arXiv subjects

Sanved Kolekar

Publications and source records attributed to Sanved Kolekar.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Field Theory in Successive Rindler Spacetimes

We study successive Rindler-like transformations in Minkowski spacetime and the corresponding sequence of vacuum states perceived by observers restricted to respective wedges. Extending the standard Rindler construction to an $n$-fold iteration, we find via Bogoliubov transformations that the vacuum of the $(n-1)^{th}$ Rindler observer appears thermal to the $n^{th}$ one. The characteristic trajectories, confined to nested wedges, exhibit characteristic accelerations and horizon shifts depending on transformation parameters ${g_1, g_2, \ldots, g_{n}}$. For the second-level transformation (\emph{Rindler Rindler} case), the late time acceleration asymptotically approaches $2g_2$ for one branch and diverges for the other. We study Minkowski, Rindler, and Rindler Rindler vacuum states from the perspective of Unruh DeWitt (UDW) detectors along inertial, Rindler, and Rindler--Rindler trajectories. The response of the UDW detector coupled to a real massless scalar field confirms the thermality: the transition rate of Rindler--Rindler observer in Minkowski vacuum matches that of a standard Rindler detector with acceleration $2g_2$, yielding a Planckian spectrum at late times. The conclusions are discussed.

gr-qc↗

arXiv2026

Hawking-Page phase transition for pure Lovelock black holes

We investigate the thermodynamic properties of static, spherically symmetric Anti-de Sitter (AdS) black holes, focusing on the interplay between characteristic temperatures, as well as on the universality of Ruppeiner scalar curvature at the Hawking-Page (HP) phase transition. In particular, we study the relation between the minimum temperature and the HP phase transition temperature for static, spherically symmetric AdS black holes in pure Lovelock gravity. For the electromagnetically neutral case in Einstein gravity, the minimum temperature in $(d+1)$ dimensions coincides with the HP transition temperature in $d$ dimensions, while in higher pure Lovelock theories this relation is modified by a dimension- and order-dependent factor, reducing to the Einstein result in appropriate limits. For charged AdS black holes, in the grand canonical ensemble, in general relativity, the two temperatures differ by a simple dimension-dependent factor, whereas no universal relation persists in higher curvature pure Lovelock theories. We further analyze the normalized Ruppeiner scalar curvature at the HP transition and show that it is a universal constant depending only on the spacetime dimension for electromagnetically neutral black holes in pure Lovelock theories. The normalized scalar curvature remains a constant, under appropriate conditions, even for the charged static spherically symmetric black holes in the grand canonical ensemble for the Einstein theory case, whereas in general pure Lovelock theories it depends on thermodynamic parameters such as pressure and electrostatic potential, asymptotically approaching a constant in the large-pressure or simultaneous large-potential and large-pressure limits.

gr-qc↗

arXiv2026

Stationary Stars Are Axisymmetric in Higher Curvature Gravity

The final equilibrium stage of stellar evolution can result in either a black hole or a compact object such as a white dwarf or neutron star. In general relativity, both stationary black holes and stationary stellar configurations are known to be axisymmetric, and black hole rigidity has been extended to several higher curvature modifications of gravity. In contrast, no comparable result had previously been established for stationary stars beyond general relativity. In this work we extend the stellar axisymmetry theorem to a broad class of diffeomorphism invariant metric theories. Assuming asymptotic flatness and standard smoothness requirements, we show that the Killing symmetry implied by thermodynamic equilibrium inside the star uniquely extends to the exterior region, thereby enforcing rotational invariance. This demonstrates that axisymmetry of stationary stellar configurations is not a feature peculiar to Einstein gravity but a universal property of generally covariant gravitational theories, persisting even in the presence of higher curvature corrections.

gr-qc↗

arXiv2026

Time delay as a probe of multiple photon spheres

Black hole shadow images are primarily determined by the properties of photon spheres and can exhibit degeneracies across different spherically symmetric spacetime geometries. We show that time delay observables associated with higher-order images of transient sources provide a robust probe to break such degeneracies in spacetimes admitting multiple photon spheres. Adopting a model-independent, parametrized, static, spherically symmetric framework that captures the generic features of double-peaked effective potentials, we investigate photon geodesics and quantify them in terms of angular deflection, travel time, and the order of the image. We identify distinctive signatures of trajectories probing the region between the unstable photon spheres. In particular, we find that these trajectories are characterized by the nontrivial temporal behavior, including a minimum travel time, a minimum angular deflection, and a characteristic triplet structure of higher-order images with a specific arrival sequence. We further show that the influence of the depth of the potential well, between the two photon spheres, on the observed time delays provides a direct handle on otherwise inaccessible regions of the spacetime. Our results highlight that time-domain lensing observables encode information beyond static shadow images and offer a promising avenue for probing the structure of compact objects and the strong-field regime of gravity.

