arXiv subjects

Sumanta Chakraborty

Publications and source records attributed to Sumanta Chakraborty.

At least 37 records · Page 2Linked to original sources

arXiv2025

Singularity avoidance from path integral

We have demonstrated that the wave functional describing the quantum nature of the spacetime inside the black hole horizon, vanishes near the singularity, using the path integral formalism. This is akin to the DeWitt criterion, applied to the interior of a Schwarzschild black hole. To achieve the same we have expressed the interior of a Schwarzschild black hole as a Kantowski-Sachs spacetime and have applied the minisuperspace formalism to determine an exact form of the propagator, and hence the wave function near the singularity, using path integral over the reduced phase space. It is to be emphasized that our result is exact and not a saddle point approximation to the path integral.

gr-qc↗

arXiv2025

Tidal Love numbers and quasi-normal modes of the Schwarzschild-Hernquist black hole

We derive the model of the Schwarzschild black hole immersed into a dark matter halo with a relativistic Hernquist profile, the Schwarzschild-Hernquist black hole, and obtain its tidal Love numbers and quasi-normal modes. We thoroughly compare our odd and even parity perturbation equations with the literature and point out that two distinct choices of matter perturbations lead to distinct spectra. We establish that the quasi-normal modes admit qualitatively distinct scaling laws in terms of dark matter densities for non-relativistic and relativistic halos. We develop a stable numerical scheme for computing tidal Love numbers based on asymptotic series expansions. We further comment upon the existence of matter configurations obeying the dominant energy condition that lead to multiple light rings.

gr-qc↗

arXiv2025

On supertranslation invariant Lorentz charges

In recent papers, Fuentealba, Henneaux, and Troessaert (FHT) gave definitions for supertranslation invariant Lorentz charges in the ADM Hamiltonian formalism and showed that their definitions match with the Chen, Wang, Yau (CWY) definitions of Lorentz charges at null infinity which are free from ``supertranslation ambiguities''. In this brief note, motivated by the analysis of FHT, we write expressions for the supertranslation invariant Lorentz charges in Beig-Schmidt variables at spacelike and timelike infinity. We present calculations, building upon the work of Compère, Gralla, and Wei (CGW), to show that our expressions for supertranslation invariant Lorentz charges match the CWY definitions at null infinity.

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arXiv2025

Rotating black holes experience dynamical tides

We find the dynamical tidal response of a Kerr black hole (BH) and demonstrate its tidal Love numbers to be non-vanishing when present in a non-axisymmetric external tidal field. To leading order, they depend quadratically on the black hole spin and linearly on the mode frequency. This implies that Kerr BHs are deformable under certain external, time-dependent perturbations. Since non-vanishing tidal Love numbers have been used in compact binary coalescences observed by the LIGO-Virgo-KAGRA (LVK) Collaboration to infer their non-BH nature, our findings have important implications on such inferences from future gravitational wave observations.

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arXiv2025

Scalar-tensor theories of gravity from a thermodynamic viewpoint

In any diffeomorphism invariant theory of gravity, one can define a Noether charge arising from the invariance of the Lagrangian under diffeomorphisms. We have determined the Noether charge for scalar-tensor theories of gravity, in which case the gravity is mediated by the metric tensor as well as by a scalar degree of freedom. In particular, we demonstrate that the total Noether charge within an appropriate spatial volume can be related to the heat content of the boundary surface. For static spacetimes, in these theories, there exist an ``equipartition" between properly defined bulk and surface degrees of freedom. While the dynamical evolution of spacetime in these theories of scalar-tensor gravity arises due to the departure from the equipartition regime. These results demonstrate that thermodynamical interpretations for gravitational theories transcend Einstein and Lovelock theories of gravity, holding true for theories with additional scalar degrees of freedom as well. Moreover, they hold in both the Jordan and the Einstein frames. However, it turns out that there are two dynamically equivalent representations of the scalar-tensor theory in the Jordan frame, differing by total derivatives in the action, which are thermodynamically inequivalent. This depicts the importance of having a thermodynamic description, which can be used in distinguishing various dynamically equivalent representations of gravity theories beyond Einstein.

