arXiv subjects

Chiranjeeb Singha

Publications and source records attributed to Chiranjeeb Singha.

At least 19 recordsLinked to original sources

arXiv2026

Boltzmann Dynamics in K-essence Cosmology: Photon Propagation in an Emergent Spacetime

Recent cosmological tensions, notably the Hubble and $S_{8}$ tensions, motivate extensions of the conventional FLRW framework in which additional dynamical fields can alter the effective spacetime experienced by matter and radiation. In K-essence cosmology, the scalar field induces an emergent FLRW geometry disformally related to the gravitational metric, leading to a \emph{tilted causal structure} in which the photon light cone differs from that of gravity. We develop a covariant Boltzmann formalism in a homogeneous K-essence background and derive the modified mass-shell condition, geodesic equations, and collision integrals for both massless and massive species. The photon distribution remains thermal in the emergent frame, while appearing geometrically rescaled in the gravitational frame. Microphysical Thomson/Compton scattering is unmodified; instead, the emergent geometry induces effective rest-frame energies and a geometrically rescaled scattering rate, conveniently encoded in a transport coefficient $σ_T^{(\mathrm{geom})}$. Focusing on the tightly coupled pre-recombination era, we obtain modified acoustic dynamics and, for a purely kinetic DBI-type model, find an effective interaction rate scaling $n_{e}σ_T^{(\mathrm{geom})}a\propto a^{-8}$ and a diffusion scale scaling $k_{D}^{-2}\propto a^{29/2}$, enhancing the sensitivity of small-scale anisotropies to the evolving geometry. Our results provide a self-consistent kinetic description of particle transport in the emergent K-essence spacetime, and they lay the groundwork for a full numerical Einstein-Boltzmann implementation to compute post-recombination free streaming and the CMB angular power spectrum. A detailed numerical analysis will be pursued in future work.

gr-qc↗

arXiv2026

On The Regularized McVittie Black Bounce and a Family of Traversable Wormholes

The McVittie metric describes a self-gravitating compact object embedded in an expanding universe. It inherits two singularities: a curvature singularity at $r \to 0$ and a cosmological singularity at $a(t) \to 0$. Motivated by the construction of black-bounce geometries, we regularize both of these singularities and propose a regularized McVittie metric. The central singularity is regularized by replacing $r \to \sqrt{r^{2}+b^{2}}$, leading to a geometry that interpolates between a cosmological black hole, a black bounce, and a traversable wormhole. Similarly, the cosmological singularity is regularized by introducing a non-vanishing scale factor $a \rightarrow \sqrt{a^{2} + a_b^{2}}$. The resulting spacetime is supported by an effective imperfect fluid with finite anisotropic stress. We investigate the formation of the apparent horizon, analyze the circular geodesic structure, and argue that the geometry can also be interpreted as a conformally evolving Morris-Thorne wormhole embedded in a regular cosmological background.

gr-qc↗

arXiv2026

Tidal deformation of an accreting compact object

Tidal deformation of a compact object serves as a sensitive probe of the strong-gravity regime and nature of the compact object. It captures how a compact object responds to the external perturbing field of a companion. In realistic astrophysical settings, compact objects are typically immersed in matter-rich environments, which can significantly alter the response. In this work, we investigate the static deformability of Schwarzschild-like exotic compact objects (ECOs) embedded in a quasi-stationary, self-gravitating thin accretion disk. By modelling the external spacetime with a relativistic thin-disk solution, we isolate environmental contributions to the scalar and spin-1 response while maintaining analytical control. We show that, for perfectly reflecting ECOs, the characteristic logarithmic dependence of the scalar and spin-1 response on compactness, set by near-horizon physics, remains intact even in the presence of accretion. The disk primarily amplifies the overall magnitude of the response significantly. These findings highlight that environmental effects can seriously impact tidal signatures, while still permitting, under suitable conditions, the distinguishability of horizonless compact objects from black holes in gravitational-wave observations.

gr-qc↗

arXiv2026

Geometric properties of slowly rotating black holes embedded in matter environments

