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Is Dark Matter made of Primordial Massive Black Holes?
Author/s: E. Mediavilla, J. Jiménez-Vicente, J. A. Muñoz, H. Vives-Arias and J. Calderón-Infante
Reference: 2017 ApJL 836 L18 | Link
Probability density function (PDF) for the abundance (alpha=1 means that all the galaxy mass is in microlenses) and mass of the microlenses, obtained from the analysis of microlensing data from 24 gravitationally lensed quasars. Notice that the likelihood of objects in the range of masses and abundances needed to explain dark matter in terms of massive black holes is very low.
The quest for dark matter is one of the most important efforts in nowadays Physics. The lack of experimental evidence, which could allow us to identify dark matter with one or other of the new elementary particles predicted by the theorists, as well as the recent discovery of gravitational waves by LIGO (Laser Interferometer Gravitational Wave Observatory) have revived interest in the possibility that dark matter might take the form of primordial black holes. If there were an appreciable number of black holes in the halos of galaxies, some of them may intercept the light coming towards us from a distant quasar, causing an increase in the apparent brightness of the quasar. This effect, quasar gravitational microlensing, is sensitive to any type of compact objects in the lens galaxy, to their abundance, and to their mass. The analysis of optical and X-ray microlensing data from 24 gravitationally lensed quasars reveals that the strength of the effect is relatively low, as would be expected from objects with a mass between 0.05 and 0.45 times that of the Sun, and well below that of intermediate mass black holes. In addition these microlenses form roughly 20% of the total mass of a galaxy, equivalent to the expected mass in stars. So these results show that, with high probability, it is normal stars and not primordial intermediate mass black holes which are responsible for the observed microlensing. Therefore, is very unlikely that black holes with masses between 10 and 100 times the mass of the Sun make up a significant fraction of the dark matter. For that reason the black holes whose merging was detected by LIGO were probably formed by the collapse of stars, and were not primordial black holes.