In this lecture we'll discuss the role of
dark matter in galaxy formation and learn
about the largest structures in the universe.
It's likely that dark matter is responsible
for pulling together galaxies and galaxy clusters.
There's just too much of it for it not to
have played a role.
You may recall that our understanding of galaxy
formation is that hydrogen and helium gas
in the protogalactic clouds collapse inward
and give birth to stars.
The dark matter is weakly interacting, so
it is unable to radiate away orbital energy.
It therefore stays in place and will not collapse
into a disk like the rest of the material.
This model tells us that the luminous matter
in each galaxy- the disk and globular clusters-
must still be nestled inside the larger spherical
halo of dark matter.
Observations of the motions of stars within
our own Milky Way and other galaxies support
this idea.
The formation of clusters of galaxies is likely
to be similar to that of the formation of
individual galaxies.
Early on, all the galaxies that will eventually
make up a cluster are flying apart with the
expansion of the universe, but the gravity
of the dark matter associated with the cluster
eventually reverses the trajectories of these
galaxies.
The galaxies ultimately fall back inward and
start orbiting each other with random orientations,
much like the stars in the halo of our galaxy.
Some galaxy clusters apparently have not yet
finished forming.
Their enormous gravity is still drawing in
new galaxies.
For example, the Virgo Cluster of galaxies
appears to be drawing in the Milky Way and
other galaxies of the Local Group.
The arrows on this diagram indicate the motions
of individual galaxies over a space of hundreds
of millions of light years.
We can see that the galaxies flow in regions
where the density of galaxies is already high.
It's dark matter that helps full galaxies
together.
We call the resulting vast, high-density regions
superclusters.
Beyond about 300 million light-years from
Earth, Hubble's law becomes our primary method
for measuring distances to galaxies.
With Hubble's law, astronomers can make maps
of the distribution of galaxies in space.
This endeavor requires a vast amount of data.
In particular astronomers need to measure
the redshifts of each individual galaxy so
that the distance can be estimated using Hubble's
Law.
Measurements have been made for millions of
distant galaxies.
The Sloan Digital Sky Survey has created detailed
three-dimensional maps of the Universe, with
images of one third of the sky, and spectra
for more than three million astronomical objects.
The maps show that galaxies are not scattered
randomly through space but are instead arranged
in huge chains and sheets that span many millions
of light-years.
Clusters of galaxies are located at the intersections
of these chains.
Between these chains and sheets of galaxies
are giant empty regions called voids.
The overall distribution of galaxies appears
nearly uniform on very large scales, and the
structures we see appear to mirror the original
distribution of dark matter in the early universe.
Supercomputers can simulate the growth of
galaxies, clusters, and large structures,
providing models of extremely enormous regions
of space.
These models show how dark matter, assumed
to be WIMPS, should be distributed throughout
the entire observable universe.
The results of these models look remarkably
similar to the slices of the universe from
the Sloan Digital Sky Survey.
This gives astronomers confidence in our understanding
of the role of dark matter in the formation
of large-scale structures.
I'll leave you with these simulations from
the Max-Planck-Institute for Astrophysics.
Here a large-scale simulation of galaxy distribution
fades to the the corresponding simulated distribution
of dark matter.
For me it can be overwhelming to think about
the enormity of our universe and how tiny
we are.
But at the same time, it's so beautiful that
we can know so much about the structure of
the cosmos.
It's also humbling that there are still so
many enormous mysteries to be solved.
Take care, I will talk to you again soon.
