Hot Dark Matter vs. Cold Dark Matter

Hot Dark Matter vs. Cold Dark Matter :
Each scenario of structure formation has its own predictions for the appearance of the
Universe today. Both require a particular form for dark matter, a particular type of
particle that makes up the 90% of the Universe not visible to our instruments. These two
forms of dark matter are called Hot and Cold.

HDM produces large, smooth features since it travels at high velocity. Massive neutrinos
move at near the speed of light, yet interact very weakly with matter so can serve to
smooth out large density enhancements.
CDM, on the other hand, is slow moving and, therefore, clumps into small regions. Large
scale features are suppressed since the small clumps grow to form small galaxies.
There is strong evidence that galaxies formed before clusters, in the sense that the stars in
galaxies are 10 to 14 billion years old, but many clusters of galaxies are still forming
today. This would rule against the top-down scenario and support the bottom-up process.

Large Scale Structure :
Galaxies in the Universe are not distributed evenly, i.e. like dots in a grid. Surveys of
galaxy positions, e.g. maps of galaxies, have shown that galaxies have large scale
structure in terms of clusters, filaments and voids.
The clusters, filaments and voids reflect the initial fluctuations at recombination, plus any
further evolution as predicted by HDM or CDM models. CDM and HDM models have
particular predictions that can be tested by maps or redshift surveys that cover 100's of
millions of light-years.

Interestingly enough, the real distribution of galaxies from redshift surveys is exactly
in-between the HDM and CDM predictions, such that a hybrid model of both HDM and
CDM is needed to explain what we see.
The mapping of large scale structure also has an impact on determining is the Universe is
open or closed. Galaxies on the edges of the filaments will move in bulk motion towards
concentrations of other galaxies and dark matter. These large scale flows can be used to
determine the density of large regions of space, then extrapolated to determine the mean
density of the Universe.

Universe Today :
The present-day Universe is a rich collection of galaxies of many types,
clusters of galaxies, large scale structure and exotic phenomenon (e.g. Galactic
black holes). The galaxies themselves contain stars of all sizes, luminosities
and colors, as well as regions of gas and dust where new stars form. We
suspect that many stars have planets, solar systems in their own right, possible
harbors of life.
So what's going to happening in the future??
Time Reversal:
If the Universe is closed, then we might expect the arrow of time, as defined by
entropy to reverse. There appears to be a natural connection between the
expanding Universe and the fact that heat moves from hot areas (like stars) to
cold areas (like outer space). So if the expansion of space were to reverse, then
would entropy run the other way?

This kind of Universe has no real beginning or end, and is refered to as an
oscillating Universe. Notice that it's impossible to determine which side you
currently are on since time reverses and all appears normal to the observer.