September 13, 2012
Astronomers Claim Dark Energy Is The Real Deal
April Flowers for redOrbit.com - Your Universe Online
Since the 1990's, dark energy has been the most accepted hypothesis to explain the expansion rate of the Universe. A hypothetical form of energy, dark energy is thought to permeate all of space and accounts for about 73% of the total mass-energy of the universe.
Tommaso Giannantoni and Robert Crittenden led a two-year study, after which they concluded that the likelihood of its existence stands at 99.996 percent.
Professor Bob Nichol, a member of the Portsmouth team, said: "Dark energy is one of the great scientific mysteries of our time, so it isn't surprising that so many researchers question its existence. But with our new work we're more confident than ever that this exotic component of the Universe is real — even if we still have no idea what it consists of."
Astronomers observing the brightness of distant supernovae more than a decade ago realized that the expansion of the Universe seemed to be accelerating. They attributed this acceleration to the repulsive force associated with dark energy, and though the researchers who made this discover received the Nobel Prize for Physics in 2011, the existence of dark energy remains a topic of hot debate.
Previous techniques used to try to confirm the reality of dark energy have either been indirect probes or they have been susceptible to their own uncertainties. Clear evidence for dark energy comes, at this time, from the Integrated Sachs Wolfe effect, named after Rainer Sachs and Arthur Wolfe.
The Cosmic Microwave Background (CMB), the radiation of the residual heat of the Big Bang, is seen all over the sky. In 1967, Sachs and Wolfe proposed that light from this radiation would become slightly bluer as it passed through the gravitational fields of lumps of matter, an effect known as gravitational redshift.
In 1996, Robert Crittenden and Neil Turok, now at the Perimeter Institute in Canada, took this idea to the next level, suggesting that astronomers could look for these small changes in the energy of the light, or photons, by comparing the temperature of the radiation with maps of galaxies in the local Universe.
In the absence of dark energy, or a large curvature in the Universe, there would be no correspondence between these two maps (the distant CMB and relatively closer distribution of galaxies), but the existence of dark energy would lead to the strange, counter-intuitive effect where the CMB photons would gain energy as they traveled through large lumps of mass.
In 2003, the Integrated Sachs Wolfe effect was first detected. This was immediately seen as corroborative evidence for dark energy, featuring in the "Discovery of the Year" in Science magazine. The signal was weak, however, and the expected correlation between maps is small, leading some scientists to suggest it was caused by other sources such as the dust in our galaxy.
Since those first papers, several astronomers have cast doubts on the original detections and have called into question the strongest evidence yet of dark energy.
In this most recent paper, published in the journal Monthly Notices of the Royal Astronomical Society, the team has re-examined all the arguments against the Integrated Sachs Wolfe detection as well as improving the maps used in the original work. They conclude that there is a 99.996 percent chance that dark energy is responsible for the hotter parts of the CMB maps (or the same level of significance as the recent discovery of the Higgs boson).
"This work also tells us about possible modifications to Einstein's theory of General Relativity", notes Tommaso Giannantoni.
"The next generation of cosmic microwave background and galaxy surveys should provide the definitive measurement, either confirming general relativity, including dark energy, or even more intriguingly, demanding a completely new understanding of how gravity works."
Image 2 (below): A visual impression of the data used in the study. The relevant extra-galactic maps are represented as shells of increasing distance from Earth from left to right. The closest thing seen is our Milky Way galaxy, which is a potential source of noise for the analysis. After this are six shells containing maps of the millions of distant galaxies used in the study. These maps are produced using different telescopes in different wavelengths and are color-coded to show denser clumps of galaxies as red and under-dense regions as blue. There are holes in the maps due to data quality cuts. The last, largest shell shows the temperature of the cosmic microwave background from the WMAP satellite (red is hot, blue is cold), which is the most distant image of the Universe seen, some 46 billion light-years away. The team have detected (at 99.996% significance) very small correlations between these foreground maps (on the left) and the cosmic microwave background (on the right). Image credits: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP. Click for a high resolution image.