WEBVTT

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Right now, the best estimate
of when the Big Bang occurred--

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and once again, I don't like
the term that much because it

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kind of implies some
type of explosion.

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But what it really is
is kind of an expansion

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of space, when space
started to really start

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to expand from a singularity.

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But our best estimate
of when this occurred

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is 13.7 billion years ago.

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And even though we're used
to dealing with numbers

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in the billions,
especially when we

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talk about large amounts
of money and what not,

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this is an unbelievable
amount of time.

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It seems like something that is
tractable, but it really isn't.

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And in future
videos, I'm actually

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going to talk about
the time scale.

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So we can really
appreciate how long,

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or even start to
appreciate, or appreciate

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that we can't appreciate how
long 13.7 billion years is.

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And I also want to emphasize
that this is the current best

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estimate.

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Even in my lifetime, even in
my lifetime that I actually

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knew about the Big Bang and
that I would pay attention

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to what the best estimate
was, this number's

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been moving around.

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So I suspect that in
the future, this number

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might become more accurate
or might move around some.

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But this is our best guess.

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Now with that said, I want to
think about what this tells us

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about the size of the
observable universe.

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So if all of the expansion
started 13.7 billion years ago,

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that 13.7 billion
years ago, everything

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we know in our
three-dimensional universe

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was in a single
point, the longest

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that any photon of light could
be traveling that's reaching us

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right now-- so let's say that
that is my eye right over here.

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That's my eyelashes, just like
that-- so some photon of light

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is just to getting to
my eye or maybe it's

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just getting to the
lens of a telescope.

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The longest that that
could have been traveling

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is 13.7 billion years.

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So it could be traveling
13.7 billion years.

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So when we looked
at that depiction--

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this I think was two
or three videos ago,

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of the observable universe--
I drew, it was this circle.

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And when we see light coming
from these remote objects--

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that light is getting
to us right here.

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This is where we are.

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This is where I guess
in the depiction

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the remote object was.

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But the light from
that remote object

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is just now getting to us.

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And that light took 13.7
billion years to get to us.

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Now, what I'm going
to hesitate to do,

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because we're talking
over such large distances

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and we're talking on such large
time scales and time scales

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over which space
itself is expanding--

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we're going to see in this video
that you cannot say that this

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object over here, this is
not necessarily, this is NOT,

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I'll put it in caps, this is NOT
13.7 billion light years away.

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If we were talking about
smaller time scales

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or I guess smaller
distances, you

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could say approximately that.

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The expansion of the universe
itself would not make as much

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of a difference.

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And let me make it
even more clear.

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I'm talking about an
object over there.

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But we could even talk about
that coordinate in space.

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And actually, I should say
that coordinate in space-time,

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because we're viewing it at
a certain instant as well.

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But that coordinate is not
13.7 billion light years away

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from our current coordinate.

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And there's a couple of
reasons to think about it.

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First of all, think
about it, that light

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was emitted 13.7
billion years ago.

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When that light was
emitted, we were

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much closer to that coordinate.

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This coordinate was
much closer to that.

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Where we are in the
universe right now

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was much closer to that
point in the universe.

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The other thing
to think about is

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as this-- let me
actually draw it.

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So let's go 300,000 years
after that initial expansion

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of that singularity.

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So we're just 300,000 years
into the universe's history

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right now.

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So this is roughly 300,000
years into the universe's life.

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I guess we could
view it that way.

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And first of all, at that point
things haven't differentiated

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in a meaningful
way yet right now.

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And we'll talk more
about this when

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we talk about the cosmic
microwave background radiation.

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But at this point
in the universe,

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it was kind of this almost
uniform white-hot plasma

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of hydrogen.

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And then we're going
to talk about it.

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It was emitting
microwave radiation.

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And we'll talk more about
that in a future video.

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But let's just think about two
points in this early universe.

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So in this early universe,
let's say you have that point.

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And let's say you have the
coordinate where we are right

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now.

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You have the coordinate
where we are right now.

