Astronomers are now half-way towards the largest (and deepest) map of our universe. Since the invention of multiplexing spectroscopy (which means taking many galaxy spectra simultaneously) in the 1980s, astronomers have been creating detailed maps of the nearby universe. These maps show how galaxies are distributed and how they are evolving with cosmic time over several billion years. Some recent surveys include 2dF Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS).
However since the late 1990s it is believed that a yet unknown dark energy is causing the expansion of the universe to accelerate. This dark energy seems to have ‘turned on” when the universe was half its present age (13.7 billion years). The current preferred cosmological model, the Lambda CDM (Lambda = Dark Energy + Cold Dark Matter) model is supported by the form of galaxy large-scale structure seen. The 2dFGRS and SDSS surveys are limited by the collecting area of their telescopes which makes it hard to probe to large distances and hence earlier times (~7 billion years ago) where the purported dark energy seems to have arisen.
The European Southern Observatory (ESO) has been undertaking a survey project to map the universe in three dimensions. In order to probe to larger distances ESO is using their 8m Very Large Telescope (VLT) to carry out this mapping project and the survey is called VIPERS (VIMOS Public Extragalactic Redshift Survey). VIPERS will survey the large-scale distribution of galaxies between 5 and 8 billion years ago, when the Universe was about half its current age. It will focus on measuring galaxy clustering and related cosmological quantities to understand the origin of cosmic acceleration.
Click here for the animations of the first release data from VIPERS, showing ~55,000 galaxy redshifts in two fields from z=0.45 to z=0.95, spanning 4.6 billion light years in co-moving length.
ESO astronomers have just announced the first results of VIPERS consisting of ~55,000 galaxy redshifts, about half of the final survey. (The complete survey will measure redshifts for ~100,000 galaxies between 0.5 < z < 1.2. The spectroscopic observations should be completed by 2014.)
One of the critical aspects of the survey will be measuring the peculiar velocities of galaxies (the velocity component due to nearby gravitational overdensities – i.e. groups, clusters, and not due to the overall expansion of the universe) to see how large-scale structure evolves with time.
Such deviations from the general expansion produce what is called Redshift Space Distortions (RSD). Tests will show if dark energy is correct, or perhaps if gravity (and thus the General Theory of Relativity, GTR) needs revising. The rate of growth of cosmic structure, f(z), is predicted by GTR. The venerable cosmologist Jim Peebles determined under GTR we expect to measure f(z) = [Ωm(z)]0.55 (Peebles 1980). Analysis of RSDs at large look-back times (~7 billion years) is crucial to testing whether the repulsive dark energy is valid.
For more information, see
- Huge Map of the Distant Universe Reaches Halfway Point, ESO press release
- VIPERS website
- The cosmological constant and dark energy, Peebles (1980), Rev. Mod. Phys. 2003, 75, p559
[Syed A. Uddin and Glen Mackie]