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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.” |
