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?

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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.