Probing Modified Gravity Theories with Scalar Fields Using Black-Hole Images

Antoniou, Georgios; Papageorgiou, Alexandros; Kanti, Panagiota

doi:10.3390/universe9030147

Cited by 5 publications

(2 citation statements)

References 159 publications

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“…The linear scalar-Gauss-Bonnet term φG [93][94][95] is special in the sense that it is shift-symmetric φ → φ + const, as G is famously a total derivative. Significant efforts have been put into constraining the Wilson coefficient of this operator with the gravitational wave and X-ray data from binary compact stars [98][99][100][101][102][103][104][105][106][107]. These observations capitalize on the fact that the scalar-Gauss-Bonnet coupling alters the star configurations and as well as induces significant dipole radiation in binaries, thanks to the scalar degree of freedom.…”

Section: Scalar-tensor Eftmentioning

confidence: 99%

“…The class of models involving the Gauss-Bonnet invariant JHEP10(2023)135 2 stand out, as they are low orders in the EFTs and can give rise to hairy black holes [92][93][94][95] and the phenomenon of (spontaneous) scalarization [96,97]. These operators have been confronted with gravitational wave observations and beyond [98][99][100][101][102][103][104][105][106][107]. Wheeler famously coined the phrase that a black hole has no hair [108].…”

Section: Introductionmentioning

confidence: 99%

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Causality bounds on scalar-tensor EFTs

Hong,

Wang,

Zhou

2023

J. High Energ. Phys.

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We compute the causality/positivity bounds on the Wilson coefficients of scalar-tensor effective field theories. Two-sided bounds are obtained by extracting IR information from UV physics via dispersion relations of scattering amplitudes, making use of the full crossing symmetry. The graviton t-channel pole is carefully treated in the numerical optimization, taking into account the constraints with fixed impact parameters. It is shown that the typical sizes of the Wilson coefficients can be estimated by simply inspecting the dispersion relations. We carve out sharp bounds on the leading coefficients, particularly, the scalar-Gauss-Bonnet couplings, and discuss how some bounds vary with the leading (∂ϕ)4 coefficient and as well as phenomenological implications of the causality bounds.

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Section: Scalar-tensor Eftmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Causality bounds on scalar-tensor EFTs

Hong,

Wang,

Zhou

2023

J. High Energ. Phys.

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Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A ∗

Vagnozzi

Roy

Tsai

et al. 2023

Class. Quantum Grav.

220

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Horizon-scale images of black holes (BHs) and their shadows have opened an unprecedented window onto tests of gravity and fundamental physics in the strong-field regime. We consider a wide range of well-motivated deviations from classical General Relativity (GR) BH solutions, and constrain them using the Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), connecting the size of the bright ring of emission to that of the underlying BH shadow and exploiting high-precision measurements of Sgr A*’s mass-to-distance ratio. The scenarios we consider, and whose fundamental parameters we constrain, include various regular BHs, string-inspired space-times, violations of the no-hair theorem driven by additional fields, alternative theories of gravity, novel fundamental physics frameworks, and BH mimickers including well-motivated wormhole and naked singularity space-times. We demonstrate that the EHT image of Sgr A* places particularly stringent constraints on models predicting a shadow size larger than that of a Schwarzschild BH of a given mass, with the resulting limits in some cases surpassing cosmological ones. Our results are among the first tests of fundamental physics from the shadow of Sgr A* and, while the latter appears to be in excellent agreement with the predictions of GR, we have shown that a number of well-motivated alternative scenarios, including BH mimickers, are far from being ruled out at present.

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Testing the speed of gravity with black hole ringdowns

Sirera

Noller

2023

Phys. Rev. D

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We investigate how the speed of gravitational waves, cGW, can be tested by upcoming black hole ringdown observations. We do so in the context of hairy black hole solutions, where the hair is associated with a new scalar degree of freedom, forecasting that LISA and TianQin will be able to constrain deviations of cGW from the speed of light at the O(10−4) level from a single supermassive black hole merger. We discuss how these constraints depend on the nature of the scalar hair, what different aspects of the underlying physics they are sensitive to in comparison with constraints derived from gravitational wave propagation effects, which observable systems will place the most stringent bounds, and that constraints are expected to improve by up to two orders of magnitude with multiple observations. This is especially interesting for dark energy-related theories, where existing bounds from GW170817 need not apply at lower frequencies and where upcoming bounds from lower-frequency missions will therefore be especially powerful. As such, we also forecast analogous bounds for the intermediate-frequency AEDGE and DECIGO missions. Finally, we discuss and forecast analogous black hole ringdown constraints at higher frequencies (so from LVK, the Einstein Telescope and Cosmic Explorer) and in what circumstances they can yield new information on top of existing constraints on cGW. All calculations performed in this paper are reproducible via a companion Mathematica notebook [1]. Published by the American Physical Society 2023

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Probing Modified Gravity Theories with Scalar Fields Using Black-Hole Images

Cited by 5 publications

References 159 publications

Causality bounds on scalar-tensor EFTs

Causality bounds on scalar-tensor EFTs

Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A ∗

Testing the speed of gravity with black hole ringdowns

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