NASA - National Aeronautics and Space Administration

It may seem counterintuitive, but GLAST — a gamma-ray observatory — may provide crucial information about the amount of visible and ultraviolet light being emitted by the first stars to form in the Universe. GLAST will be able to study this problem because, as LAT science team member David Thompson of NASA's Goddard, explains, "Einstein's equation E = mc² works both ways."

In other words, matter can be converted into energy (in the form of electromagnetic radiation), but energy can also be converted into matter. If a gamma-ray photon interacts with another photon, and if the two photons combined have a high enough energy, they can combine to become an electron and its antimatter counterpart, a positron. Visible-light and ultraviolet photons have enough energy to combine with gamma rays to make these interactions possible. The first generations of stars and galaxies must have bathed the early Universe in visible and ultraviolet photons, making such interactions probable enough that GLAST's Large Area Telescope (LAT) should easily be able to detect this effect by seeing blazars fading and disappearing as it looks further and further back in time.

The effect depends greatly on a gamma ray's energy, since the higher a photon's energy, the more likely it is that it will meet another photon that it can interact with (or smash into) and produce matter particles. For example, below the energy of 1 GeV, a gamma ray can reach Earth from a redshift of 6 or higher, so LAT should be able to detect gamma-ray sources at those distances. (Redshift is a measure of how much an object's light has been "stretched" by cosmic expansion, so the higher an object's redshift, the further back we see it in time. A redshift of 6 corresponds to an era about 1 billion years after the Big Bang.) But gamma rays with energies of 100 GeV or higher should be absorbed en route to Earth if they originated from a source (such as a blazar) with a redshift of 6 or higher.

"GLAST will be able to measure how the amount of visible and ultraviolet light in the Universe changed over time by observing the gamma-ray flux as a function of distance and energy," says GLAST Project Scientist Steve Ritz of NASA Goddard. "This effect kicks in at lower energies and greater distances."

The EGRET instrument on NASA's Compton Gamma-ray Observatory wasn't sensitive enough to see enough very distant blazars and other gamma-ray sources to study this problem. In addition, it couldn't detect gamma rays with high enough energies to provide meaningful results. "But the LAT has both capabilities, so it can do this experiment," says Thompson. "It will give us a clue about the source of light in the very early Universe. This is an exciting prospect."

"The more stars that formed in the early Universe, the more absorption of gamma rays," adds Floyd Stecker of NASA Goddard. "These measurements will give us a handle on what happened in the distant past, which is why I call it photon archaeology." by Robert Naeye