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Chiranjeeb Singha

Publications and source records attributed to Chiranjeeb Singha.

At least 37 records · Page 2Linked to original sources
arXiv2025

Tidal Love numbers and quasi-normal modes of the ECO in a Dark Matter halo

It is well-known that exotic compact objects (ECOs) are a class of objects categorised as Black Hole (BH) mimickers. ECOs have been shown to possess signatures distinguishing them from BHs. However, in our universe, no object exists in complete isolation. Consequently, any compact object, whether a BH or an ECO, must reside within some environment that inevitably influences the surrounding spacetime geometry due to back-reaction. In this paper, we investigate a scenario where an ECO is embedded in an environment of dark matter (DM). In this work, we assume two different models of the DM halo profile. We compute the Love numbers and GW echoes of this composite system to assess the impact of the surrounding dark matter halo. To analyze the echoes, we focus on odd-parity perturbations, while for calculating the tidal Love numbers, we consider both even and odd parity perturbations. We aim to understand how the DM properties couple to the ECO signatures in the Love number or the GW-echo signal, both of which have a strong bearing as observables in future-generation detectors.

gr-qc↗
arXiv2025

Can a pure state remain pure in the Unruh effect?

A uniformly accelerated detector in an inertial vacuum undergoes an unavoidable dissipation, and the final steady-state becomes thermal. However, to attain such a mixed state, there is no bound for the acceleration of the single atomic detector. Here we show that the scenario is entirely different for two atoms with the same energy levels. There exists a critical limit of the acceleration for two atomic detectors, below which the purity of a particular initial state can be preserved. We observe that the generator of the dissipative dynamics (Lindbladian) is invariant under a weak symmetry transformation at this limit. Hence one of the eigenstates of the symmetry operator is unchanged during the evolution. This kind of state is called a quantum dark state, which is essentially a decoherence-free subspace. As a consequence, the system becomes localized, and it can skip the Unruh thermalization. Beyond the critical limit, the symmetry is explicitly broken. Therefore our results suggest that the system goes through a first-order dissipative phase transition from a localized to a thermal phase.

quant-ph↗
arXiv2025

From Nonextremal to Extremal: Entropy of Reissner-Nordström and Kerr black holes Revisited

In this paper, we derive the entropy of Reissner-Nordström (RN) and Kerr black holes using the Hawking-Gibbons path integral method. We determine the periodicity of the Euclidean time coordinate using two approaches: first, by analyzing the near-horizon geometry, and second, by applying the Chern-Gauss-Bonnet (CGB) theorem. For non-extremal cases, both these methods yield a consistent and unique periodicity, which in turn leads to a well-defined expression for the entropy. In contrast, the extremal case exhibits a crucial difference. The absence of a conical structure in the near-horizon geometry implies that the periodicity of the Euclidean time is no longer uniquely fixed within the Hawking-Gibbons framework. The CGB theorem also fails to constrain the periodicity, as the corresponding Euler characteristic vanishes. As a result, the entropy cannot be uniquely determined using either method.

gr-qc↗
arXiv2025

Hawking radiation from a hot NUT-Kerr-Newman-Kasuya-Anti-de Sitter black hole

In this work, we investigate the tunneling phenomenon of a charged Dirac particle emerging from a thermal horizon of a hot NUT-Kerr-Newman-Kasuya-Anti-de Sitter (HNKNK-AdS) black hole. Considering the tunneling formalism, we report the Hawking temperature of the charged Dirac particle through the horizon and the heat capacity of the HNKNK-AdS black hole. It is found that the effective Hawking temperature for the black hole depends on the black hole's parameters, such as electric and magnetic charges, magnetic mass, the cosmological constant, and the angular momentum of the black hole. The Hawking radiation for the Kerr-Newman black hole is reduced. It is shown that there is a small correction to the Hawking thermal spectrum. The heat capacity for the HNKNK-AdS black hole is found different from the previous work, and for the Kerr-Newman black hole is also reduced.

gr-qc↗
arXiv2025

Traversable Wormholes with Spontaneous Symmetry Breaking

We argue that a spherically symmetric traversable wormhole solution of the Einstein field equations can be supported by minimally coupled self-interacting scalar field which allows a spontaneous symmetry breaking of the field around the wormhole throat. We study two cases : (i) the phantom wormhole solution of Bronnikov and (ii) a generalized Kiselev wormhole. We study the property of radial null geodesics and show that the metric can describe either a two-way or a one-way traversable wormhole depending on certain parameter ranges. The scalar field exhibits spontaneous symmetry breaking within the coordinate range where a wormhole throat forms and helps one suggest that spontaneous symmetry breaking may act as a threshold for wormhole throat formation. We also compute the radius of the photon sphere, the Lyapunov exponent, the shadow radius, and the innermost stable circular orbits for the geometries.

gr-qc↗
arXiv2025

Probing Chaos in Schwarzschild-de Sitter Spacetime: The Role of Black Hole and Cosmological Horizons

