Are black holes the dark matter?
A few months ago the news about the discovery of gravitational waves from two merging black holes with about 30 solar masses each rippled through the scientific community and caused a lot of excitement even among non-scientists. Almost exactly 100 years after Albert Einstein predicted them, gravitational waves were finally observed, and also gave proof that black holes exist — something that Einstein himself never believed in.
The discovery by the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Hanford, WA, and Livingston, LA was a huge scientific breakthrough, but immediately opened a slew of astrophysical questions. The fact that LIGO discovered two black hole mergers within a short period of time means that a very large number of these objects must be out there, and we do not have a clue where they came from.
Black holes or dark matter
This may have changed after a new publication suggested that the black holes discovered by LIGO may actually be the mysterious dark matter. It is well known that dark matter dominates the total mass of the universe. Its gravitational pull is responsible for keeping the stars together in the galaxies, and keeping the galaxies together in clusters, groups and the cosmic web of large-scale structure.
Despite several decades of experimental searches, there is still no glimpse of what could comprise the dark matter. On the contrary, the space where dark matter particles could still be found has dramatically diminished over the past few years.
Clues to the evolution of the universe
Our collaborator Alexander Kashlinsky from the NASA Goddard Space Flight Center has now linked the two mysterious dark phenomena black holes and dark matter together. If true, this would solve several major questions in our understanding of the Big Bang and the evolution of the universe.
If black holes of about 30 solar masses are actually dark matter, they must have been born in the first split second of the universe, right after the Big Bang. Every galaxy, like our own Milky Way, would be surrounded by a huge halo of black holes, and it is likely that the supermassive black holes in the centers of galaxies are descendants of some of these.
As his main argument, Kashlinsky uses a discovery that our group made a few years ago. We have found significant similarities in the faint cosmic diffuse background radiation in the X-ray and infrared wavebands, which indicates that the first population of objects emitting light in the early universe must have a substantial contribution from black holes. In our detective story, this may well be the first fingerprint of the dark matter black holes!
I am leading a group at the University of Hawaiʻi Institute for Astronomy that, together with Yale University and the NASA Goddard Space Flight Center, is expanding the X-ray/infrared cross-correlation studies to other large cosmological survey fields that have been observed with the NASA Chandra and Spitzer telescopes. We hope to measure the spectral shape of the X-ray emission from these early black holes, which may well give us a first proof about the nature of these fascinating objects.
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