Extreme mass-ratio inspirals (EMRIs) provide a precise probe of strong-field gravity, where small deviations from vacuum Kerr geometry can accumulate over many orbital cycles. In realistic astrophysical settings, black holes are embedded in surrounding dark and baryonic matter whose presence and motion can perturb the spacetime. A systematic semi-analytic framework incorporating the rotation of the environment itself into a black-hole geometry, and propagating its effects consistently into conserved quantities and epicyclic observables, has not been explored exhaustively, limiting consistent assessments of environmental effects on precision observables. In this work, we construct a slowly rotating black hole spacetime embedded in an anisotropic matter distribution and explicitly include the angular velocity of the surrounding medium within a controlled slow-rotation expansion. We demonstrate that the environment's velocity field induces corrections to the metric coefficients that propagate into modifications of conserved quantities governing geodesics. We also derive semi-analytic shifts in the innermost stable circular orbit, light-ring location, and radial and vertical epicyclic frequencies, showing that environmental rotation produces systematic and in certain regimes qualitatively distinct behavior relative to static configurations. Consequently, we explicitly show that the environment's nature and motion shift the positions of the epicyclic resonances. The formalism applies to generic anisotropic matter profiles and is not restricted to the specific halo model adopted for numerical illustration. These results establish a direct and quantitatively controlled link between environmental rotation and strong-field orbital observables, enabling consistent incorporation of rotating matter environments into precision EMRI modeling.

gr-qc↗

arXiv2026

Oppenheimer-Snyder Collapse in f(R) Gravity : Stalemate or Resolution?

We study the Oppenheimer--Snyder (OS) collapse problem in metric $f(R)$ gravity by matching a homogeneous dust Friedmann--Lemaître--Robertson--Walker (FLRW) interior to a generalized Vaidya exterior across a timelike hypersurface. In metric $f(R)$ gravity, regular matching requires the continuity not only of the induced metric and extrinsic curvature, but also of the Ricci scalar and its normal derivative. These additional conditions generically exclude the usual Ricci-flat exteriors, such as the Schwarzschild solution. We show that, for an unrestricted generalized Vaidya exterior, the matching conditions fix the boundary data but do not uniquely determine the bulk extension, leaving open the possibility of a physical resolution of the collapse problem. However, once the exterior matter content is restricted to the generalized Vaidya form, the field equations impose a strong constraint, forcing $f_{,R}$ to be linear in the areal radius, $f_{,R}=A(v)\,r+B(v)$. For locally invertible $f_{,R}$ with $f_{,RR}\neq 0$, this sharply reduces the admissible class of exteriors, so that the matching data uniquely determine the exterior solution on each interval where the boundary map is locally invertible. We further show that, for generic viable $f(R)$ models, the branch with $A(v)\neq 0$ does not admit a global extension with finite asymptotic curvature, while the branch $A(v)=0$ places the interior on a constant-curvature sector. This excludes nontrivial dust collapse, although it does not rule out collapse for more general interior matter with constant trace. Thus, generalized Vaidya exteriors reopen the collapse problem at a formal level, but within the restricted matter sector considered here, the OS dust collapse problem remains unresolved and the physically acceptable branch is highly constrained.

gr-qc↗

arXiv2026

The Third Law of Black Hole Dynamics in Lovelock Gravity

The third law of black hole dynamics states that it is impossible, through any classical perturbation of a stationary configuration, to reduce the surface gravity of a black hole to zero. In this work, we examine the validity of this law for static, spherically symmetric charged black holes in the Lovelock theory of gravity. By studying infinitesimal variations in mass and charge, we derive a set of inequalities that constrain these variations. Our analysis shows that as the surface gravity approaches zero ($κ\to 0$), the range of admissible perturbations gradually diminishes, thereby forbidding the attainment of extremality through any finite classical process. The saturation of the inequality is interpreted as the emergence of a dynamical barrier near extremality, which prevents further evolution toward the extremal configuration.

gr-qc↗

arXiv2026

Bose-Einstein Condensate Dark Matter in the Core of Neutron Stars: Implications for Gravitational-wave Observations

We investigate neutron stars admixed with dark matter (DM) in the form of a finite-temperature Bos-Einstein condensate (BEC) within a general relativistic two-fluid framework in which the nuclear and dark components interact only gravitationally. Using realistic nuclear matter equations of state (EOS), APR4, MPA1, and SLy, we construct equilibrium configurations and compute mas-radius relations, tidal Love numbers, and dimensionless tidal deformabilities. We quantify how the presence of a BEC dark component modifies the mas-$Λ$ relation relevant for gravitational wave observations, finding that increasing the DM mass fraction generically reduces the maximum mass, radius, and tidal deformability of neutron stars. By comparing theoretical mass-$Λ$ curves with EOS-insensitive posteriors from GW170817, we evaluate, in a conditional sense, the dark matter fractions that would align a given nuclear EOS with the observed tidal constraints; for example, under the assumption that APR4 describes nuclear matter and that the GW170817 components were dark-matter admixed neutron stars, our study favors dark matter fractions of order a few percent, whereas stiffer EOSs require larger fractions to achieve comparable agreement. This interpretation assumes that inspiral waveforms are adequately characterized by tidal deformability and should therefore be regarded as structural rather than a direct detection of dark matter. We also examine finite-temperature effects in the BEC sector and find that, for moderate dark matter fractions, temperature has a negligible impact on the stability and tidal properties of admixed configurations. Our results demonstrate how even modest DM admixtures can influence neutron star structure and tidal observables, highlighting the importance of considering non-standard matter components in multimessenger constraints on dense matter.