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And in fact, I'll just make
that roughly-- I won't make it

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the center just because I
think it makes it easier

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to visualize if
it's not the center.

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And let's say at that very
early stage in the universe,

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if you were able to just take
some rulers instantaneously

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and measure that, you
would measure this distance

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to be 30 million light years.

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And let's just say
right at that point,

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this object over here--
I'll do it in magenta--

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this object over here
emits a photon, maybe

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in the microwave
frequency range.

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And we'll see that that was the
range that it was emitting in.

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But it emits a photon.

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And that photon is traveling
at the speed of light.

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It is light.

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And so that photon, says,
you know what, I only

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got 30 million light
years to travel.

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That's not too bad.

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I'm going to get there
in 30 million years.

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And I'm going to do it discrete.

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The math is more complicated
than what I'm doing here.

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But I really just
want to give you

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the idea of what's
going on here.

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So let's just say,
well, that photon

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says in about 10 million years,
in roughly 10 million years,

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I should be right about
at that coordinate.

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I should be about one
third of the distance.

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But what happens over the course
of those 10 million years?

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Well, over the course of
those 10 million years,

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the universe has expanded some.

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The universe has expanded
maybe a good deal.

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So let me draw the
expanded universe.

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So after 10 million years, the
universe might look like this.

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Actually it might even
be bigger than that.

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Let me draw it like this.

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After 10 million
years, the universe

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might have expanded a good bit.

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So this is 10 million
years into the future.

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Still on a cosmological time
scale, still almost at kind

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of the infancy of the
universe because we're

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talking about 13.7
billion years.

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So let's say 10 million years.

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10 million years go by.

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The universe has expanded.

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This coordinate, where
we're sitting today

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at the present time, is
now all the way over here.

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That coordinate where the
photon was originally emitted

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is now going to be
sitting right over here.

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And that photon, it said, OK,
after 10 million light years,

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I'm going to get over there.

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And I'm approximating.

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I'm doing it in a
very discrete way.

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But I really just want
to give you the idea.

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So that coordinate,
where the photon roughly

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gets in 10 million light years,
is about right over here.

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The whole universe has expanded.

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All the coordinates have gotten
further away from each other.

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Now, what just happened here?

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The universe has expanded.

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This distance that was 30
million light years now--

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and I'm just making
rough numbers here.

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I don't know the
actual numbers here.

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Now, it is actually--
this is really just

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for the sake of giving you the
idea of why-- well, giving you

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the intuition of
what's going on.

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This distance now is no
longer 30 million light years.

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Maybe it's 100 million.

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So this is now 100 million light
years away from each other.

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The universe is expanding.

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These coordinates, the space
is actually spreading out.

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You could imagine it's
kind of a trampoline

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or the surface of a balloon.

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It's getting stretched thin.

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And so this coordinate
where the light happens

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to be after 10
million years, it has

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been traveling for
10 million years,

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but it's gone a much
larger distance.

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That distance now might
be on the order of-- maybe

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it's on the order of
30 million light years.

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And the math isn't exact here.

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I haven't done the
math to figure it out.

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So it's done 30
million light years.

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And actually I shouldn't even
make it the same proportion.

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Because the distance it's gone
and the distance it has to go,

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because of the
stretching, it's not

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going to be completely linear.

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At least when I'm thinking about
it in my head, it shouldn't be,

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I think.

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But I'm going to make a
hard statement about that.

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But the distance that it
reversed, maybe this distance

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right here is now 20
million light years

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because it got there.

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Every time it moved some
distance, the space that it

00:09:20.960 --> 00:09:23.020
had traversed is now stretched.

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So even though its traveled
for 10 million years,

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the space that it
traversed is no longer

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just 10 million light years.

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It's now stretched to
20 million light years.

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And the space that it
has left to traverse

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is no longer only 20
million light years.

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It might now be 80
million light years.

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It is now 80
million light years.

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And so this photon might
be getting frustrated.

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There's an optimistic
way of viewing it.

