BRIAN: Welcome everyone to our course on cosmology,
where we're going to be looking at the issues that are literally the size
of the universe, as big as you can imagine.
PAUL: We're going to start off with two fundamental observations
or principles that actually determine most of our cosmological thinking.
One is the principal of homogeneity or uniformity.
And this is saying that, basically, the universe is the same everywhere.
The second, which we'll come to in the next video,
is the principal of expanding universe, which
comes from redshifts and distances.
And for this video, we're going to talk about what's
often called the extended Copernican principle, the principle
that every bit of the universe is the same.
Nowhere is special.
I guess this came out of history.
In history, people always thought that wherever they lived was special.
My village is the center of the universe.
And eventually, people got better at exploring,
they realized that didn't make sense, but then
their country was the center of the universe.
And then the Earth was the center of the universe,
and then the solar system was the center of universe.
And time after time, we felt that where we are
is special, and been proven wrong.
BRIAN: So you want to say that we're no place special,
but you and I know we're both special.
And you know, clearly, where we are here seems special to us,
and it's not like an average place in the universe, as near as I can tell.
Average place in the universe is going to be a lump of cold empty space.
PAUL: Yeah, so this principle that the universe is all the same
seems pretty perverse, because we know it's not the case locally.
And where this picture here, the density is 20 orders of magnitude
more than it is 100 kilometers above this place.
That's pretty not the same.
BRIAN: OK, so locally, clearly not the same.
So let's go out a little bit and think a bit about this.
PAUL: So the idea isn't that everything's identical in small scales,
but on big enough scales.
The question is, how big?
Maybe stars would be a good place.
Look at the night sky, and you see stars in every direction.
So maybe the universe is just full of stars,
uniformly spread all over the place.
BRIAN: But already, it doesn't.
Look, I see the Milky Way here, and if I look at that with a telescope,
then I'll realize that there's a billion stars in a tiny little piece there.
So it's clearly not--
PAUL: Yeah.
BRIAN: --uniform there.
That's different than here.
PAUL: Yeah.
So that's the first failure of uniformity.
Stars are not spread through the universe.
They're grouped in clusters called galaxies.
BRIAN: Right.
PAUL: About 10 to the 11 of them in typical galaxy,
us in the outskirts of one.
OK, so that's failure.
BRIAN: And we know that this galaxy, not our own, is separated,
and there's not going to be anything really near it,
and then there'll be another one, all on its own.
So again--
PAUL: So maybe there's a uniform-ness, maybe, universe just full
of galaxies spread all around the place.
BRIAN: Oh, so you think the galaxies will be uniformly distributed?
PAUL: Even though the stars are concentrated just in the galaxies,
maybe the galaxies are spread around uniformly.
BRIAN: So let's look at a map of the nearby universe.
PAUL: Yeah, so you've got us in the middle,
and we are in the local group, which is two
big galaxies and a whole bunch of small ones.
And there are other little groups around.
But then we get a bit to the side, you start
seeing a cluster, the Virgo cluster, which
is an enormous number of galaxies.
So clearly, galaxies are not spread uniformly.
They live in groups and clusters.
BRIAN: So we're sort of on the outside of this metropolis,
sort of in the suburbs of the Virgo cluster.
PAUL: Yeah.
BRIAN: And so, we're in a special place in the universe.
We're on the edge of a cluster.
But then there are voids, right?
Up here are these voids where there's almost nothing.
So--
PAUL: Yeah.
I mean, how about clusters?
Are cluster spread uniformly?
Well, no.
Just clusters have lined up.
Here's an even bigger image.
There's the Virgo, and then we've got Centaurus, Hydra-Centaurus cluster
and other clusters.
So we get super clusters of clusters, and they're
arranged in, like, strings or filaments.
BRIAN: A neighborhood of clusters string around, and once again,
so we haven't really found homogeneity yet.
OK.
PAUL: Yeah.
So this is beginning to look like a failed plan before you even started.
But if we just go to even bigger scale.
So one of the biggest maps was the 2dF Galaxy Redshift Survey.
What they did was, we are here, and they did a wedge across the sky,
and measured the distance to a whole bunch of galaxies, about 200,000
of them.
And each blue dot here is a galaxy.
What we see here.
BRIAN: So this almost kind of reminds me of the foam on a cappuccino.
So yeah, I see little lumps and bubbles, but it's beginning to kind of look
like when you look at a cappuccino for a long ways away.
It just looks like white if you don't mix up the coffee.
So it's sort of maybe a hint that it's beginning to look like that.
PAUL: Yeah.
So in every previous scale, you found things and they've been clustered,
so stars are clustered
in galaxies.
BRIAN: Yeah.
PAUL: Galaxies in clusters, clusters in super clusters.
But super clusters don't seem to be clustered in hyper clusters,
or whatever you want to call them.
BRIAN: So we're at a scale of about one to one and a half billion light years
now.
So we're getting pretty big.
PAUL: Yeah.
I mean, we see lumps that are maybe 100 million light years across,
but then there's another one, and another one, another one.
And they're not seemingly lined up.
And we can actually to even bigger scale still.
Piggy backing off the same survey was a quasar survey.
And quasars are so bright and so distant,
that we can extend out to 10 billion light years.
This is just a fly-through of that.
You can see the pattern here.
BRIAN: All right.
So it really is almost like a sponge, or a spider web, or something so--
PAUL: Yeah.
But again, looking fairly uniform.
It fades out at the end.
That's because you can't see any further.
BRIAN: OK.
PAUL: But if we now look at quasars--
BRIAN: Ah.
That's really looking boring.
This is the stuff you work on, boring stuff like this,
but I don't understand.
That all looks pretty random, but then, why-- maybe we are special.
We seem to be a place there aren't any quasars.
PAUL: Yeah, the quasars don't like us.
So we're now looking at a distance out to 12, 13 billion light years,
and it is pretty uniform in different directions across the sky,
but we do seem to be in a local hole.
The quasars are avoiding us.
And there's also an edge out here.
Now, it could be that we are in a special place,
and it just is-- this is the place that quasars hate.
But more likely what we're looking at here is an effect of time.
Because, bear in mind, if we see a quasar over there,
we see it not as it is now, but as it was, in that case,
about 6 billion years ago.
Because it's taken the light 6 billions years to reach us.
So what this is actually telling us, we don't see any right at the edge
here, because that's back where the Big Bang happened
and there hadn't been time for them to form.
And then there's a great era of quasars, and there's
giant black holes eating the middle of galaxies and shining like crazy.
But more recently, the black holes are still there,
but they've eaten everything near them.
So by and large, they've--
BRIAN: Oh, so--
PAUL: --stopped shining.
BRIAN: --there's like this age of quasars,
sort of analogous to the age of dinosaurs, and for whatever reason,
they've faded out.
And so, we are sort of at a special place in time, but not necessarily
in space.
PAUL: But odds are, if we actually lived on that galaxy over there--
BRIAN: Yep.
PAUL: --we'd see the same thing.
We'd see an apparent hole around us, because we're already
seeing the things that were nearby recently--
and there are not many quasars around it, we'd see lots of quasars over here.
So once again, it looks like special time, but one place in the universe
probably is very much like another.
BRIAN: That's convenient.
PAUL: Yes.
It turns out to be a really powerful constraint on the cosmological models.
That the idea that, quite opposite of the old idea
that we are the center of the world, except not only
are we not in the center, but nowhere is in the center.
Every place, we see pretty much the same thing,
once you're on these scales of billions of light years.
Yes, there are clusters and things on, pathetically, the small scale,
like 100 million light years.
But when you get up to several hundred million light years,
things really are uniform.