gr-qc↗

arXiv2026

Chaotic Dynamics in Extremal Black Holes: A Challenge to the Chaos Bound

We investigate chaotic dynamics in extremal black holes by analyzing the motion of massless particles in both Reissner-Nordström and Kerr geometries. Two complementary approaches (i) taking the extremal limit of non-extremal solutions and (ii) working directly in the extremal background, yield consistent results. We find that, contrary to naive extrapolation of the Maldacena-Shenker-Stanford (MSS) chaos bound, the Lyapunov exponent remains positive even at zero temperature. For Reissner-Nordström black holes, chaos diminishes but persists at extremality, while for Kerr black holes it strengthens with increasing spin. These results demonstrate that extremal black holes exhibit residual chaotic dynamics that violate the MSS bound, establishing them as qualitatively distinct dynamical phases of gravity.

gr-qc↗

arXiv2026

On the stability of the objects of limiting compactness: Black hole and Buchdahl star

In General Relativity, there exist two objects of limiting compactness, one with a null boundary defining the horizon of a black hole and the other with a timelike boundary defining a Buchdahl star. The two are characterized by gravitational energy equal to or half the mass. Since non-gravitational mass-energy is the source of gravitational energy, both of these objects are manifestly stable. We demonstrate in this letter, in a simple and general way, that the equilibrium state defining the object is indeed stable, independent of the nature of the perturbation.

gr-qc↗

arXiv2026

Tidal deformation of black holes in Lovelock gravity

It is well established that black holes in four-dimensional, vacuum, general relativity exhibit vanishing static tidal Love numbers, indicating no multipolar response to the external tidal fields in the static limit. This intriguing feature does not extend to higher-dimensional spacetimes within general relativity, where static black holes can possess non-zero static tidal Love numbers (TLNs). In this work, we have examined the tidal deformation of black holes in Lovelock gravity. We find that, in certain cases within pure Lovelock gravity, the static TLN vanishes, extending the four-dimensional result to specific higher-dimensional settings. On the other hand, black holes in Einstein-Gauss-Bonnet gravity consistently exhibit non-zero static TLNs, with their magnitude depending on the Gauss-Bonnet coupling constant. Exceptions occur only in certain special cases associated with axial perturbations. These results highlight the sensitivity of the multipolar response of a black hole under tidal field, to the underlying theory of gravity and the spacetime dimensions.

gr-qc↗

arXiv2025

On the Limitations of a Generalized Vaidya Metric

We prove that there can not be a smooth matching of the Generalized Vaidya metric with an exterior Schwarzschild/Vaidya patch across a finite boundary hypersurface unless the mass function is a function of the null coordinate alone. By explicitly deriving the extrinsic curvature components, we show that for $\partial m / \partial r \neq 0$ one has a discontinuity in the curvature and induces a surface stress-energy tensor, corresponding to a thin shell of matter. This discontinuity also appears in the geometric invariant $\mathcal{K} = K_{ab}K^{ab}$ and in the Kodama current, indicating a mismatch in quasi-local energy flux across the boundary. The analysis of timelike geodesics leads to the same condition, reinforcing that the generalized Vaidya geometry with $\partial m / \partial r \neq 0$ cannot represent a consistent stellar interior bounded by a regular surface. We therefore note that the generalized Vaidya spacetime should be interpreted as an unbounded geometry with intrinsic heat flux rather than a viable bounded source.

gr-qc↗

arXiv2025

On electrogravity duality and black hole with global monopole

By resolving the Riemann curvature into electric and magnetic parts, Einstein's equation can accordingly be written in terms of electric (active and passive) and magnetic parts. The electrogravity duality is defined by the interchange of active and passive parts. It turns out that in static and stationary spacetimes, there is a subset of the equations (that identifies the effective vacuum equation) that is sufficient to yield the vacuum solution. In spherically symmetric spacetime, the electrograv dual of the effective equation solves to give the Schwarzschild black hole with a global monopole. Interestingly, this is not so for axial symmetry, where the Kerr vacuum solution turns out to be electrograv self-dual. However, in the asymptotic limit where the effect of rotation dies out, the situation reverts to the static case, admitting a global monopole. This is also what follows when we apply the Newman-Janis transformation to the static black hole with a global monopole.