00:09:48.880 --> 00:09:50.670
It is like, wow, I
was able to cover

00:09:50.670 --> 00:09:53.387
20 million light years
in only 10 million years.

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It looks like I'm moving
faster than the speed of light.

00:09:55.720 --> 00:09:58.270
The reality is it's not
because the space coordinates

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themselves are spreading out.

00:10:00.610 --> 00:10:01.630
Those are getting thin.

00:10:01.630 --> 00:10:04.496
So the photon is just moving
at the speed of light.

00:10:04.496 --> 00:10:05.870
But the distance
that it actually

00:10:05.870 --> 00:10:10.090
traversed in 10 million
years is more than 10 million

00:10:10.090 --> 00:10:10.590
light years.

00:10:10.590 --> 00:10:12.090
It's 20 million light years.

00:10:12.090 --> 00:10:15.480
So you can't just
multiply a rate times time

00:10:15.480 --> 00:10:19.090
on these cosmological scales,
especially when the coordinates

00:10:19.090 --> 00:10:23.100
themselves, the distance
coordinates are actually

00:10:23.100 --> 00:10:24.690
moving away from each other.

00:10:24.690 --> 00:10:27.430
But I think you see,
or maybe you might see,

00:10:27.430 --> 00:10:29.520
where this is going.

00:10:29.520 --> 00:10:33.150
OK, this photon says, oh, in
another-- let me write this.

00:10:33.150 --> 00:10:35.430
This is 80 million light
years-- in another 40 million

00:10:35.430 --> 00:10:40.190
light years, maybe I'm
going to get over here.

00:10:40.190 --> 00:10:43.110
But the reality is over that
next 40 million light years--

00:10:43.110 --> 00:10:47.440
sorry, in 40 million years,
I might get right over here,

00:10:47.440 --> 00:10:49.650
because this is 80
million light years.

00:10:49.650 --> 00:10:52.420
But the reality is
after 40 million years--

00:10:52.420 --> 00:10:56.030
so another 40 million years
go by-- now, all of a sudden,

00:10:56.030 --> 00:10:59.750
the universe has
expanded even more.

00:10:59.750 --> 00:11:01.710
I won't even draw
the whole bubble.

00:11:01.710 --> 00:11:04.680
But the place where the
photon was emitted from

00:11:04.680 --> 00:11:07.630
might be over here.

00:11:07.630 --> 00:11:11.850
And now our current
position is over here.

00:11:11.850 --> 00:11:18.250
Where the light got after 10
million years is now over here.

00:11:18.250 --> 00:11:24.070
And now, where the light
is after 40 million years,

00:11:24.070 --> 00:11:27.500
maybe it's over here.

00:11:27.500 --> 00:11:29.520
So now this distance,
the distance

00:11:29.520 --> 00:11:32.240
between these two
points, when we started,

00:11:32.240 --> 00:11:33.590
it was 10 million light years.

00:11:33.590 --> 00:11:36.380
Then it became 20
million light years.

00:11:36.380 --> 00:11:39.100
Maybe now, it's on the order
of-- I don't know-- maybe it's

00:11:39.100 --> 00:11:41.140
a billion light years.

00:11:41.140 --> 00:11:43.240
Maybe now it's a
billion light years.

00:11:43.240 --> 00:11:45.512
And maybe this distance
over here-- and I'm

00:11:45.512 --> 00:11:46.720
just making up these numbers.

00:11:46.720 --> 00:11:49.450
In fact, that's probably be too
big for that point-- maybe this

00:11:49.450 --> 00:11:52.010
is now 100 million light years.

00:11:52.010 --> 00:11:55.390
This is now 100
million light years.

00:11:55.390 --> 00:11:59.050
And now, maybe
this distance right

00:11:59.050 --> 00:12:01.730
here is-- I don't know--
500 million light years.

00:12:01.730 --> 00:12:04.080
And maybe now the total
distance between the two points

00:12:04.080 --> 00:12:05.950
is a billion light years.

00:12:05.950 --> 00:12:08.730
So as you can see, the photon
might getting frustrated.

