Black holes with scalar hair in light of the Event Horizon Telescope. (arXiv:2005.05992v3 [gr-qc] UPDATED)
<a href="http://arxiv.org/find/gr-qc/1/au:+Khodadi_M/0/1/0/all/0/1">Mohsen Khodadi</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Allahyari_A/0/1/0/all/0/1">Alireza Allahyari</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Vagnozzi_S/0/1/0/all/0/1">Sunny Vagnozzi</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Mota_D/0/1/0/all/0/1">David F. Mota</a>
Searching for violations of the no-hair theorem (NHT) is a powerful way to
test gravity, and more generally fundamental physics, particularly with regards
to the existence of additional scalar fields. The first observation of a black
hole (BH) shadow by the Event Horizon Telescope (EHT) has opened a new direct
window onto tests of gravity in the strong-field regime, including probes of
violations of the NHT. We consider two scenarios described by the
Einstein-Maxwell equations of General Relativity and electromagnetism, to which
we add a scalar field. In the first case we consider a minimally-coupled scalar
field with a potential, whereas in the second case the field is
conformally-coupled to curvature. In both scenarios we construct charged BH
solutions, which are found to carry primary scalar hair. We then compute the
shadows cast by these two BHs as a function of their electric charge and scalar
hair parameter. Comparing these shadows to the shadow of M87* recently imaged
by the EHT collaboration, we set constraints on the amount of scalar hair
carried by these two BHs. The conformally-coupled case admits a regime for the
hair parameter, compatible with EHT constraints, describing a so-called mutated
Reissner-Nordstr”{o}m BH: this solution was recently found to effectively
mimic a wormhole. Our work provides novel constraints on fundamental physics,
and in particular on violations of the no-hair theorem and the existence of
additional scalar fields, from the shadow of M87*.
Searching for violations of the no-hair theorem (NHT) is a powerful way to
test gravity, and more generally fundamental physics, particularly with regards
to the existence of additional scalar fields. The first observation of a black
hole (BH) shadow by the Event Horizon Telescope (EHT) has opened a new direct
window onto tests of gravity in the strong-field regime, including probes of
violations of the NHT. We consider two scenarios described by the
Einstein-Maxwell equations of General Relativity and electromagnetism, to which
we add a scalar field. In the first case we consider a minimally-coupled scalar
field with a potential, whereas in the second case the field is
conformally-coupled to curvature. In both scenarios we construct charged BH
solutions, which are found to carry primary scalar hair. We then compute the
shadows cast by these two BHs as a function of their electric charge and scalar
hair parameter. Comparing these shadows to the shadow of M87* recently imaged
by the EHT collaboration, we set constraints on the amount of scalar hair
carried by these two BHs. The conformally-coupled case admits a regime for the
hair parameter, compatible with EHT constraints, describing a so-called mutated
Reissner-Nordstr”{o}m BH: this solution was recently found to effectively
mimic a wormhole. Our work provides novel constraints on fundamental physics,
and in particular on violations of the no-hair theorem and the existence of
additional scalar fields, from the shadow of M87*.
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