Old and heavy: a simulated dark-matter halo the size of the Earth. Brighter regions show denser dark matter.© J. Diemand et al.Long before the first stars ignited, ghostly blobs of dark matter were forming in our Universe, according to astrophysicists who have used a supercomputer to replay the early history of the cosmos.
Dark matter produced just after the Big Bang formed into haloes as heavy as the Earth and as wide as the Solar System, the simulation found. The haloes' gravity would have pulled other matter together, which eventually formed stars and galaxies.
"These structures, the building blocks of all we see today, started forming only 20 million years after the Big Bang," says astrophysicist Ben Moore of the University of Zurich, Switzerland, one of the team who made the calculations.
The researchers estimate that there are now more than a quadrillion (1015) of these haloes in our Galaxy alone, enough for the Earth to pass through one every 10,000 years, they report in Nature1.
Moore believes astronomers have a good chance of detecting flashes of gamma rays emitted by the haloes. Others are more sceptical. "I wouldn't put my money on it," says astrophysicist Abraham Loeb of Harvard University in Cambridge, Massachusetts. The evidence for the haloes depends on unproven physics, he points out.
Elusive stuff
Dark matter makes up more than 80% of the Universe's mass. Although invisible, dark matter betrays its presence by its gravitational pull. For example, without dark matter to hold them together, rotating galaxies would simply fly apart.
“If you could detect these halos, you can backtrack to work out the precise conditions that generated them.”
Abraham Loeb
Harvard University, Cambridge, Massachusetts
A leading candidate for dark matter is a particle called the neutralino, which Moore's team used in its simulation.
The neutralino has never been detected, but a branch of particle physics called supersymmetry predicts its existence, as a massive partner to a known particle called the neutrino. Supersymmetry models predict the neutralino's mass and how it can be created, which has allowed Moore and his team to simulate its role in the early Universe.
The supercomputer that the Zurich team built in 2003 to perform the calculations was, for its size, the world's most powerful. "This has never been attempted before, because we haven't had the computing power," says Loeb.
Flash of light
The neutralino is its own antiparticle, so when two collide they annihilate each other in a blast of gamma rays. "We expect that gamma rays are constantly being emitted from the dense central regions of the halo," says Moore, so astronomers should be able to find physical evidence for the ancient congregations of neutralinos.
"And if you can detect these halos, you can backtrack to work out the precise conditions that generated them," adds Loeb.
Neutralino collisions would be extremely rare. "It would be like trying to see the light of a single candle on Pluto," says Jürg Diemand, another member of the Zurich team.
Nevertheless, Moore thinks that the HESS (High Energy Stereoscopic System) telescope in Namibia could pick up the flashes of light in the atmosphere caused when the gamma rays reach Earth. "As the gamma rays enter the Earth's atmosphere they create a shower of photons that sensitive telescopes can observe," Moore says.
The team also say that NASA's Gamma-ray Large Area Space Telescope, scheduled for launch in 2007, should be able to detect the gamma rays from space.
Back on Earth, the Large Hadron Collider currently under construction at CERN, the European particle-physics laboratory near Geneva, Switzerland, will hunt for supersymmetric particles such as neutralinos when it opens in 2007.
Harvard University, Cambridge, Massachusetts
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