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?
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.
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
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
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
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
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
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
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.