Press Releases
Apr. 24, 2009

Black-Hole Dark Matter can be observed by Gravitational Waves.

  • Jun'ichi Yokoyama (Professor, Research Center for the Early Universe, The University of Tokyo)
  • Ryo Saito (Graduate Student, Department of Physics, The University of Tokyo)

Black holes with a mass smaller than the lunar mass, which may have been formed through gravitational collapse of extremely over dense regions in the hot primordial Universe, are a viable candidate of the dark matter, an invisible and unknown constituent of the Universe other than atoms accounting for 23% of the cosmic mass density. Unlike elementary-particle candidates of dark matter, which can be probed by laboratory experiments, no observational or experimental means were known to prove their existence.

We have calculated the amount of gravitational waves resulting from collisions of high density clumps in the early Universe and found that their characteristic amplitude and frequency are closely related with the abundance and the mass of these primordial black holes. This means that the black-hole dark matter can be confirmed by the planned space-based gravitational wave detectors such as LISA, DECIGO, and BBO as well as atomic gravitational wave interferometric sensors. In addition, our result already rules out the possibility that the intermediate-mass black holes, which are known as a source of ultra-luminous X-ray, are of primordial origin, because the gravitational waves associated with their formation would be larger than the upper bound imposed by millisecond pulsar timing.

Figure 1

Figure 1. Logarithmic energy density of gravitational waves as a function of their frequency and the corresponding mass of the primordial black holes. Dotted horizontal line indicates the amount of gravitational waves produced in the case mass density of primordial black holes at each mass accounts for 20% of the total cosmic density. Wedge-shaped curves represent the frequency profile of gravitational waves in the case primordial black holes accounts for the intermediate-mass black holes (left) and dark matter (right). Curves marked with DECIGO and LISA represent sensitivity of gravitational waves for each detector.

Figure 2

Figure 2. Constraints on the mass fraction of the Universe that may collapse to primordial black holes. Dotted line represents the mass range to be constrained by future gravitational-wave detectors. The right V-shaped region is the newly excluded region by the timing data of a millisecond pulsar.