. We now see this light as
an almost perfectly smooth microwave buzz across
the entire sky. That smoothness
tells us that all of the material in the universe
when the CMB was released was almost exactly
the same temperature, around 3,000 Kelvin,
varying from one patch to the next by at most
one part in 100,000 across the entire
observable universe. Why is this so weird? If you have a cup of coffee,
and drop in some cold milk, it will all smooth out and
become the same temperature after a bit of time. Well, the universe
works in the same way. But based on the
simplistic expansion you predict from
general relativity, when the CMB was
released, there just hadn't been enough time for
this mixing to have occurred. See, in order for the
most distant patch of the universe we can
see in that direction to have the same temperature
and density as the most distant patch in
that direction, there needs to have been
enough time for something to travel between those
points to diffuse and even out that heat. And there just wasn't, not even
for light, the fastest thing that there is. Let's take this
grain-of-sand-sized universe at 10 to the minus 32 seconds. A photon emitted on
one side of that grain wouldn't have time to get
to the other side, not even in 400,000 years. See, although light is
fast, those opposite edges of the universe were
traveling apart even faster. Another way to say this is that
those edges of the universe have always been beyond each
other's particle horizons. A quick refresher. Here's a nice review
episode on cosmic horizons. So those edges shouldn't be
in each other's observable universes, not then, not now. This serious issue is
called the horizon problem. And it's a big deal that
we need to sort out. The only way around
this problem is to somehow have the
universe, once upon a time, be small enough
so it could easily get all nicely mixed together,
and then pow, blow it up in size much faster than general
relativity would normally allow. The theory that describes
this pow is called inflation. The idea is the universe
started subatomic, small enough that it was able to even
out its temperature and then enter the state of
insane, exponentially accelerating
expansion in which it increased in size by a factor of
at least 10 to the power of 26. So 100 trillion
trillion to something like our grain of sand
size at which point it slowed down to its
regular expansion rate. But its edges are thrown way
out of causal connection. Yet, they look the same
because they were once causally connected. This whole idea fixes
our horizon problem. In fact, inflation
solves a number of vexing problems with the
Big Bang Theory so well, in fact that most cosmologists
accept that something like this must have happened even
though we don't have any direct evidence for it. There are a number of
explanations for how inflation might have happened. And some of them actually call
into question our understanding of that very first
instant of the Big Bang. In this sense, it may be more
accurate to think of the Big Bang Theory not as a theory
of the origin of the universe, but instead as a
theory describing the period of expansion from
a subatomic to a cosmic size. Aspects of this theory
have such hard evidence that we know that the
basic picture is right. However, as with every really
well-established theory, there are boundaries to what
the Big Bang Theory currently explains. Those boundaries are being
chipped away at all the time. Perhaps the theory
will eventually encompass a true origin
for this universe. Or perhaps that
question will take us far beyond the Big Bang. One thing I stand by. We can science anything, the
origin of everything included. We'll get back to that on
future episodes of Space Time. In the last episode, we
laid out the evidence for why the Big Bang
definitely happened. You guys let us know
what you thought. ElectroMechaCat asks why if
the universe is expanding, doesn't matter also get
stretched with that expansion. OK, this is a genuinely
tricky question. It's surprising the matter would
not be stretched by expansion because the bonds
between and within atoms are vastly stronger than
any degree of expansion on the scale of any material
object in the universe. That's if it were even
valid to extrapolate that expansion to the scale of
objects or even to galaxies. What we call the Hubble
expansion of the universe arises from the
FriedmannLemaîtreRobertsonWalker metric, which describes a space
time in which all matter is perfectly smoothly distributed. That works on the largest scales
in which galaxies and galaxy clusters are a speckled foam
on top of a much vaster space time. It doesn't work in
galaxies or solar systems. For example, the solar
system is better described with the Schwarzschild
metric, dominated by the sun's gravitational field. In that metric, space is
most certainly not expanding. So you can have regions of
non-expanding space embedded in a globally
expanding universe. Kalakashi asks, does
this mean if you were to take every
proton, neutron, and electron in
the universe, you could fit them all into a space
the size of a grain of sand? This would be correct if you
add the word "observable" before the word "universe." Everything that we can see
to our cosmic horizon, so the observable
universe, was once compacted into something
smaller than a grain of sand, and indeed, much, much
smaller than that. However, this isn't
the same as saying that everything that exists
was compacted into that volume. If our universe is
infinite, then you can compact it as
much as you like and it will still be infinite. We don't know how large
the greater universe is. So we restrict ourselves
to talking about the size of the observable part of it. James Beech writes,
in the beginning there was nothing,
which exploded. Now, a lot of non-scientists
like to repeat this statement. But no credible scientist has
ever said this nor believes it. So it's unfortunate that
this statement is used to deride the Big Bang Theory. The Big Bang describes
a series of events that happened to the
universe following its existence in an
extremely hot, dense state. We have a ton of evidence
that the universe was once in such a state. Perhaps our understanding
of this state will eventually lead to a theory
of the origin of the universe. But the Big Bang
Theory as it stands does not claim to
explain such an origin. [MUSIC PLAYING]
