that we cannot see the entire universe right now.

And that's because there's such a thing that we quantify

as the observable universe.

The universe has existed, we think, since the Big Bang,

about, say, 13.8 billion years.

So as you look farther and farther out into space,

you necessarily have to look back in time.

If something is a million light years away from you,

like the Andromeda galaxy

is about two million light years away,

the light that you see through binoculars tonight,

as you look up at the Andromeda galaxy

left two million years ago.

We now have instruments that see so far,

we can actually look back to about 400,000 years

after the Big Bang.

We can see so far out into space

that the light has taken that long to get to us.

The whole idea of the Big Bang has been given

I think a real disservice.

There are so many misconceptions,

and certainly, one of the biggest misconceptions

is that people think that scientists feel

that the Big Bang came out of nothing.

I mean, how did all of this energy

and all of this matter that made up the universe,

you're saying it just came out of nothing.

No, I don't think any scientist actually believes that.

The problem is when you think

about the condition the universe was in at that point,

I mean, take our observable universe, right?

I mean, you can look from one side of the universe

to the other, you'll back 13.5 billion light years or more.

All of the stuff that we see was actually compressed

into a space smaller than an atom,

volume smaller than an atom.

We don't have the physics that describes

how that would work.

That is so much mass, so much energy and so little volume,

at this point, there wasn't even mass,

just basically pure energy,

that right now, our physics doesn't go there.

As we get a better idea about how gravity works

under very extreme circumstances, huge energy densities,

we may have some idea what set off the Big Bang,

and possibly what came before the Big Bang.

So today we actually have telescopes that are so powerful,

they can see back to a time about 400,000 years

after the Big Bang.

That's amazing, we can see so far away in space

that the light has taken that long to get to us,

nearly 13.8 billion years.

And when we look back to that time,

the universe looks very different.

For one thing, it's very hot. (chuckles)

It's actually about as hot as the surface of the sun.

And it's so dense and hot

that we actually can't see any farther.

Literally, in any direction you look around the sky,

anywhere you look,

if you look to that distance,

you see the universe as it was at that time,

400,000 years after the Big Bang,

and everything becomes just hot hydrogen gas.

That's incredible.

But what that means is that there's a limit.

There's a bubble around us that we can see

just because there's been time for light to come to us

from those areas.

Think about my arm being the universe before the Big Bang,

in some kind of state

that we can't even describe through modern physics.

The entire observable universe that we can see now

used to be a tiny volume of it, maybe an atom in my arm.

One atom expanded

and became the entire observable universe that we see.

But that's not the whole universe.

There are trillions of atoms in my arm.

Each one of those could expand

to actually be its own entirely observable universe.

So we can't tell yet how big

the universe was before the Big Bang,

or even what shaped the universe is,

because all we're seeing is a tiny little bit of it

that expanded to become everything that we see,

but that's not the whole universe.

That's our observable universe.

There's far more out there than what we can see.

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