During its first year of operations, NASA's Fermi GammaRay Space Telescope mapped the extreme sky with unprecedentedresolution and sensitivity. It captured more than one thousanddiscrete sources of gamma rays -- the highest-energy form of light.Capping these achievements was a measurement that provided rareexperimental evidence about the very structure of space and time,unified as space-time in Einstein's theories.
"Physicists would like to replace Einstein's vision of gravity -- asexpressed in his relativity theories -- with something that handlesall fundamental forces," said Peter Michelson, principal investigatorof Fermi's Large Area Telescope, or LAT, at Stanford University inPalo Alto, Calif. "There are many ideas, but few ways to test them."
Many approaches to new theories of gravity picture space-time ashaving a shifting, frothy structure at physical scales trillions oftimes smaller than an electron. Some models predict that the foamyaspect of space-time will cause higher-energy gamma rays to moveslightly more slowly than photons at lower energy.
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, eliminating some approaches to a new theory of gravity. The animation link below shows the delay scientists had expected to observe.
(Photo Credit: NASA/Sonoma State University/Aurore Simonnet)
Such a model would violate Einstein's edict that all electromagneticradiation -- radio waves, infrared, visible light, X-rays and gammarays -- travels through a vacuum at the same speed.
On May 10, 2009, Fermi and other satellites detected a so-called shortgamma ray burst, designated GRB 090510. Astronomers think this typeof explosion happens when neutron stars collide. Ground-based studiesshow the event took place in a galaxy 7.3 billion light-years away.Of the many gamma ray photons Fermi's LAT detected from the2.1-second burst, two possessed energies differing by a milliontimes. Yet after traveling some seven billion years, the pair arrivedjust nine-tenths of a second apart.
"This measurement eliminates any approach to a new theory of gravitythat predicts a strong energy dependent change in the speed oflight," Michelson said. "To one part in 100 million billion, thesetwo photons traveled at the same speed. Einstein still rules."
Fermi's secondary instrument, the Gamma ray Burst Monitor, hasobserved low-energy gamma rays from more than 250 bursts. The LATobserved 12 of these bursts at higher energy, revealing three recordsetting blasts.
GRB 090510 displayed the fastest observed motions, with ejected mattermoving at 99.99995 percent of light speed. The highest energy gammaray yet seen from a burst -- 33.4 billion electron volts or about 13billion times the energy of visible light -- came from September'sGRB 090902B. Last year's GRB 080916C produced the greatest totalenergy, equivalent to 9,000 typical supernovae.
Scanning the entire sky every three hours, the LAT is giving Fermiscientists an increasingly detailed look at the extreme universe."We've discovered more than a thousand persistent gamma ray sources-- five times the number previously known," said project scientistJulie McEnery at NASA's Goddard Space Flight Center in Greenbelt, Md."And we've associated nearly half of them with objects known at otherwavelengths."
Blazars -- distant galaxies whose massive black holes emit fast-movingjets of matter toward us -- are by far the most prevalent source, nownumbering more than 500. In our own galaxy, gamma ray sources include46 pulsars and two binary systems where a neutron star rapidly orbitsa hot, young star.
"The Fermi team did a great job commissioning the spacecraft andstarting its science observations," said Jon Morse, AstrophysicsDivision director at NASA Headquarters in Washington. "And now Fermiis more than fulfilling its unique scientific promise for making novel, high-impact discoveries about the extreme universe and the fabric of space-time."
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.
(Photo Credit: NASA/DOE/Fermi LAT Collaboration)
Source: NASA/Goddard Space Flight Center