Dark Matter May Be Black Hole Pinpoints

Scientists at Stanford University say that dark matter, a mysterious substance that contributes to about 80 percent of the mass of the Universe, might be at least partially in the form of minuscule black holes.

These black holes, created in the early moments of the Big Bang during a theorized epoch called inflation, perhaps dot the entire Universe like holes in a fine sieve.

Dr. Pisin Chen of the Stanford Linear Accelerator Center discussed this theory at a meeting at Stanford University entitled "Beyond Einstein: From Big Bang to Black Holes." Chen said that future missions studying the Big Bang afterglow, called the cosmic microwave background, as well as the cosmic gravitational wave background, could find traces of these sub-atomic-sized black holes, which themselves are remnants of "evaporated" black holes born smaller than the width of a proton.

"If a sufficient amount of small black holes can be produced in the early Universe, then the resultant remnants, which are stable and interact only through gravity, can be an interesting candidate for dark matter," said Chen. "This math certainly doesn't rule out this possibility."

The nature of dark matter remains one of the great, unanswered questions of physics and astronomy. All the visible matter we can detect directly does not come close in accounting for the motion of stars in galaxies. Some unseen matter that is attracted by gravity, like ordinary matter, yet does not emit or reflect light must be present. There are many black hole candidates, each with charms and flaws. What is clear is that ordinary matter follows the gravitational pull of dark matter.

Black holes are often thought of as the endpoint of massive stars, formed in a star explosion, and most scientists agree that such black holes exist. Scientists speculate also on the existence of atomic-sized black holes containing several hundreds of kilograms of mass confined within a speck. Such black holes, often called primordial black holes, may have formed moments after the Big Bang.

Physicist Stephen Hawking postulates that black holes radiate energy and hence evaporate. This radiation arises from quantum fluctuations in space just outside the black hole. Atomic-sized black holes might evaporate extremely quickly, within one billionth of a second, as opposed to 10⁶⁰ years for stellar-sized black holes.

Chen and his collaborators at Stanford, Drs. Ronald Adler and David Santiago, find that the generalized uncertainty principle (GUP), which is supported by string theory, might prevent a black hole from evaporating completely. As a result there should exist a Planck-sized black hole remnant at the end of its evaporation. The Planck scale, 10^-35 meter, is the shortest distance conceivable. Essentially, evaporation stops when the black hole event horizon -- the theoretical black hole border -- approaches the Planck scale. Such a black hole remnant would be a spacetime "nugget" of finite density containing about a fraction of a milligram of mass inside a Planck volume. Chen points out that the stability of the black hole remnant may be further protected by the supersymmetry theory.

Chen said that certain inflation models, such as the hybrid inflation model proposed by Dr. Andre Linde of Stanford University, can indeed generate the right amount of small black holes at the end of the inflation needed for dark matter. Being born small, these black holes would rapidly evaporate down to their remnants within a tiny fraction of a second. (This notion implies the existence of a new "black hole epoch" immediately following the inflation epoch, where the Universe is dominated by small black holes and their evaporation.)

Lots of tiny black holes are needed to be a true contender for dark matter, perhaps several thousands within the volume of the Earth throughout the Universe. Because the black hole remnants are so small and interact so weakly with ordinary matter, direct detection is doubtful. However, signatures of primordial black hole formation might be frozen within the cosmic microwave background or the cosmic gravitational wave background.

Finding evidence for these primordial black holes requires extreme sensitivity and resolution, Chen said. The black holes would have produced gravitational perturbations at the end of the inflationary epoch, sort of like a specific kind of ripple. These ripples would be "frozen" in time today in the ancient light of the cosmic microwave background, like tiny waves on a frozen pond. The presence of black holes would reveal themselves in the degree of polarization in this light -- polarization detectable only at very small angular resolution.

NASA's Wilkinson Microwave Anisotropy Probe (WMAP) currently observes the cosmic microwave background. Chen said that successors to this mission, such as the European Space Agency's Planck mission or NASA's proposed CMBPol mission might be able to detect this subtle effect in the polarized background. Similarly, the black holes would have created ripples in the cosmic gravitational wave background, yet too faint and perhaps of too high a frequency to be detected by the joint NASA-ESA Laser Interferometer Space Antenna (LISA), proposed for launch next decade. We would likely need a follow-up to this mission.