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The dark matter: hot vs cold

The non-baryonic dark matter consists of moving particles. Depending, on how fast particles move, it can be separated into two main categories:

hot dark matter: particles move fast.
cold dark matter: particles move slow.

Since temperature measures how fast atoms in the gas move, we can also use temperature to measure how fast dark matter particles move. Hot dark matter has a temperature about the CMB temperature or slightly less (say, 2 degrees Kelvin). The cold dark matter has a very low temperature, perhaps 0.0001 degrees Kelvin or even less.

For a cosmologists, the difference between the hot and the cold dark matter is that in a hot dark matter universe galaxies form only at z=1 or after that, quite recently in the life of the universe.

The cold dark matter

The cold dark matter consists of WIMPs: slow moving massive particles which do not interact with baryons (except by gravity).

Particle physics has many candidates for such a particle: light gaugino, sneutrino, higgsino, gluino, glueball etc

There are now several experiments under way that can detect some of those particles. So, perhaps soon we will have a direct experimental evidence of the existence of the dark matter. But in meantime we have to rely on astronomical evidence.

Hierarchical clustering

From observations we know that the universe is not homogeneous on small scales. It has a well developed structure on a range of scales. It is believed that this structure forms as gravity pulls the mass concentrations on various scales together. This process is called hierarchical clustering.

There are two main types of hierarchical clustering:

top-down: large structures form first, then fragment into small ones.
bottom-up: small structures form first, then assemble into larger ones.

$\framebox{\Huge\bf ?}$ The process when galaxies form before the clusters of galaxies is a

A.: top-down clustering.
B.: bottom-up clustering.

The type of the dark matter determines what kind of clustering our universe has.

$\framebox{\Huge\bf ?}$ Can you guess which is correct?

A.: hot dark matter $\rightarrow$ top-down clustering.
B.: hot dark matter $\rightarrow$ bottom-up clustering.
C.: cold dark matter $\rightarrow$ top-down clustering.
D.: cold dark matter $\rightarrow$ bottom-up clustering.

Now we have evidence that galaxies started forming very early, at z=5or even earlier (the farthest galaxy is at z=5.64). We also see that they were small, much smaller than the present day galaxies.

$\framebox{\Huge\bf ?}$ What does it tell you about the kind of clustering in our universe? About the type of the dark matter in the universe?

Growth of inhomogeneities

The inhomogeneities in the CMB are only 1 part in 100,000. In the expanding universe the inhomogeneities grow in proportion to the scale factor. We see the CMB at the surface of last scattering at z=1000, when the universe was 1000 times smaller than today. Thus, the inhomogeneities in the universe today should be 1 part in 100, or 1%.

Galaxies today are inhomogeneities in the universe with the amplitude up to 1 billion percent. They managed to grow billion times more than they should.

$\framebox{\Huge\bf ?}$ What is wrong?

Perturbations in the expanding universe can only grow when they are smaller than the horizon. Since the horizon grows with time, small scale perturbations can begin growing earlier than larger scale perturbations.

Thus, perturbation on a scale of galaxies started to grow when the universe was one millionth of its current size.

High redshift galaxies

Thanks to the Hubble space telescope, we now have a glimpse on high redshift galaxies. In December 1995 Hubble looked for ten days in a row at a tiny piece of the sky - 1/30th of the diameter of the full Moon - to get the deepest picture of the universe ever, the so called Hubble Deep Field, or HDF.

It really saw galaxies being born!

That was the best proof up-to-date that early galaxies are small, and that the structure builds bottom-up.

The most distant galaxy (discovered in the HDF) today is at z=5.64. One years ago it was at z=5.34.

$\framebox{\Huge\bf ?}$ It moved away because the universe expanded during the last year.

A.: Agree
B.: Do not agree

The modern picture of galaxy formation

The universe consists of about 60-70% of lambda-matter, 30-40% of cold dark matter (WIMPs), and 4% of baryons.

First galaxies began to form at z about 20. Stars formed within them.
At z about 8 to 10 they produced enough UV radiation to ionize the universe. The dark ages ended.
At z about 2 the galaxy formation reached its peak, most of galaxies and stars that we see today formed then.
Since z=1 galaxy formation did not proceed very fast, but most of large groups and clusters formed since then.
Right now, at z=0, superclusters are forming.