In this paper, we study the motion of a massless, chargeless particle in Schwarzschild-de Sitter spacetime, revealing exponential radial growth and potential chaos in an integrable system. Poincaré sections show regular Kolmogorov-Arnold-Moser (KAM) tori when black hole and cosmological horizons are distant, but distortions and chaos emerge as they converge. As the horizons coincide, the Poincaré sections fully contract and vanish, marking the system's transition to Nariai spacetime. \textit{Our analysis also suggests that, within the parameter range explored, the event horizon exerts a comparatively more substantial chaotic influence on the system, primarily due to its consistent proximity.} Additionally, we analyze the Lyapunov exponents to quantify the degree of chaos in the system. Our findings indicate that as the closeness of the two horizons increases, the most prominent Lyapunov exponent also increases, signifying a rise in chaotic behavior. By examining the long-term saturation values of the Lyapunov exponents, we confirm that they consistently comply with the Maldacena-Shenker-Stanford (MSS) bound.

gr-qc↗
arXiv2025

Can Rotating Black Holes Have Short Hairs?

Despite the no-hair theorem, several notable hairy black hole (BH) solutions exist in both General Relativity and modified gravity theories. For such hairs to be detectable, they must extend sufficiently beyond the event horizon. This idea has been rigorously formalized by the no-short hair theorem, which dictates that all existing hairs of a static spherically symmetric BH must extend at least to the innermost light ring (LR). However, the theorem's applicability to the astrophysically relevant rotating BHs remains elusive as yet. To address this gap, we examine its validity for rotating BHs in the Konoplya-Rezzolla-Zhidenko-Stuchlík and Johannsen classes. Interestingly, for Klein-Gordon separable BHs in these classes that are solutions of non-vacuum GR, the no-short hair property continues to hold. However, unlike in static cases, this result may not apply in a theory-agnostic fashion due to the rotation-induced repulsive effects. Consequently, we identify a minimal set of additional criteria on the metric and matter content needed for such a generalization in other theories. Our study marks an important first step toward establishing general results on the extent of rotating BH hairs, reinforcing their observational detections. Further extension of this novel result for rotating horizonless objects is also discussed.

gr-qc↗
arXiv2024

Scalar tidal response of a rotating BTZ black hole

We study the response of a rotating BTZ black hole to the scalar tidal perturbation. We show that the real component of the tidal response function isn't zero, indicating that a rotating BTZ black hole possesses non-zero tidal Love numbers. Additionally, we observe scale-dependent behaviour, known as log-running, in the tidal response function. We also conduct a separate analysis on an extremal rotating BTZ black hole, finding qualitative similarities with its non-extremal counterpart. In addition, we present a procedure to calculate the tidal response function of a charged rotating BTZ black hole as well.

gr-qc↗
arXiv2024

Implications of Fermionic Dark Matter Interactions on Anisotropic Neutron Stars

The presence of Dark matter (DM) within a neutron star (NS) can substantially influence the macroscopic properties. It is commonly assumed that the pressure inside an NS is isotropic, but in reality, pressure is locally anisotropic. This study explores the properties of anisotropic NS with a subfraction of DM (isotropic) trapped inside. Implementing a two-fluid formalism with three Equations of State (EOS): AP3 (a realistic nucleon-nucleon interaction model), BSk22 (modeling atomic nuclei and neutron-matter), and MPA1 (considering relativistic effects in nuclear interactions). The properties of NS, such as mass ($M$), radius ($R$), and dimensionless tidal deformability ($Λ$), for various DM-anisotropic configurations, have been rigorously tested against observational constraints. These constraints include data from the binary NS merger GW170817, NICER x-ray measurements, and pulsar mass-radius observations. We observe that with increasing DM subfraction, higher anisotropies could also satisfy the observational constraints. Furthermore, increasing the coupling ($g$) between DM and its mediator leads to the formation of a core-halo structure, with a DM halo surrounding the baryonic matter (BM). Specifically, for coupling values of $g = 10^{-4}$, $10^{-3.7}$, and $10^{-3.5}$, we observe that the maximum radius ($R_{max}$) decreases with increasing anisotropy, which contrasts with the behavior at $g = 10^{-5}$ and in scenarios with no DM. Our analysis indicates that binary pulsar systems could potentially constrain the extent of admixed anisotropic NS or, more optimistically, provide evidence for the existence of DM-admixed anisotropic NS.

astro-ph.CO↗
arXiv2024

Galactic Pure Lovelock Blackholes: Geometry, stability, and Hawking temperature

In this article, we will first-time model galactic black holes in pure Lovelock gravity. Even though working with higher spacetime dimensions, we assume (implicitly) the Hernquist-type mass profile for the galaxy in such a way that the horizon structure of a pure LoveLock black hole remains intact. In this way, we will model the galactic pure Lovelock black hole with arbitrary dimension ($d$) and order ($N$). Then, we will specialize this technique for critical dimension $d=3N+1$. We want to see how the galactic parameters affect the time domain single, quasinormal modes, photon sphere, innermost stable circular orbits (ISCO), and shadow radius. The time domain signal may allow us to identify the galactic parameters as well as to distinguish them from their isolated pure Lovelock counterparts if it is observed in future generations of gravitational wave measurements. We also calculate Hawking temperature for the same setup and want to see how Hawking's temperature will be affected due to the presence of a galaxy. It shows that the presence of a galactic halo can quench Hawking temperature.

