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MARK HARTMAN: Cluster,
from the image,

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using the distance to the
center of the cluster, which

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was kind of the average of
all the redshifts of all

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the other ones that make
up the cluster, we got

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a linear diameter of 8 times
10 to the 21st meters, which

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we saw was about 8 to 10
times the size of one galaxy.

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That makes sense because
there's at least 10

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to 100 galaxies packed in there.

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When we look at
the closest galaxy

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to us using the same
prediction from Hubble's law,

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and we look at the
farthest galaxy from us

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still in the cluster--

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and some of you were noticing
that some of the galaxies

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are named Abell 2029, number 55.

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Those are actual
members of the cluster.

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Some of the other ones
may or may not have been.

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But then we took the
difference between these two

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to get the linear depth.

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And these are the numbers
that we were coming up.

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We were saying that from
the front of the cluster

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to the back of the cluster
is 10 to the 24th meters--

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9 times 10 to the 24th,
2 times 10 to the 24th,

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2 times 10 to the 24th.

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How many times longer
or deeper are we

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00:01:30,630 --> 00:01:33,450
saying this cluster is
compared to how wide it is?

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00:01:40,230 --> 00:01:44,790
What is the ratio between, say,
this number and this number?

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00:01:44,790 --> 00:01:47,940
AUDIENCE: Isn't it almost
1,000 times, about?

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MARK HARTMAN: We are saying that
this galaxy cluster is 1,000

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times deeper than it is wide.

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00:01:55,680 --> 00:01:57,962
That would be like
taking a galaxy cluster,

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stretching it across
the room, and having

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it be about that wide.

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Does that make sense?

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AUDIENCE: [INAUDIBLE]

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MARK HARTMAN: Did you
actually draw it over there?

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AUDIENCE: Yeah.

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00:02:07,500 --> 00:02:09,396
MARK HARTMAN: All
right, so project it.

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AUDIENCE: It doesn't
fit on one board.

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MARK HARTMAN: It doesn't
fit on one board.

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AUDIENCE: 300 times.

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00:02:14,744 --> 00:02:16,660
MARK HARTMAN: So that's
only 300 times longer.

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00:02:16,660 --> 00:02:19,740
So yeah, if we stretch
it across this whole room

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and it was only that wide,
what is going on here?

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We made a prediction
that, yeah, maybe,

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but how common would that be?

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If you have just a
clump of galaxies

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that formed from
some cloud, you might

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expect it to be maybe twice
as long, or maybe even 10

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times longer and it
would be like a cigar,

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but not 1,000 times longer.

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This is a prediction
that doesn't make sense.

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How did we get this
prediction again?

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Let's try to figure
out whats going on.

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How did we get the distances
to the front and to the back?

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How did you learn this?

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That question's going to come
back and bite you in the ass

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so many times.

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AUDIENCE: Hubble's law?

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MARK HARTMAN: How did
we get the distance

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to the front and the back?

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AUDIENCE: Hubble's law.

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MARK HARTMAN: Hubble's law.

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So what did we actually
measure using Hubble's law?

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AUDIENCE: Distance.

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MARK HARTMAN: We
predicted the distance.

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What did we measure?

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AUDIENCE: Velocity.

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MARK HARTMAN: The velocity.

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So we're taking the
velocities of these guys

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and turning them into a distance
and taking the difference

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of those distance.

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So we said that this
one is moving away

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from us because of the
expansion of the universe.

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This one's moving
away even further

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from the expansion
of the universe.

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Jaylen?

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AUDIENCE: How do you
measure the size of it

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by the velocity
of when it moves?

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MARK HARTMAN:
That's Hubble's law.

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We said that if things
are further away,

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we can look at their
velocity if they're

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expanding with the
universe's expansion

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00:04:05,630 --> 00:04:09,230
because we're using
velocity to get to distance.

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Why else might these
galaxies be moving?

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00:04:13,370 --> 00:04:15,821
Because we took motion, and
we turned it into distance.

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David?

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AUDIENCE: Motion
within the cluster.

