A LONE astronomer says observations of the cosmos go further towards ruling out one of physicists’ most popular theories than the most expensive particle accelerators have so far. His claims are causing consternation among particle physicists, who say that his work relies on assumptions that may not be true.
Most physicists and astronomers agree that for rotating galaxies not to fly apart, they must contain much more matter than we actually see. It’s believed that more than 90 per cent of the Universe is made up this “dark matter”. Dark matter particles are called “WIMPs” – weakly interacting massive particles – because it’s thought they interact with each other too feebly to produce observable radiation.
The most popular WIMP candidate is the hypothetical neutralino. It’s predicted by a theory called “supersymmetry”, which has been proposed by particle physicists to explain observations that the Standard Model of physics cannot. One of its main attractions is that it can explain dark matter. It says that every particle we know has a host of heavier cousins – particles with the same characteristics but much greater mass. The neutralino is the lightest of these supersymmetric particles.
Advertisement
Particle accelerators have so far been unable to create or detect any of these particles. And as for ruling out some of the theoretical masses for the neutralino, they’ve hardly made a dent. But now astronomer Craig Tyler says he may have excluded at least half of all the possible values in one sweep, by peering into the heart of a galaxy.
Although we would never normally see neutralinos, they might give themselves away if squashed together tightly enough. Depending on their mass, they would collide and annihilate each other, giving off gamma rays. So for his PhD thesis at the University of Chicago in Illinois, Tyler focused on a tiny galaxy called Draco. It contains far more dark matter than most galaxies – 99.8 per cent of its total – yet the space-based Compton Gamma Ray Observatory looked at Draco last year and saw no gamma rays coming from the galaxy’s heart.
In Physical Review D (vol 66, p 023509) Tyler says that if Draco’s core is as dense as he predicts, then the lack of gamma rays severely restricts the possible mass of the neutralino. What’s more, if Draco’s magnetic field is similar to those detected in comparable galaxies, it should deflect other decay products of the neutralinos, producing radio waves. But no radio waves have been detected either, in theory ruling out almost every possible mass for the neutralino – bad news for supersymmetry.
“If you make his assumptions, this would be a problem for neutralinos,” says astrophysicist Katherine Freese at the University of Michigan in Ann Arbor. But she says Tyler has extrapolated too far from the available data on Draco: he assumes that its core is very dense, yet we only have data on regions further out, she says. “You’re relying on an untested assumption that almost everyone in the dark matter community thinks is wrong.”
Tyler is undaunted. “This method has the potential to eliminate most popular WIMPs,” he says. “There will be fewer and fewer things you can imagine doing with direct detection.” Indeed, the Gamma Ray Large Area Space Telescope, due for launch in 2005, should provide enough data to rule out or detect most WIMPs, even given doubts over where they are.
But Thomas Dent, who works on supersymmetry theory at the University of Michigan, insists stargazing could never replace particle accelerators. “Looking at annihilation in galaxies might be more powerful than direct detection, but there is a trade-off since the results are subject to more theoretical uncertainty,” he says.