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

Self-Similar Collapse in Painleve-Gullstrand Coordinates

We report a family of self-similar exact solutions in General Relativity. The solutions are found in a Painleve-Gullstrand coordinate system but can also be transformed smoothly into a diagonal form. The solutions represent a gravitational collapse leading to three possible outcomes, depending on the parameter space : (i) a collapse followed by a bounce and dispersal of the clustered matter distribution, (ii) a rapid collapse followed by a bounce and an eventual re-collapse, and (iii) a standard collapse leading to zero proper volume. Profiles of the energy conditions are studied for all of the scenarios, and it is noted that a bounce is usually associated with a violation of the Null Energy Condition. It is found that more than one null surfaces (apparent horizons) can develop during the collapse. We also discuss that for a general metric tensor having a conformal symmetry, some regions of the parameter space allows a formation of null throat, much like a wormhole. Matching the metric with a Schwarzschild metric in Painleve-Gullstrand form leads to the geodesic equation for a zero energy falling particle in the exterior.

gr-qc↗
arXiv2023

Hawking temperature of black holes with multiple horizons

There are several well-established methods for computing thermodynamics in single-horizon spacetimes. However, understanding thermodynamics becomes particularly important when dealing with spacetimes with multiple horizons. Multiple horizons raise questions about the existence of a global temperature for such spacetimes. Recent studies highlight the significant role played by the contribution of all the horizons in determining Hawking temperature. Here we explore the Hawking temperature of a rotating and charged black hole in four spacetime dimensions and a rotating BTZ black hole. We also find that each horizon of those black holes contributes to the Hawking temperature. The effective Hawking temperature for a four-dimensional rotating and charged black hole depends only on its mass. This temperature is the same as the Hawking temperature of a Schwarzschild black hole. In contrast, the effective Hawking temperature depends on the black hole mass and angular momentum for a rotating BTZ hole.

gr-qc↗
arXiv2023

Hawking radiation in multi-horizon spacetimes using Hamilton Jacobi method

It has been recently shown that the contribution between the horizons determines the Hawking temperature for a multi-horizon spacetime. In this article, we apply the Hamiltonian Jacobi method to compute the Hawking temperature for some multi-horizon spacetimes like Schwarzschild-de Sitter spacetime (SdS), Reissner-Nordstrom-de Sitter spacetime (RNdS), and rotating BTZ black hole spacetime (RBTZ) and also arrive at the same conclusion. There are two contributions to the tunneling process of radiation. The combination of these two contributions gives the radiation with the Hawking temperature with an effective surface gravity.

gr-qc↗
arXiv2022

Extreme electron acceleration with fixed radiation energy

We examine the extreme situation of radiation from an electron that is asymptotically accelerated to the speed of light, resulting in finite emission energy. The analytic solution explicitly demonstrates the difference between radiation power loss and kinetic power loss (null).

gr-qc↗
arXiv2022

Strong cosmic censorship conjecture for a charged AdS black hole

The strong cosmic censorship conjecture states (SCCC) that one cannot extend spacetime beyond the Cauchy horizon with a square-integrable connection. This conjecture was postulated to save the deterministic nature of the most successful theory of gravitation, general relativity. In order to explore the validation/violation of the SCCC for the charged anti-de Sitter black hole spacetime, we compute the ratio of the imaginary part of the quasinormal mode frequencies and the surface gravity at the Cauchy horizon both analytically and numerically. The lowest value of which defines the key parameter $β$ determining the fate of SCCC where $β< 1/2$ indicates validation and else violation. We show that $β> 1/2$ for a charged AdS black hole with the dissipative boundary conditions in the near extremal region. Thus the SCCC is violated for this spacetime.

gr-qc↗
arXiv2022

Hawking radiation as quantum mechanical reflection

In this article, we explore an alternative derivation of Hawking radiation. Instead of the field-theoretic derivation, we have suggested a simpler calculation based on quantum mechanical reflection from a one-dimensional potential. The reflection coefficient shows an exponential fall in energy which, in comparison with the Boltzmann probability distribution, yields a temperature. The temperature is the same as Hawking temperature for spherically symmetric black holes. The derivation gives an exact local calculation of Hawking temperature that involves a region lying entirely outside the horizon. This is a crucial difference from the tunneling calculation, where it is necessary to involve a region inside the horizon.

gr-qc↗