The figure above shows the results from the
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.