If dark matter fills the universe, astronomers should see the gamma rays it produces. That evidence has so far failed to materialise
Among the most dramatic events in the universe are the death of stars as they collapse into black holes and the collision of black holes themselves. These events are so violent that they shake the firmament, generating gravity waves that ripple across the cosmos. They also generate huge blasts of neutrinos that can sometimes be picked up by giant neutrino telescopes on Earth.
But while these events are fascinating, not least because they almost certainly involve physics beyond our ken, they are hugely difficult to observe. That's because neutrinos and gravity waves are notoriously shy.
Neutrinos usually pass straight through the Earth. In fact, astronomers have only once detected neutrinos from beyond the Solar System and that was almost 25 years ago during a supernova called SN1987A.
But neutrinos are veritable party animals compared to gravity waves. Physicists have never seen a gravity wave, despite spending hundreds of millions of dollars on machines designed to find them.
Thankfully, there is a third way to study these extreme 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.
So what better time to take stock of its findings, say Luca Baldini at Italy's National Institute of Nuclear Physics in Pisa and few buddies. These guys represent the international collaboration behind Fermi so they ought to know.
Fermi has a unique view of the universe. Through its eyes, the sky is ablaze with a constant diffuse gamma ray light. About 70 per cent of this is generated by high energy cosmic rays smashing into stuff in our galaxy. The rest comes from beyond, from processes that we don't yet understand.
Superimposed on this background, Fermi also sees the occasional burst of gamma rays from distant violent events that are, albeit briefly, among the brightest things in the Universe. This cosmic fireworks display is our window into the most extreme conditions in the cosmos.
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 of these bursts 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.
Exactly what mechanism makes such high energy light is not known but more data will surely help.
Fermi is also transforming our understanding of active galactic nuclei: supermassive black holes at the centre of galaxies. By combining Fermi's observations of flares from these objects with observations at other wavelengths, astronomers have shown that whatever mechanism generates gamma rays, also generates other light too.
But 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.
More evidence comes from dwarf galaxies that are not easy to see in the visible part of the spectrum because they are mainly made up mostly of dark matter. But Fermi ought to be able to pick up the gamma rays this dark matter generates. So far it has seen little evidence of this and Baldini and co so this negative evidence will soon be published.
This is tantalising evidence that physicists and astronomers alike are still digesting. Their task is to work out whether the evidence is there and Fermi can't see it or that they it isn't there at all.
Fermi's view of the universe is a unique way of studying dark matter that provides a fascinating counterpoint to Earth-based experiments. That's something worth keeping an eye on.
Ref: arxiv.org/abs/1106.3416: Science highlights from the Fermi Large Area Telescope
"
