Euclid

European scientists have combined two proposed missions to study dark energy from space into a single project, named Euclid. The mission would combine two major techniques to look for dark energy.

The first is weak gravitational lensing, in which the satellite would look for the signature of dark energy in tiny distortions in the appearances of distant galaxies. In this technique, the path of light rays from distant objects is altered a bit by the gravity of intervening objects. As the light from galaxies that are billions of light-years away speeds through the universe, its path is deflected by the gravity of galaxy clusters and vast concentrations of dark matter between those galaxies and Earth. This produces small distortions in the appearance of the distant galaxies.

This Hubble Space Telescope image of a galaxy cluster shows the effects of gravitational lensing. The thin arcs around the bright ‘blobs’ are images of distant galaxies that have been compressed by the galaxy cluster’s powerful gravity. [NASA/ESA]

Astronomers use these distortions to plot the concentrations of dark matter in the universe. The distribution of dark matter depends in part on the expansion of the universe, which is affected by dark energy. Three-dimensional maps of the dark matter concentrations helps astronomers plot the expansion of the universe at different epochs, which in turn helps them choose the correct explanation for dark energy.

The second technique is known as baryon acoustic oscillations, and it looks at the imprint of sound waves from the early universe.

Scientists say the mission would produce a 3D map of about 500 million galaxies at distances of up to 10 billion light-years from Earth, as well as measuring the motions of each of these galaxies. The locations and motions of these galaxies would allow astronomers to chart the changing expansion rate as the universe ages, revealing the nature of dark energy.

Euclid would use a 1.2-meter telescope and digital detectors that are sensitive to both visible and near-infrared wavelengths of light, which are just below the range of the human eye. The light of distant galaxies is “stretched” to longer wavelengths by the expansion of the universe, which means they shine stronger in the infrared than in optical wavelengths.

The European Space Agency is considering Euclid as part of a long-range science planning process. A decision on whether to build the satellite is planned for 2011, with launch no earlier than 2017.