Distortions to galaxy clustering yield clues to the puzzle of the accelerated expansion of the Universe

Observations of distant Type Ia supernovae and of the cosmic background radiation show that the universe is now in a phase of accelerated expansion. Spacetime itself is expanding, causing the galaxies to fly apart ever more quickly. The physical cause of this acceleration is not yet understood. Luigi Guzzo, a visiting researcher at the Max Planck Institutes for Astrophysics (MPA) and Extraterrestrial Physics (MPE), has led a team studying a new method to clarify the cause of the acceleration. This method is based on a distortion of the clustering pattern of galaxies, caused by gravitationally induced motions. The team compared data gathered with ESO telescopes to virtual universes created in MPA computers, showing that their method may indeed help solve one of the great puzzles of current physics (Nature, January 31, 2008).

Fig: 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
Managing Director
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2211
Fax: +49 89 30000-2235
email: swhitempa-garching.mpg.de