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.