Dark energy and the stretching of the universe
Observations have shown that the expansion of the universe is accelerating. Perhaps the simplest solution is to postulate a new component of energy density with an effective negative pressure that forms 75 % of the present energy density of the universe. There are many other theoretical ideas for explaining this phenomenon, which is often referred to as dark energy. Funded by the EU, the MDEPUGS (Measuring dark energy properties using galaxy surveys) project used measurements of the 3D positions for over 250 000 galaxies, covering the largest volume of the universe ever surveyed, by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) and included in the SDSS Data Release 9 (DR9). In total, the SDSS DR9 BOSS data includes spectroscopic redshifts for over 400 000 galaxies spread over a footprint of more than 3 000 deg2. The map can be used to understand how and why the universe's expansion is accelerating. It provides details on the position of galaxies out to some 6 billion light years from Earth and gives those measurements to within with an uncertainty of only 1.7 %. Project members used two techniques to try to understand the acceleration. One examined baryon acoustic oscillations. These are pressure-driven waves that passed through the very early universe and which were imprinted on the distribution of matter once conditions had cooled. Because this pattern can be traced all the way through the universe, it reveals a lot about its content. The waveforms observed constrain the values for the content and expansion rates in the early universe. The other technique used involved redshift space distortions. These describe the component in the velocity of galaxies that stems from the growth of structure in the universe. The team looked to see if neighbouring galaxies are clustering in a way that would be expected solely from the action of gravity. Results are consistent with Einstein's theory of general relativity, coupled with the cosmological constant that he put into his equations and that can be interpreted as a representation of dark energy. The team has also completed predictive work for the next generation of galaxy surveys, including the European Space Agency (ESA) Euclid mission.
Keywords
Dark energy, universe, galaxies, MDEPUGS, redshifts