Max-Planck-Institut
  für extraterrestrische Physik

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

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.

Image: Klaus Dolag, MPA

Snapshot from a computer simulation of structure formation in the Universe, showing a patch of 100 million-light-years size and the resulting coherent motions of galaxies flowing towards the highest mass concentration in the center. The snapshots refers to an epoch about 10 billion years back in time. The color scale describes the mass density, with the highest density regions painted in red and the lowest in black. The tiny yellow lines describe the intensity and direction of galaxy velocities. Like compass needles, they map the infall pattern measuring the rate of growth of the central structure. This depends on the subtle balance between dark matter, dark energy and the expansion of the Universe.

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.

Contact:

Dr. Mona Clerico, Press Officer
Max Planck Institute for Extraterrestrial Physics and
Max Planck Institute for Astrophysics, Garching
Tel. +49 89 30000-3980
Fax: +49 89 30000-3569
email: clerico@mpe.mpg.de

Dr. Luigi Guzzo
Max Planck Institute for Extraterrestrial Physics and
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-3780
Fax: +49 89 30000-3569
email: luigi.guzzo@brera.inaf.it

Prof. Dr. Ralf Bender
Max Planck Institute for Extraterrestrial Physics, Garching
Tel.: +49 89 30000-3702
Fax: +49 89 30000-3351
email: bender@mpe.mpg.de

Prof. Dr. Simon White
Managing Director
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2211
Fax: +49 89 30000-2235
email: swhite@mpa-garching.mpg.de

MPE web pages: http://www.mpe.mpg.de/main.html

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