June 4, 2004  




Is Dark Matter Actually Black?

By Mason Inman

Gravity is the glue that holds together huge objects such as planets and galaxies. After looking at scores of galaxies, however, physicists realized something was amiss. On the outskirts of rotating galaxies, for example, stars were moving too fast for the galaxies to hold together by the gravity from the stars alone. They proposed that there must be a much greater amount of so-called dark matter that orbits alongside the stars, carrying them along by gravitational attraction, but otherwise showing little sign of its presence.

Though this notion of dark matter is now well established, the nature of this material is still up for grabs. Theorists have advanced many exotic particles and bodies like neutralinos and axions as candidates, but experiments haven’t pinned it down yet. At the Beyond Einstein conference at SLAC in May, Pisin Chen (ARDA) put a new spin on an old candidate for dark matter: black holes. Rather than giant black holes that voraciously consume matter and light, Chen is proposing much more benign objects, vastly smaller than atoms, called black hole remnants.

Several Big Bang theories propose that at the end of a phase called inflation in which the early universe grew enormously, multitudes of tiny black holes naturally arose, Chen says. According to Steven Hawking’s theory of black hole evaporation, however, it seemed these tiny black holes would quickly disappear. But the behavior of black holes as they approach nil hadn’t been worked out, Chen says. “It’s been a long standing question how the Hawking evaporation would end.”

Chen and his collaborators—Ronald Adler and David Santiago (both at Stanford)—began work on this problem simply to understand black holes better. They addressed black hole evaporation using the so-called generalized uncertainty principle, which is supported by string theory and melds quantum properties with gravity. The outcome: small black holes would evaporate almost completely, but would stop when they reached the Planck length, a theoretical lower limit on the size of anything. The Planck length is so miniscule, the gulf between it and an atom’s breadth is roughly the same as the difference between your height and the size of the visible universe.

The generalized uncertainty principle alone may not be enough to stabilize the black hole remnants so they’d survive indefinitely. Chen, along with his other collaborators Keshav Dasgupta (Stanford) and Marina Shmakova (ARDA), are now checking whether survival of black hole remnants requires additional symmetry principles from theories of supersymmetry and supergravity. These theories modify the idea of black hole evaporation, with the end result called an extremal black hole.

Several models of inflation, a hypothesized stage of the very early universe in which it quickly grew by leaps and bounds, naturally produce small black holes. These would then quickly evaporate until only tiny remnants were left. The hybrid inflation model, proposed by Andrei Linde (Stanford), in principle can produce just the right amount of small black holes so their remnants could account for all the dark matter that’s needed to explain the structure of galaxies, Chen says. But he speculates dark matter may turn out to consist of a variety of particles and bodies.

It’s unclear whether Planck-size black holes can be detected. They could collide with other objects or each other, thus growing into somewhat larger black holes. But since they’re so small, these collisions would be very rare. Most of the time these remnants would simply fly, ghost-like, through all other objects. This makes them good candidates for dark matter, but also poses a challenge for physicists who want to find them. “How can you ever capture such elusive objects?” Chen asks.

To detect these objects, if they exist, physicists may have to rely on indirect cosmological signs. If tiny black holes were created in the early universe, they would leave behind gravity waves. These could show up as subtle fluctuations in polarization of the cosmic microwave background, and might be visible in the data from the highly-sensitive Planck Surveyor, a NASA-European Space Agency joint project in the works. “Hopefully that will show something,” Chen says. But he cautions that his idea is just a plausible hypothesis, and not yet a self-consistent theory. “Before the theory of black hole remnants is further developed,” Chen adds, “it may be premature to ponder too much on its observation.”


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Thursday June 03, 2004 by Emily Ball