“L2” – Ground Zero in the Search for Dark Energy a Million Kilometers from Earth

080516-dark-energy_big The spiral galaxy NGC 4258, 25 million light-years away, shown in the image left, is one of many that contain visible Type Ia supernovae, which astronomers use to tell how fast the universe is expanding. Ten years ago in May astronomers announced that the universe's expansion is speeding up thanks to dark energy, a mysterious "antigravity" that makes up 74 percent of the universe.

The Decadal Survey produced by the National Research Council has listed what it believes the big-picture priorities should be in the coming 10 years for American astronomical and astrophysical research. At the top of the list is the Wide-Field Infrared Survey Telescope or WFIRST, a 1.5m telescope costing about $1.6 billion, regarded as a must-have because it would help settle some fundamental questions about the nature of "dark energy."

WFIRST would be despatched to a gravitational "sweetspot" in space known as L2, a location more than a million kilometres from Earth on its night side where the observatory could have an unencumbered view of the sky. A major objective would be to spy as many stellar explosions, or supernovas, as possible. Scientists will want to see how their light has been stretched by the expansion of space. This will give them greater insight into how dark energy has worked through time and perhaps give them some clues as to how it might operate in the future.

WFIRST would get complementary information by mapping the distribution of some two billion galaxies. The pattern in the great voids that exist between the galaxies can be used as a kind of "yardstick" also to probe the expansion through time. And a third observation technique, known as weak lensing, would look at how the light from far-distant galaxies has been subtly shaped by intervening matter, again giving insights into the influence of dark energy through the epochs.

WFIRST wouldn't be just a dark energy mission, though. It would address broader issues, too, including undertaking a survey of exoplanets in the Milky Way to try to put some better statistics on how many Earth-like objects might be out there.

Not surprisingly, a European Space Agency mission (ESA) called Euclid is in the works do much of what WFIRST aspires to do. Euclid is currently in a competitive selection process with a couple of other  ESAspace concepts for two launch opportunities, in 2017 and 2018.

Europe expects to make a decision on whether to fly Euclid within the next 12 months; the US probably wouldn't make up its mind on whether to go with WFIRST for three or four years for a 2020 launch. 

Technically, Europe needs American involvement in Euclid. The European telescope's infrared detectors would have to be sourced from the US. But Europe is in the advantageous position of having a far-more advanced design which is now undergoing detailed industrial assessment.  Beginning in September, NASA will engage with  ESA in talks about how their dark energy ambitions can both be met.

Casey Kazan via www.bbc.co.uk/blogs/thereporters/jonathanamos/2010/08/the-dark-science-and-poker-in.shtml

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