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Fermi caps first year with glimpse of space-time
Posted: October 29, 2009

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In its first year of operations, NASA's Fermi telescope has captured more than one thousand gamma ray sources and mapped the gamma ray sky in unprecedented detail, providing rare evidence on the structure of space and time.

“Physicists would like to replace Einstein’s vision of gravity – as expressed in his relativity theories – with something that handles all fundamental forces,” says Peter Michelson, principal investigator of Fermi’s Large Area Telescope, LAT. “There are many ideas, but few ways to test them.”

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. Image: NASA/Sonoma State University/Aurore Simonnet.

Many suggestions suppose space-time as having a shifting, frothy structure at physical scales trillions of times smaller than an electron. In this scenario the foamy aspect will cause higher-energy gamma rays to move slightly more slowly than photons at lower energy, but such a model would violate Einstein's theory that all electromagnetic radiation travels through a vacuum at the same speed.

In May of this year, a gamma-ray burst originating from a galaxy 7.3 billion light years away provided proof that Einstein's theory is solid. During the 2.1 second blast from GRB 090510, Fermi's LAT detected two gamma ray photons displaying energies that varied by a million times, yet despite the vast distance travelled, the pair arrived just nine-tenths of a second apart.

“This measurement eliminates any approach to a new theory of gravity that predicts a strong energy dependent change in the speed of light,” says Michelson. “To one part in 100 million billion, these two photons traveled at the same speed. Einstein still rules.”

This view of the gamma-ray sky constructed from one year of Fermi LAT observations is the best view of the extreme Universe to date. The map shows the rate at which the LAT detects gamma rays with energies above 300 million electron volts – about 120 million times the energy of visible light – from different sky directions. Brighter colors equal higher rates. Image: NASA/DOE/Fermi LAT Collaboration.

Fermi's Gamma-ray Burst Monitor has also had a fine first year, observing low-energy gamma rays from over 250 bursts of which the LAT simultaneously observed 12 at higher energy, revealing some record-breaking properties within three of the blasts.

GRB 090510 boasted material racing through space at 99.99995 percent the speed of light, while GRB 090902B yielded the highest energy gamma ray yet seen from a burst, at 33.4 billion electron volts, some 13 billion times the energy of visible light. The greatest total energy emitted came from GRB 080916C, equivalent to 9,000 typical supernovae.

Scanning the entire sky every three hours, Fermi's LAT has been mapping the gamma ray Universe in unprecedented detail, probing the "extreme Universe" to previously unseen depths. Amongst the bounty are numerous blazars, distant galaxies whose black holes are directing jets of matter towards us. Fermi has brought the blazar count to over 500.

“We’ve discovered more than a thousand persistent gamma ray sources – five times the number previously known,” says project scientist Julie McEnery at NASA’s Goddard Space Flight Center. “And we’ve associated nearly half of them with objects known at other wavelengths.”

The space telescope will continue to contribute to studies of the extreme Universe and the fabric of space-time throughout its expected lifetime of at least five years. Fermi resides in a low-Earth circular orbit at 550 kilometres altitude.