00:12:08.730 --> 00:12:10.230
As it covers more
and more distance,

00:12:10.230 --> 00:12:13.200
it looks back and says, wow,
in only 50 million years,

00:12:13.200 --> 00:12:16.020
I've been able to cover
600 million light years.

00:12:16.020 --> 00:12:16.910
That's pretty good.

00:12:16.910 --> 00:12:18.701
But it's frustrated
because what it thought

00:12:18.701 --> 00:12:22.510
was it only had to cover
30 million light years

00:12:22.510 --> 00:12:23.050
in distance.

00:12:23.050 --> 00:12:25.070
That keeps stretching
out because space

00:12:25.070 --> 00:12:26.560
itself is stretching.

00:12:26.560 --> 00:12:32.190
So the reality, just going
to the original idea,

00:12:32.190 --> 00:12:36.050
this photon that is
just reaching us,

00:12:36.050 --> 00:12:38.870
that's been traveling
for-- let's say

00:12:38.870 --> 00:12:41.690
it's been traveling
for 13.4 billion years.

00:12:41.690 --> 00:12:43.180
So it's reaching us is just now.

00:12:43.180 --> 00:12:46.690
So let me just fast
forward 13.4 billion years

00:12:46.690 --> 00:12:49.620
from this point now to
get to the present day.

00:12:49.620 --> 00:12:54.842
So if I draw the whole visible
universe right over here,

00:12:54.842 --> 00:12:56.300
this point right
over here is going

00:12:56.300 --> 00:13:00.340
to be-- where it was emitted
from is right over there.

00:13:00.340 --> 00:13:04.890
We are sitting right over there.

00:13:04.890 --> 00:13:06.640
And actually, let me
make something clear.

00:13:06.640 --> 00:13:08.770
If I'm drawing the whole
observable universe,

00:13:08.770 --> 00:13:10.922
the center actually
should be where we are.

00:13:10.922 --> 00:13:12.630
Because we can observe
an equal distance.

00:13:12.630 --> 00:13:14.640
If things aren't
really strange, we

00:13:14.640 --> 00:13:16.620
can observe an equal
distance in any direction.

00:13:16.620 --> 00:13:19.210
So actually maybe we should
put us at the center.

00:13:19.210 --> 00:13:21.650
So if this was the entire
observable universe,

00:13:21.650 --> 00:13:26.180
and the photon was emitted from
here 13.4 billion years ago--

00:13:26.180 --> 00:13:29.800
so 300,000 years after
that initial Big Bang,

00:13:29.800 --> 00:13:35.620
and it's just getting
to us, it is true

00:13:35.620 --> 00:13:41.150
that the photon
has been traveling

00:13:41.150 --> 00:13:47.050
for 13.7 billion years.

00:13:47.050 --> 00:13:50.900
But what's kind of nutty about
it is this object, since we've

00:13:50.900 --> 00:13:53.970
been expanding away from each
other, this object is now,

00:13:53.970 --> 00:13:56.180
in our best
estimates, this object

00:13:56.180 --> 00:14:06.420
is going to be about 46 billion
light years away from us.

00:14:06.420 --> 00:14:08.785


00:14:08.785 --> 00:14:10.160
And I want to make
it very clear.

00:14:10.160 --> 00:14:13.220
This object is now 46 billion
light years away from us.

00:14:13.220 --> 00:14:16.781
When we just use light to
observe it, it looks like,

00:14:16.781 --> 00:14:18.530
just based on light
years, hey, this light

00:14:18.530 --> 00:14:21.280
has been traveling 13.7
billion years to reach us.

00:14:21.280 --> 00:14:22.981
That's our only way
of kind with light

00:14:22.981 --> 00:14:24.480
to kind of think
about the distance.

00:14:24.480 --> 00:14:27.740
So maybe it's 13.4
or whatever-- I

00:14:27.740 --> 00:14:31.340
keep changing the decimal-- but
13.4 billion light years away.

