In the recent paper, Jiao and colleagues suggested that cosmic strings may provide a joint resolution of two puzzles in astrophysics. On one hand, large radius string loops may explain the origin of the seeds about which super-massive black holes (SMBHs) accrete. On the other hand, smaller radius loops can lead to the formation of intermediate mass black holes (IMBHs), in particular black holes in the “mass gap” range between 65M and 135M where standard stellar black hole formation scenarios predict that no black holes should exist. Such black holes, however, have been detected by the LIGO/VIRGO collaboration.
Cosmic strings exist as solutions of the field equations in a wide range to particle physics models beyond the Standard Model. If Nature is described by such a theory, then a network of strings inevitably forms in the early universe and persists to the present time. The network of strings contains loops with a continuous range of radii. String loops represent nonlinear density fluctuations. Hence, string loops may seed black holes with a continuous range of masses.
Jiao and colleagues in their recent paper have studied the implications of the proposal that both super-massive black holes (SMBHs) and intermediate black holes (IMBHs) could originate from cosmic string loop seeds. The cosmic string model contains (in principle) one free parameter, namely the string tension. Researchers normalize the string tension to yield one candidate seed per galaxy which can develop into a SMBH of mass greater or equal to 106 M.
They then, predicted the number per galaxy of IMBHs capable of seeding black holes in the “mass gap” window of 65 M − 130 M. This number is 106 modulo Eddington accretion factors.
We find that there could be as many as 106 of intermediate mass black holes per galaxy, providing a tantalizing target for gravitational wave observatories to look for.— concluded authors of the study.
According to authors, black holes with a smaller mass than this (i.e. modulo Eddington accretion factor) would have to be formed from string loops that would not undergo any efficient accretion.
“We expect that the accretion onto these cosmic string loops collapses into a black hole before structure formation takes place. Therefore, the formation and merger history of binary systems of these IMBHs will closely resemble that of primordial black holes.”— told Jiao, third author of the study.
Reference: Robert Brandenberger, Bryce Cyr, Hao Jiao, “Intermediate Mass Black Hole Seeds from Cosmic String Loops”, ArXiv, pp. 1-8, 25 Mar, 2021. https://arxiv.org/abs/2103.14057
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