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arXiv2025

Probing charge of compact objects with gravitational microlensing of gravitational waves

Gravitational microlensing of gravitational waves (GWs) opens up the exciting possibility of studying the spacetime geometry around the lens. In this work, we investigate the prospects of constraining the `charged' hair of a charged gravitating object from the observation of a GW signal microlensed by the same. The charge can have electromagnetic or modified gravity origin. We compute the analytic form of the lensing potential with charge and construct the lensed waveforms for a range of mass and charge of the charged object, assuming them to be non spinning. Using an approximate likelihood function, we explore how future observations of microlensed GWs can constrain the charge of the lens. We conclude that lensing observations are unlikely to be able to constrain the electromagnetic charge of black holes. However, we might be able to put modest constraints on certain modified gravity models (e.g., the brandworld scenario) or the possibility of the lenses being exotic objects (e.g., naked singularities) .

gr-qc↗

arXiv2025

Gravitational physics in the context of Indian astronomy: A vision document

Contributions from the Indian gravity community have played a significant role in shaping several branches of astronomy and astrophysics. This document reviews some of the most important contributions and presents a vision for gravity research in the context of astronomy \& astrophysics in India. This is an expanded version of one of the chapters in the recently released Vision Document of the Astronomical Society of India.

astro-ph.IM↗

arXiv2025

Connecting quasi-normal modes with causality in Lovelock theories of gravity

The eikonal correspondence between the quasi-normal modes (QNMs) of asymptotically flat static spherically symmetric black holes and the properties of unstable null circular geodesics is studied in the case of higher dimensional Lovelock black holes (BHs). It is known that such correspondence does not generically hold for gravitational QNMs associated with BHs in Lovelock theories. In the present work, we revisit this correspondence and establish the relationship between the eikonal QNMs and the causal properties of the gravitational field equations in Lovelock theories of gravity.

gr-qc↗

arXiv2024

Accreting Schwarzschild-like compact object: Plasma-photon interaction and stability

Accretion is a common phenomenon associated with any astrophysical compact object, which is best described by plasma, a state of matter composed of electrons and heavy ions. In this paper, we analyze the linear dynamics of electromagnetic (EM) fields propagating through the accreting plasma around static and spherically symmetric horizon-less, exotic compact objects (ECOs). The general equations governing the propagation of EM waves in such a background exhibit quasi-bound states whose characteristic frequencies differ from the BH values for both the axial and the polar modes, as well as for homogeneous and inhomogeneous plasma distributions. Moreover, the real and imaginary parts of these quasi-bound frequencies depict an oscillatory behaviour with the plasma frequency, characteristic of the ECOs considered. The amplitude of these oscillations depends on the non-zero reflectivity of the surface of the compact object, while the oscillation length depends on its compactness. This results in slower decay of the quasi-bound states with time for a certain parameter space of the plasma frequency, compared to BHs, making these ECOs more prone to instabilities.

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arXiv2024

Probing the quantum nature of black holes with ultra-light boson environments

Quantum black holes (BHs), thought of as an excited multilevel system, can be effectively modelled by replacing an event horizon with a partially reflective membrane. This emergent feature affects their interaction with hosting environments, with the most pronounced effects happening for particles with mass~$m_{\rm p}\lesssim (10 M_{\odot}/M)\,10^{-11}\mathrm{\,eV}$, where~$M$ is the BH mass. We show that ultra-light bosons -- a viable dark matter candidate -- can be used to probe the quantum nature of BHs. We derive analytical expressions for the accretion rate and dynamical friction acting on exotic compact objects moving through an ultra-light scalar field, finding that while the accretion rate is sensitive to the quantum BH's reflectivity, the dynamical friction is the same as for classical BHs. We then use these expressions to estimate the orbital dephasing in the inspiralling of different binaries in the Laser Interferometer Space Antenna (LISA) band. Our results indicate that LISA may be able to discriminate quantum from classical BHs through their different accretion rates.