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MARK HARTMAN: Maybe they're just
orbiting, just like the Earth

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does around the Sun.

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Some of these galaxies are going
to be moving towards us just

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a little bit.

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The whole cluster,
yeah, is moving away.

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But if I'm moving away,
and I have something

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that's orbiting around me--

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in this case, I'm
orbiting this way.

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Right here, my galaxy
is moving forward.

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Here, my galaxy is
moving backwards.

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When it's moving
backwards, if Bianca

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watches it move
backwards, it looks

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like it's moving backwards
faster than I'm moving.

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And if she watches this
marker move towards her,

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it kind of looks like it
stayed there for a minute.

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[LAUGHTER]

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So the motion of these
galaxies might not

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be due to the expansion
of the universe

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just like the motion the Earth
is indifferent because we're

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going around the Sun.

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It's not like the Earth and
the Sun are getting larger.

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So we get confounded, which
means we get confused.

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How much of that
recessional velocity

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is due to the fact that
the universe is expanding?

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How much of that is just due to
the fact that the galaxies are

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moving around?

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There might be a galaxy
that's doing this,

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maybe a galaxy
that's doing that.

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Some galaxies have weird orbits.

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So how would you
summarize why we might not

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be able to get an
estimate for the distance

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from the front and the back?

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00:05:59,232 --> 00:05:59,940
Go ahead, Bianca.

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AUDIENCE: Because
Hubble's law doesn't

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00:06:01,523 --> 00:06:04,500
apply to this model
of the [INAUDIBLE]..

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00:06:04,500 --> 00:06:07,619
MARK HARTMAN: Does Hubble's
law not apply at all?

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AUDIENCE: The
theory of expanding

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universe doesn't apply.

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MARK HARTMAN: When you have
objects that are orbiting,

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that are graivationally bound--

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I know a lot of you have
been using that phrase when

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you're talking about stuff--

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Hubble's law doesn't
hold for stuff that's

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gravitationally bound together.

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That object might be moving
away because of the expansion

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of the universe.

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But if there are
things orbiting around,

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it's not that that's
expanding too.

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So in this case, what
we can do, though,

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and what the galaxy
clusters group might do,

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00:06:41,960 --> 00:06:45,470
if you look at how those
objects are orbiting,

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you can actually get
an estimate for what is

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the mass of that whole cluster.

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And remember, we
looked at what's

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the X-ray luminosity
compared to what's

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the visible light luminosity.

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We can also get an
estimate for what's

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the visible light mass--
what mass in galaxies

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is giving off that light,
what mass in X-ray gas

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is giving off that light.

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And then by watching
the objects move,

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we can get an estimate of
how much mass is there.

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And remember, we said that
all matter, whether it's

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regular matter, or dark
matter, or anything,

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causes gravitational attraction.

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And by watching how
things move, you

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can actually get an estimate
of how much dark matter

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and regular matter there is
inside that galaxy cluster.

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So even though this
motion of these galaxies

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didn't help us to
figure this out,

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it gave us a clue that
the motion then might not

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be due to Hubble expansion.

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But instead, it's
due to that orbiting.

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And that orbiting can also
tell us something useful.

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So in this case,
how could we get

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an estimate of how deep it is?

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You can't.

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00:08:02,160 --> 00:08:04,650
You can assume that it's
about the same depth

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as it is a width if you
assume that it's spherical.

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Now, you can look at the
distribution of light

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and try to figure out
something about, well,

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if it was a football shape,
then it should be really, really

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bright in the middle because
there's all kinds of stars

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and galaxies and hot gas there.

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But essentially, what I
wanted you guys to see

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00:08:22,430 --> 00:08:26,660
was by making a prediction
from a model that didn't apply

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00:08:26,660 --> 00:08:29,900
to this situation,
we got a prediction

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that doesn't make any sense,
which means that you always

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have to double-check yourself.

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Always compare it to something
else that you have measured

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00:08:37,700 --> 00:08:39,710
or something that you
know to make sure,

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does this number
actually make sense.

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OK.