gr-qc↗

arXiv2025

Gravitational collapse and singularity avoidance of a homogeneous dust fluid on a brane with timelike extra dimension

We investigate the gravitational collapse of a homogeneous dust cloud in the Shtanov Sahni braneworld model, which incorporates an extra timelike dimension. The interior of the collapsing configuration is modeled by a Friedmann Lemaitre spacetime, while the exterior is described by a Vaidya radiation envelope that eventually settles into a static Reissner Nordstrom (RN) geometry with a positive tidal charge. Although a smooth matching between the interior and the static exterior is precluded by the breakdown of Birkhoff's theorem in the braneworld scenario, we show that as long as braneworld effects remain significant, the brane tension stays finite. Consequently, the scalar curvature remains bounded, thereby preventing the formation of a singularity.

gr-qc↗

arXiv2025

Revealing Dark Matter's Role in Neutron Stars Anisotropy: A Bayesian Approach Using Multi-messenger Observations

Dark matter (DM) continues to evade direct detection, but neutron stars (NSs) serve as natural laboratories where even a modest DM component can alter their structure. While many studies have examined DM effects on NSs, they often rely on specific choices of equations of state (EOS) models, assume isotropy, and lack a Bayesian statistical framework, limiting their predictive power. In this work, we present a Bayesian framework that couples pressure-anisotropic nuclear EOS to a self-interacting fermionic DM component, constrained by NICER and GW170817 data. Our results show that DM mass fractions up to $\sim10\%$ remain consistent with current data, which softens the high-density EOS, leading to reduced stellar radii and tidal deformabilities while requiring negligible pressure anisotropy. Bayesian model comparison reveals no statistically significant preference between pure baryonic and DM-admixed NSs, indicating that DM inclusion enhances physical realism without complexity penalties. However, existing data cannot tightly constrain the DM parameters, and our empirical radius definition introduces a systematic bias toward the DM core configurations. To address this, we therefore introduce the DM radius span $ΔR_χ\equiv R_{χ,\mathrm{max}} - R_{χ,\mathrm{min}}$ as a unified diagnostic for DM distributions. This parameter simultaneously characterizes core-halo transition features while exhibiting strong linear correlations ($ΔR_χ< 4\,\mathrm{km}$) with both DM and BM parameters, providing a clear avenue for future constraints. Our approach bridges current limitations and future potential in probing DM through compact star observations.

astro-ph.HE↗

arXiv2025

Effect of dark matter halo environment on GW memory signal

In this paper, we study the gravitational wave (GW) memory effect for a black hole embedded in a dark matter halo described by a Hernquist-type density profile, both with and without a spike. We first solve the geodesic equations in this spacetime under the influence of a GW pulse to examine how the combined effects of the dark matter halo and the GW pulse modify the geodesic deviation equation and particle trajectories. We then investigate how the memory effect manifests in the waveform in the presence of the dark matter halo. To do that, we analyze the memory contribution at asymptotic null infinity using the Bondi-Sachs formalism and, in particular, the Bondi-Metzner-Sachs (BMS) flux balance laws associated with BMS symmetries. This framework allows us to quantify the GW memory contribution to the waveform, incorporate it into the ringdown waveform templates, and thereby provide a possible avenue for extracting information about the dark matter halo parameters.

gr-qc↗

arXiv2025

Emergence of Unruh prethermalization for uniformly accelerating many-atom system

A uniformly accelerated atom in an inertial vacuum generally thermalizes and reaches a Gibbs state. This phenomenon is commonly known as the Unruh effect. Here, we show that the situation is entirely different for the many-atoms problem. In the case of non-interacting accelerating atoms, we show that a regime exists where the entire system reaches a prethermal generalized Gibbs state before it thermalizes. The prethermal state is protected by emergent conserved quantities; hence, the system behaves like a nearly-integrable one, which shows a sharp distinction from the Unruh effect. We coin the term ``Unruh prethermalization" to characterize this phenomenon. The measure of entanglement is a good estimation of the lifetime of the prethermal state and is consistent with previous studies. Finally, we show that in such a regime, the dynamics show a Dicke superradiance-type radiation burst before reaching the prethermal state. In contrast, only a mono-exponential decay is observed for Unruh thermalization. In addition, to highlight the significance of our results, we compare them with existing experimental observations.

quant-ph↗

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↗