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arXiv2024

Dynamical tidal Love numbers of Kerr-like compact objects

We develop a framework to compute the tidal response of a Kerr-like compact object in terms of its reflectivity, compactness, and spin, both in the static and the frequency-dependent case. Here we focus on the low-frequency regime, which can be solved fully analytically. We highlight some remarkable novel features, in particular: i) Even in the zero-frequency limit, the tidal Love numbers (TLNs) depend on the linear-in-frequency dependence of the object's reflectivity in a nontrivial way. ii) Intriguingly, the static limit of the frequency-dependent TLNs is discontinuous, therefore the static TLNs differ from the static limit of the (phenomenologically more interesting) frequency-dependent TLNs. This shows that earlier findings regarding the static TLNs of ultracompact objects correspond to a measure-zero region in the parameter space, though the logarithmic behavior of the TLNs in the black hole limit is retained. iii) In the non-rotating case, the TLNs generically vanish in the zero-frequency limit (just like for a black hole), except when the reflectivity is ${\cal R}=1+{\cal O}(Mω)$, in which case they vanish with a model-dependent scaling, which is generically logarithmic, in the black-hole limit. The TLNs initially grow with frequency, for any nonzero reflectivity, and then display oscillations and resonances tied up with the quasi-normal modes of the object. iv) For rotating compact objects, the TLNs decrease when the reflectivity decreases or the rotation parameter increases. Our results lay the theoretical groundwork to develop model-independent tests of the nature of compact objects using tidal effects in gravitational-wave signals.

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arXiv2024

Probing black hole `charge' from the binary black hole inspiral

Recent gravitational wave (GW) observations have enabled us to look beyond the standard paradigm of gravitational physics, namely general relativity (GR). Along with the mass and the angular momentum, which typical astrophysical black holes (BHs) are endowed with, theories beyond GR generically induce `charge' to these BHs. Notably, for BHs carrying the extra `charge' hair, we expect the BH absorption effects to modify accordingly and alter the tidal heating terms. Hence, the inclusion of the corrections in the GW waveform model, arising from the BH `charge', allows us to test the consistency of the observed binaries with Kerr BHs in GR. We compute the explicit dependence of the binary inspiral phase on the `charge' parameter arising from the tidal heating effect and study the measurability of the same from GW observations of binary mergers. Specifically, we employ the {\tt TaylorF2} waveform model, which accurately models the inspiral evolution of an aligned-spin binary merger, and Bayesian analysis-based GW data inference to measure the `charge' parameter for a selected set of detected binaries. We also present a detailed simulation study to investigate the possibility of measuring the charge parameter from binaries with different masses, spins and source locations. The analysis of selected GW events from the third GW transient catalogue shows that the `charge' parameter constraints are poor from the observed signals with the current sensitivity. In contrast, the simulation studies indicate that the spinning binaries with significant mass asymmetry provide the best constraints on the BH `charge' parameter. Finally, we study the prospects of measuring the BH `charge' parameter from a future GW detector with improved sensitivity.

gr-qc↗

arXiv2024

Galactic wormholes: Geometry, stability, and echoes

In this work, we present the environmental effects on wormholes residing in a galaxy. By this, we propose that these wormholes are mimickers of supermassive black holes residing at the galactic centers. In particular, we consider two wormhole spacetimes classes: the Damour-Solodukhin wormhole and the braneworld wormhole. While there is no classical matter model for the Damour-Solodukhin wormhole, the braneworld wormhole, on the other hand, is supported by a scalar-tensor theory on the four-dimensional brane. Intriguingly, it turns out that the presence of a dark matter halo surrounding these wormholes can tame the violations of energy conditions present in generic wormhole spacetimes. Our results also demonstrate that the galactic Damour-Solodukhin wormhole is more stable than its isolated counterpart under linear scalar perturbation, whereas we obtain the opposite behavior for the braneworld wormhole. The perturbation of these wormholes leads to echoes in the ringdown waveform, which are sensitive to the properties of the dark matter halo. To be precise, the time delay between two echoes is affected by the galactic matter environment, and it appears to be a generic effect present for any exotic compact object living in a galaxy. This allows us to identify the galactic parameters, independently from the gravitational wave measurements, if echoes are observed in future generations of gravitational wave detectors. For completeness, we have also analyzed the impact of the galactic environment on the photon sphere, the innermost stable circular orbits, and the shadow radius. It turns out that the dark matter halo indeed affects these locations, with implications for shadow and accretion physics.