00:14:31.340 --> 00:14:36.600
But the reality is if you had a
ruler today, light year rulers,

00:14:36.600 --> 00:14:38.570
this space here has
stretched so much

00:14:38.570 --> 00:14:41.702
that this is now 46
billion light years.

00:14:41.702 --> 00:14:43.160
And just to give
you a hint of when

00:14:43.160 --> 00:14:45.100
we talk about the cosmic
microwave background

00:14:45.100 --> 00:14:47.959
radiation, what will
this point in space

00:14:47.959 --> 00:14:49.500
look like, this
thing that's actually

00:14:49.500 --> 00:14:52.010
46 billion light years
away, but the photon only

00:14:52.010 --> 00:14:54.540
took 13.7 billion
years to reach us?

00:14:54.540 --> 00:14:56.120
What will this look like?

00:14:56.120 --> 00:14:59.610
Well, when we say
look like, it's

00:14:59.610 --> 00:15:02.340
based on the photons that
are reaching us right now.

00:15:02.340 --> 00:15:05.350
Those photons left
13.4 billion years ago.

00:15:05.350 --> 00:15:07.090
So those photons are
the photons being

00:15:07.090 --> 00:15:10.040
emitted from this
primitive structure,

00:15:10.040 --> 00:15:16.460
from this white-hot
haze of hydrogen plasma.

00:15:16.460 --> 00:15:19.240
So what we're going to see
is this white-hot haze.

00:15:19.240 --> 00:15:26.930
So we're going to see this kind
of white-hot plasma, white hot,

00:15:26.930 --> 00:15:28.790
undifferentiated
not differentiated

00:15:28.790 --> 00:15:32.550
into proper stable atoms,
much less stars and galaxies,

00:15:32.550 --> 00:15:33.710
but white hot.

00:15:33.710 --> 00:15:36.220
We're going to see
this white-hot plasma.

00:15:36.220 --> 00:15:38.640
The reality today is
that point in space

00:15:38.640 --> 00:15:40.190
that's 46 billion
years from now,

00:15:40.190 --> 00:15:45.160
it's probably differentiated
into stable atoms, and stars,

00:15:45.160 --> 00:15:46.780
and planets, and galaxies.

00:15:46.780 --> 00:15:49.020
And frankly, if that
person, that person,

00:15:49.020 --> 00:15:51.210
if there is a civilization
there right now

00:15:51.210 --> 00:15:52.700
and if they're sitting right
there, and they're observing

00:15:52.700 --> 00:15:54.610
photons being emitted
from our coordinate,

00:15:54.610 --> 00:15:56.530
from our point in
space right now,

00:15:56.530 --> 00:15:57.920
they're not going to see us.

00:15:57.920 --> 00:16:01.810
They're going to see us
13.4 billion years ago.

00:16:01.810 --> 00:16:05.140
They're going to see the
super primitive state

00:16:05.140 --> 00:16:07.310
of our region of
space when it really

00:16:07.310 --> 00:16:09.164
was just a white-hot plasma.

00:16:09.164 --> 00:16:11.580
And we're going to talk more
about this in the next video.

00:16:11.580 --> 00:16:12.700
But think about it.

00:16:12.700 --> 00:16:15.510
Any photon that's coming
from that period in time,

00:16:15.510 --> 00:16:17.670
so from any direction,
that's been traveling

00:16:17.670 --> 00:16:21.170
for 13.4 billion years
from any direction,

00:16:21.170 --> 00:16:23.720
it's going to come from
that primitive state

00:16:23.720 --> 00:16:27.390
or it would have been
emitted when the universe was

00:16:27.390 --> 00:16:29.580
in that primitive
state, when it was just

00:16:29.580 --> 00:16:32.195
that white-hot plasma,
this undifferentiated mass.

00:16:32.195 --> 00:16:33.570
And hopefully,
that will give you

00:16:33.570 --> 00:16:36.480
a sense of where the
cosmic microwave background

00:16:36.480 --> 00:16:38.920
radiation comes from.