Dark energy: the quest for galaxies

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South Pole Telescope offers fresh view of Universe's expansion.

Astronomers searching for evidence of the mysterious energy that is speeding up the expansion of the Universe have discovered three new galaxy clusters. They used a microwave survey technique that could rival existing ways of searching for dark energy.

By locating new galaxies, the South Pole Telescope should allow researchers to make a more accurate estimate of the qualities of dark energy. Credit: J. McMahon

The discovery is the first step towards a catalogue of thousands of galaxy clusters, whose evolution in the early Universe reflects the tug of war between gravity and dark energy, the repulsive force that seems to make up three-quarters of the total mass-energy in the Universe. The astronomers found the three clusters by searching for shadows in the cosmic microwave background, the relic radiation of the Big Bang, using a microwave telescope located at the Amundsen-Scott South Pole Station in Antarctica (see 'The South Pole Telescope').

"We're looking at how many clusters there are as a function of time, and that depends on the expansion factor, which leads us back to the dark energy," says John Ruhl, a collaborator from Case Western Reserve University in Cleveland, Ohio, one of seven universities and labs involved in the US$19-million telescope.

Previous dark-energy estimates relied on observations of very old and distant star explosions. Astronomers think that supernovae of a specific class explode with uniform brightness, so the observed brightness can be correlated with distance. More distant supernovae seem to be receding more quickly than they should be, given a constant expansion of the Universe, so astronomers added a fudge factor: the accelerating effect of dark energy.

The galaxy-cluster method will provide an independent check from a different direction: the growth of structure in the Universe, as hundreds of galaxies coalesce into gravitationally bound clusters, and then stop coalescing. "They'd better both give the same answers or something is wrong," says John Carlstrom, director of the Kavli Institute for Cosmological Physics at the University of Chicago in Illinois and principal investigator for the project.

He announced the discovery on 9 October at a dark-energy conference in Munich, Germany. The findings have also been published on the arXiv preprint server (Z. Staniszewski et al. http://arxiv.org/abs/0810.1578; 2008).

The telescope came into use in 2007 and will complete its second season of observations in November. By the end of four winters, the team expects to have a catalogue of several thousand galaxy clusters in the southern sky. That would be enough to constrain dark energy to a precision that could rival that of the Joint Dark Energy Mission, a space telescope planned for 2015 that is likely to push the 'supernovae method' to its limits (see _Nature_ 455, 577; 2008).

Having two viable methods "is very important", says Mario Livio, a theorist at the Space Telescope Science Institute in Baltimore, Maryland. "The nature of dark energy is arguably the biggest question physics is facing today. You absolutely want to have more than one method to get at it."

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Hand, E. Dark energy: the quest for galaxies. Nature (2008) doi:10.1038/455843a

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