Cosmic Foam


The fact that galaxies clump together has been known for quite a while. To study this clumping in detail requires observations of many galaxies. Although small surveys have provided important information about the structure of the universe, current large surveys, such as the  Sloan Digital Sky Survey (SDSS)  and the 2dF Galaxy Redshift Survey (2dFGRS) are a major step forward.
They measure the distribution of hundreds of thousand galaxies. These observations clearly demonstrate the complicated structure of our present day universe. Gravity is responsible for the web or foam like universe we see.

structure in the distribution of nearby galaxies as observed by the 2dFGRS

The figure above shows the results from the  2dF Galaxy Redshift Survey (2dFGRS). The distribution of galaxies
shows big concentrations, connected to each other through filaments. In between are large empty regions: the voids.

Numerical simulations are very good in reproducing the evolution of the dark matter distribution. The dark matter is so dominant that the results should reflect the real universe quite well. Compared to the real universe, the simulations have the advantage that we can make movies, flying through the cosmic web, and see clusters of galaxies forming. A nice collection of simulations can be found at http://www.MPA-Garching.MPG.DE/Virgo, which are based on simulations by the VIRGO consortium. Here we use some of these results to illustrate how structures form.
 


 

This movie gives an excellent indication of the foam-like structure of the universe. The structures form from very small inhomogeneaties in the (dark) matter distribution. Gravity then causes the higher density regions to collapse, and eventually
the structures we see today emerge. The movie below shows how a cluster of galaxies condenses out of "nowhere".
 


 

The final result depends on the global properties of the universe: the cosmological parameters. Astronomers have been working
for decades to determine their values. Ten years ago, measurements withfifty percent errors were considered good work.
Nowadays, the aim is more ambitious: we want accuracies of a few percent! Astronomers refer to this as "precision cosmology".
The picture below illustrates the appearance of the simulated universe changes for different cosmologies (the panels
to the right correspond to the present day universe).

These simulations only include the dark matter distribution. The key question now is whether the galaxies follow
the dark matter exactly. If true, the cosmological parameters are easily extracted from a comparison of the observed galaxy distribution and the simulations (or other predictions).

But what to do, if the situation is more complicated.... ?
 Weak gravitational lensing  can probe the dark matter directly, and essentially we can compare the results directly to the simulations. That is why weak lensing has become an important tool to study the universe.