The computer simulation shows the infall pattern of galaxies
around a forming massive supercluster of galaxies. Like compass
needles, the velocity vectors around the structure gauge the
magnitude of the gravitational growth, which depends on the
subtle balance between dark matter, dark energy and the
expansion of the Universe.
Credit: Klaus Dolag and the VVDS team.
A decade ago scientists discovered that the universe is
expanding faster now than in the recent past.
Previously everyone had assumed that the mutual
attraction of galaxies would cause the expansion
to slow with time. Thus, the acceleration was a
complete surprise and raised the question of what
might be causing it. Two main competing classes of
ideas try to provide an answer.
One possible explanation for the gravitational repulsion is
“Dark Energy”, a generalisation of the “cosmological constant”
which was proposed by Albert Einstein as part of his new theory
of gravity. Einstein himself later abandoned the cosmological constant,
but its fortunes have revived with the discovery of the cosmic acceleration.
According to this explanation 75 percent of the overall energy
density of the universe is Dark Energy, but its only measurable
effects are on the expansion of the Universe and on the rate at
which the clumping of matter grows with time.
An alternative explanation may be that the
equations of General Relativity (Einstein’s theory of gravity)
are incorrect or incomplete, and thus need to be modified.
In this case Dark Energy might be unnecessary.
Guzzo and his colleagues propose a new approach to studying this
issue which takes advantage of the fact that galaxy distances are
measured using redshifts. The spectrum of a galaxy is systematically
shifted to longer wavelengths by the motion away from us resulting
from the cosmic expansion, an effect which is called redshift.
In the first approximation the distance to a galaxy is directly
related to its redshift. However, galaxy motions are affected not
only by the cosmic expansion, but also by gravitational forces
resulting from the clumpy distribution of matter. Big concentrations
of matter attract nearby galaxies, producing additional motions.
As a result, redshifts give a distorted picture of the clustering
pattern in maps of the galaxy distribution. Guzzo and his team
demonstrated how this effect works using virtual universes created
by MPA scientists in Garching supercomputers.
By determining the strength of the distortion both in the
nearby (i.e., present-day) universe and the distant
(i.e., early) universe, it is possible to learn how
the clustering of matter has grown with time. This is
expected to be different for different kinds of Dark Energy,
and different again if Einstein's theory of gravity is modified.
Such measures thus give clues to the root cause of the cosmic acceleration.
Guzzo and his team carried out a survey of galaxies using
the ESO Very Large Telescope (VLT) on Cerro Paranal in the
Chilean Andes. With this 8-meter telescope they were able
to obtain redshifts of almost 6000 extraordinarily faint
galaxies, so distant that we see them as they were when the
universe was half its present age, about seven billion years ago.
They measured the distortion in the clustering at this time,
and compared it with values from surveys carried out nearby.
Their results are consistent with those expected if Dark Energy
is a cosmological constant.
However the uncertainties in these measurements are still large.
That’s why a new generation of surveys, covering at least
ten times the volume, is already being planned. With the
results of such surveys it may be possible to tell the
difference between a universe accelerated by a cosmological
constant and one with a different kind of Dark Energy or a new theory of gravity.
Prof. Dr. Simon White
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2211
Fax: +49 89 30000-2235