An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.
Superclusters
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Confronting The Big Questions: Highlights of Modern Astronomy
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University of Rochester
272 classificações
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What Exists Outside our Galaxy?
Galaxies and Their Environments - Galaxy types, Active Galactic Nuclei, Clusters of Clusters of Galaxies
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Adam FrankProfessor
Physics and Astronomy
Welcome back, everyone.
So, we've been moving outward in terms of scale of the universe,
and now we're sort of getting towards the largest of largest scales.
And the interesting thing is we still see a penultimate size scale.
We still see that there are structures in the universe out to extremely large
scales, and these are called superclusters, galaxy
superclusters, and they are clusters of clusters.
So the way we discover these is by
astronomers doing all sky surveys, basically looking over large
patches of the sky, measuring red shifts to
galaxies, so they can place them in three-dimensional space.
And then, you know, making maps, making
three-dimensional maps of the of the universe.
And what we find from this is that the universe that even out to
hundreds of millions of light-years, there is
still structure, that galaxies are not distributed uniformly
on these scales.
So when we've already seen there are these things called galaxy clusters
which are large collections of, you know, hundreds to thousands of galaxies.
And now what we're going to see is those
clusters themselves are a cluster that you get.
The clusters are sort of grouped together to form enormous filaments of galaxies.
So, a supercluster is a chain of clusters and groups bound together by mutual
gravitational attraction.
The whole idea that there's any structure at all
implies that gravity, since the time of the big bang,
has had time to be able to pull material
together to form structures, to form non-homoge, you know, inhomogeneities.
And that's really what we mean by structure in, in, astronomy.
So, the Milky Way part of the Virgo supercluster has about 100
galaxy clusters and groups together.
That, so, that's just, that's just our local supercluster, in some sense.
The typical cluster mass will be ten to the 15 times the mass of the sun and its
typical luminosity will be the same as taking, 10
to the 12 suns, and putting them all together.
So there's an enormous amount of mass and
an enormous amount of power in these super clusters.
You can also get lensing. We talked about lensing last time.
Lensing is the ability
for a foreground object, its gravity to distort space enough
that something in the background, the light from it will be
distorted such that we can actually that the, that background
object we will be able to see magnified images of it.
So superclusters also allow us to see gravitational lensing and
use those lensings for interesting things like doing precision cosmology.
Now one of the really interesting things about
these large amounts of mass, is the fact that if you
have a lot of mass, you're going to get gravitational force.
And if you're going to get gravitational force, you're going to get motion.
So we are sometimes able to detect the presence of large collections of mass,
sometimes dark matter, through the motions of the galaxies that we can see.
Sometimes these are called rivers of galaxies.
So as the universe expands, you know,
we know that we have the expansive motion.
And we expect that, the, what we call the Hubble flow,
just all the galaxies moving away from all the other galaxies.
But then we'll see places where they're, the galaxies depart from
the Hubble flow, and we actually see, not just expansion, but
we also see galaxies sort of pulling toward, together, together or
flowing towards one side of the sky in a nonuniform way.
And that tells us, from that we can infer the presence of large collections
of mass, of something like dark matter that we couldn't see any other way.
So, for example, there is what we call the Great Attractor.
It's a region of space where we see per they're called peli, peculiar.
Excuse me.
I had a hard time saying that. Peculiar velocities.
A peculiar velocity is any velocity which doesn't go
along with the Hubble flow, with the general expansion.
So, the great attractor is a region of space towards which we see vell,
galaxies flowing towards with velocities in excess of 700 kilometers per second.
So these, all these galaxies flowing in that direction tell us that there's a very
large collection of mass that we're not seeing
directly that must have resulted from gravitational attraction.
So these rivers of galaxies, which the Milky Way is part of,
are flowing towards even the largest,
or are flowing towards the largest superclusters.
So we know that the superclusters are part of this are the result of these
gravitational motions that are in excess of the Hubble flow.
And that is essentially the nature of how structure forms.
It's gravity collections of mass, falling into the gravitational well,
getting, making the well deeper, pulling even more material around it towards it.
And again it's really important to understand how much dark matter.
If you don't have dark matter, this is not going to make sense.
You need the dark matter in order to explain these gravitational motions.
So it's really, remember that most of the universe's mass is
in the dark form, not in the form that we can see.
And it's these things like the, the, the
rivers of galaxies that tell us this is true.
Okay.
So, is there any scale at which the mass distribution
in the universe actually smooths out,
becomes statistically uniform, meaning, if I,
you know, take a large enough region of space over here and
a large enough region of space over there, it actually just, the
amount of mass is the same and statistically, it looks the same.
And the question is, the answer to that question is yes.
When you get to large enough scales then the universe, the distribution
of mass does look uniform, and we call this the cosmic sponge.
So on the largest scale, the structure of the
universe becomes smooth, smooth like a sponge, in the sense that
if you look at a tiny piece of a sponge, you
know, on, over here and a tiny piece of the sponge
over there, statistically the number of holes and walls looks the same.
So that's what we mean.
Statistically, the pattern of superclusters
and the voids between them repeats.
So what would this means is that on the scale
of billions of light-years, there's, the universe finally begins to look
homogeneous and isotropic.
And that is essential because Einstein's theory of general relativity, when it's
applied to cosmology, only works if on the largest scales of the universe
is homogeneous, meaning smoothly de matrices,
smoothly distributed and isotropic, meaning in
every direction that you look, you're
always seeing the same kind of distribution.
So remarkably, even though there's so much structure in
the universe, once you get to the largest scales,
it does smooth out, and it allows us to do the
kind of cosmology that Einstein envisioned with his with his theories.
Okay.
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