If Dark Matter Fills the Universe, Astronomers Should Detect the Gamma Rays it Produces –But They Don’t


Among the most dramatic events in the universe are the death of stars that generate huge blasts of neutrinos that can sometimes be picked up by giant neutrino telescopes on Earth. Neutrinos usually pass straight through the Earth. Astronomers have only once detected neutrinos from beyond the Solar System and that was almost 25 years ago during a supernova called SN1987A.

The most efficent and effective way to study these supernova events using gamma rays, ultra high energy photons. The mother of all gamma ray telescopes is the Fermi Space Telescope, which has been peering into the cosmos from low Earth orbit for three years now. Fermi's eyes see the occasional burst of gamma rays from distant violent events that are, albeit briefly, among the brightest things in the Universe. These gamma rays bursts are thought to be the release of energy equivalent to the mass of our Sun in a single second, probably as giant stars collapse to form black holes or as black holes or neutron stars collide.

Fermi has so far seen several hundred bursts of gamma rays from distant violent events that are, briefly, among the brightest objects in the Universe –at energies that stretch over six orders of magnitude, the highest being an event on 10 May 2009 which produced photons with an energy of 31 GeV, the highest ever observed in space.

Fermi's most controversial result involves dark matter. The thinking is that dark matter particles should annihilate producing gamma rays. This ought to produce gamma ray lines at specific frequencies but Fermi has found no evidence of this. Fermi ought to be able to pick up the gamma rays this dark matter generates, but ao far it has seen no evidence of this.

But, as Carl Sagan used to say, "the absence of evidence isn't the evidence of absence. The task of  Fermi's physicists and astronomers is to work out whether the evidence is there and Fermi can't see it or that they it isn't there at all.

The Daily Galaxy via technologyreview.com and Ref: arxiv.org/abs/1106.3416: Science highlights from the Fermi Large Area Telescope

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