Another candidate for dark matter: primordial black holes (PBH's)[1]
Another candidate for dark matter is primordial black holes (PBHs) which may have formed shortly after the big bang. The universe’s first black holes may have been born in the earliest moments of cosmic time when the universe was just a seething, thick fog of fundamental particles. In the 1970s, theorists realised that dense regions of that fog could collapse under their own gravity just a second after the big bang, forming PBHs that would then shape the structure of the evolving, expanding universe. Emitting no light, PBHs would be a natural – albeit difficult to detect - candidate for dark matter.
The possibility that massive primordial black holes might actually be most or even all of the dark matter hinges on Alan Guth’s concept of cosmic inflation first proposed in the early 1980s. The authors describe inflation as “a hypothetical phase of prodigious expansion immediately after the big bang. In 10−35 second (when) two points separated by less than an atomic radius would have become separated by four light-years, a distance comparable to that of the closest stars” [2]. During inflation tiny quantum fluctuations are magnified to macroscopic scales by the rapid expansion, seeding the growing universe with underdense and overdense regions of matter and energy from which all cosmic structures subsequently emerge. This theory is strongly supported by observations of such density fluctuations in the CMB. As inflation ended, these fluctuations would create density perturbations that then form PBHs, and large, more powerful fluctuations would create more massive and more numerous PBHs.
The inflationary model proposed by Garcia-Bellido and Clesse predicts a broad peak of magnified fluctuations and a range of density perturbations, producing PBHs in clusters with each PBH ranging from 100th to 10,000 times the mass of our sun. These quantum fluctuations, enormously magnified by inflation, would then naturally produce particularly dense regions that would collapse to form a population of black holes less than one second after inflation ends. Such black holes would then behave as dark matter and dominate the matter content of the present-day universe.
Within half a million years after the big bang, each growing, evolving cluster could contain millions of PBHs in a volume just hundreds of light years across. The largest PBHs could be seeds for supermassive black holes (SMBHs) and galaxies formed a million years after the big bang. Today PBH clusters would lurk as unseen dark matter in and around galaxies, holding them together, as it were. In short, PBHs may not only provide the missing link between stellar conventional black holes and black holes of the supermassive variety (SMBH). They may also solve the mystery of dark matter and observations by Advanced LIGO and other gravitational wave detectors may provide the necessary evidence and confirmation of this hypothesis. [3]
[1] What follows is an edited version of the article by Juan Garcia-Bellido and Sébastien Clesse previously referred to: "Black Holes from the Beginning of Time", Scientific American, July 2017, 30 at 32. The graphic is also from the article.
[2] Ibid, 32
[3] Ibid, 33 and 35.