Analytic nondegenerate shift-symmetric Horndeski theories admit no static spherical regular black holes with a time-independent scalar; the unique marginal nonanalytic completion is sGB, whose hairy solutions remain centrally singular.
On Taking the $D\to 4$ limit of Gauss-Bonnet Gravity: Theory and Solutions
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abstract
We comment on the recently introduced Gauss-Bonnet gravity in four dimensions. We argue that it does not make sense to consider this theory to be defined by a set of $D\to 4$ solutions of the higher-dimensional Gauss-Bonnet gravity. We show that a well-defined $D\to 4$ limit of Gauss-Bonnet Gravity is obtained generalizing a method employed by Mann and Ross to obtain a limit of the Einstein gravity in $D=2$ dimensions. This is a scalar-tensor theory of the Horndeski type obtained by a dimensional reduction methods. By considering simple spacetimes beyond spherical symmetry (Taub-NUT spaces) we show that the naive limit of the higher-dimensional theory to four dimensions is not well defined and contrast the resultant metrics with the actual solutions of the new theory.
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Regular black-hole and black-bounce solutions are derived in lower-dimensional EGB gravity using nonlinear electrodynamics or unimodular extensions, with thermodynamics showing modified evaporation, remnants, and phase transitions.
Analytical construction of hairy meronic black holes in 4D Einstein-Maxwell-Yang-Mills with conformally coupled scalar, generalizing MTZ and AC solutions via non-Abelian fields and (super-)renormalizable terms.
EHT observations of Sgr A* constrain deviations from GR black hole solutions including regular BHs, string-inspired spacetimes, and BH mimickers, with some limits exceeding cosmological bounds.
Neutron stars with primary scalar hair in a DHOST subfamily are more compact than GR predictions and develop singularities above a critical scalar charge threshold.
citing papers explorer
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Static regular black holes in Horndeski theories: analytic no-go and nonanalytic obstructions
Analytic nondegenerate shift-symmetric Horndeski theories admit no static spherical regular black holes with a time-independent scalar; the unique marginal nonanalytic completion is sGB, whose hairy solutions remain centrally singular.
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Geometrically Regular Black Object Solutions in Lower-Dimensional Gauss-Bonnet Gravity and Its Unimodular Extension
Regular black-hole and black-bounce solutions are derived in lower-dimensional EGB gravity using nonlinear electrodynamics or unimodular extensions, with thermodynamics showing modified evaporation, remnants, and phase transitions.
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(Super-)renormalizable hairy meronic black holes
Analytical construction of hairy meronic black holes in 4D Einstein-Maxwell-Yang-Mills with conformally coupled scalar, generalizing MTZ and AC solutions via non-Abelian fields and (super-)renormalizable terms.
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Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$
EHT observations of Sgr A* constrain deviations from GR black hole solutions including regular BHs, string-inspired spacetimes, and BH mimickers, with some limits exceeding cosmological bounds.
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Neutron stars with primary scalar hair
Neutron stars with primary scalar hair in a DHOST subfamily are more compact than GR predictions and develop singularities above a critical scalar charge threshold.
- Rotating black holes with primary hair in five-dimensional generalized Proca theory