gr-qc↗

arXiv2024

Distinguishing cosmological models through quantum signatures of primordial perturbations

We study the evolution of various measures of quantumness of the curvature perturbation by integrating out the inaccessible entropic fluctuations in the multi-field models of inflation. In particular, we discuss the following measures of quantumness, namely purity, entanglement entropy and quantum discord. The models being considered in this work are ones that produce large scale curvature power spectra similar to those produced by single-field models of inflation. More specifically, we consider different multi-field models which generate nearly scale invariant and oscillatory curvature power spectrum and compare their quantum signatures in the perturbations with the corresponding single-field models. We find that, even though different models of inflation may produce the same observable power spectrum on large scales, they have distinct quantum signatures arising from the perturbation modes. This may allow for a way to distinguish between different models of inflation based on their quantum signatures. Intriguingly, this result generalizes to bouncing scenarios as well.

astro-ph.CO↗

arXiv2024

Bounds on tidal charges from gravitational-wave ringdown observations

Black hole solutions in the braneworld scenario are predicted to possess a tidal charge parameter, leaving imprints in the quasinormal spectrum. We conduct an extensive computation of such spectrum, and use it to construct a waveform model for the ringdown relaxation regime of binary black hole mergers observed by LIGO and Virgo. Applying a Bayesian time-domain analysis formalism, we analyse a selected dataset from the GWTC-3 LIGO-Virgo-Kagra catalog of binary coalescences, bounding the value of the tidal charge. With our analysis we obtain the first robust constraints on such charges, highlighting the importance of accounting for the previously ignored correlations with the other black hole intrinsic parameters.

gr-qc↗

arXiv2024

Lorentzian quantum cosmology with torsion

We evaluate the Lorentzian gravitational path integral in the presence of non-vanishing torsion with the application of the Picard-Lefschetz theory for minisuperspaces corresponding to a number of phenomenological bouncing cosmological models as well as for the inflationary paradigm. It turns out that the semi-classical wave function derived from the saddle points of the path integral formalism coincides with the solutions of the Wheeler-DeWitt equation. Intriguingly, our analysis showed that the relative probability, derived using these semi-classical wave functions favors universes with smaller values of torsion. Moreover, we find that in the inflationary case, non-zero values of a certain parity-violating component of the torsion enhance the power in the large physical length scales, which can have important observational implications. On the other hand, in the case of bouncing models, the power spectrum is characterized by an initial region of growth, an intermediate oscillatory region, and then again a final region of growth. The shape of the power spectrum in the initial and intermediate regions is sensitive to the abundance of the bounce-enabling matter and torsion, along with the initial wave function of the universe, while the final size modifies the behavior of the power spectrum in the smaller length scales.

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arXiv2024

Distinguishing Jordan and Einstein frames in gravity through entanglement

In general relativity, the use of conformal transformation is ubiquitous and leads to two different frames of reference, known as the Jordan and the Einstein frames. Typically, the transformation from the Jordan frame to the Einstein frame involves introducing an additional scalar degree of freedom, often already present in the theory. We will show that at the quantum level, owing to this extra scalar degree of freedom these two frames exhibit subtle differences that the entanglement between two massive objects can probe.

gr-qc↗

arXiv2024

Asymptotically de-Sitter black holes have non-zero tidal Love numbers

Love numbers of compact objects quantify their tidal deformability against external perturbations. It is expected that Love numbers of asymptotically flat black holes (BHs) in General Relativity are identically zero. We show that quite contrary to common expectations, the tidal Love numbers of asymptotically de Sitter black holes are non-zero.

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