gr-qc↗

arXiv2026

Memory Effects and Entanglement Dynamics of Finite time Acceleration

We construct a smooth trajectory in Minkowski spacetime that is inertial in the asymptotic past and future but undergoes approximately uniform acceleration for a finite duration. In a suitable limit, this trajectory reduces to the standard Rindler trajectory, reproducing the expected Bogoliubov transformations and results consistent with the thermal time hypothesis. We analyze the behavior of an Unruh-DeWitt (UDW) detector following such a trajectory and explore the dependence of complete positivity (CP) divisibility on the detector's frequency, acceleration, and the duration of acceleration. Notably, we find that the detector exhibits a memory effect due to the finite duration of acceleration, which is also quantified by the Fisher information. We further examine two UDW detectors along various trajectory combinations and show that, unlike the transition rate, both the total correlation and the entanglement harvested return smoothly to their initial values after the acceleration/deceleration phase. These correlation measures behave similarly in both accelerating and decelerating segments. Interestingly, we do not observe any measurable effect of the memory effect on negativity or mutual information. We also discuss the physical significance of the sign of the flux of acceleration-induced radiation.

gr-qc↗

arXiv2026

Kinematics of Acceleration-Induced Excitations in Confined Quantum Fields

We investigate quantum field excitations in a rigid cavity that undergoes a transition from inertial motion to uniform acceleration while maintaining constant proper length. By constructing exact Bogoliubov transformations between inertial and accelerated mode bases, we analyze the induced excitations and identify a universal power-law decay in the excitation power spectrum and an alternating pattern in particle production. The spectrum's dependence solely on the acceleration, and not on the size of the box or the observer's position, highlights a kinematic universality akin to that seen in horizon thermodynamics. Generalization to time-dependent accelerations reveals a convolution structure for the Bogoliubov coefficients, with resonant oscillations selectively enhancing mode excitations. These results provide new analytical insights into the interplay between acceleration, confinement, and quantum excitations.

gr-qc↗

arXiv2025

Probing Generalized Emergent Dark Energy with DESI DR2

As an update on the initial findings of DESI, the new results provide the first hint of potential deviations from a cosmological constant ($ω=-1$), which, if confirmed with significance $>(2-4)σ$, would challenge the validity of $Λ$ within the $Λ$CDM model. We explore the Generalized Emergent Dark Energy (GEDE) model using recent BAO measurements from DESI DR2, Type Ia supernova compilations, and CMB distance priors. Employing nested sampling, we constrain the parameter $Δ$, which characterizes deviations from $Λ$CDM. Our analysis shows that with CMB+DESI DR2 alone, GEDE tends to prefer positive values of $Δ$. However, when different SNe Ia calibrations are included, the model favors negative values of $Δ$, corresponding to an earlier injection of dark energy. The Marginalized constraints on $ω(z)$ further shows that GEDE sharply emerges but then asymptotes to $ω=-1$ without crossing it. At $z \sim 1$ data, GEDE provides a better fit than $Λ$CDM, while at $z \lesssim 0.5$ the data favor $ω> -1$, bringing the model deviate from $Λ$CDM. Bayesian model comparison shows weak support for GEDE with CMB+DESI DR2 ($\ln BF=1.96$), moderate with PP ($\ln BF=2.65$), weak-to-moderate with Union3 ($\ln BF=2.34$), and weak with DES-SN5Y ($\ln BF=1.44$). Overall, GEDE is consistent with current data and mildly favored when SNe Ia are included, making it a viable extension of $Λ$CDM that merits further investigation with future high precision measurements.

astro-ph.CO↗

arXiv2025

Photon region boundary for stationary axisymmetric black holes

The black hole shadow is fundamentally connected to the structure of light rings and the photon region in the background geometry. We investigate the photon region boundary in a generic asymptotically flat, stationary, axisymmetric black hole spacetime that admits spherical photon orbits (SPOs). Explicit expressions possessing real solutions are provided for the photon region's boundary purely in terms of the background metric functions, independent of the photon's parameters like energy or angular momentum, which are applicable to both separable and non-separable spacetimes. We further analyze its common features, including overlap with the ergoregion and rotation sense of SPOs. Additionally, light rings are identified at the extrema of the photon region boundary curves in the $(r,θ)$ plane. Our approach is validated against a few exact black hole solutions. Implications are discussed.

gr-qc↗

arXiv2025

Isentropic processes for axisymmetric Black Holes

We demonstrate that the isentropic absorption of a classical charged test particle is classically forbidden for all 3+1 dimensional stationary, non-extremal, axisymmetric black holes in any diffeomorphism invariant theory of gravity. This result is derived purely from the near-horizon geometry and thermodynamic properties of the black hole spacetime, independent of the specific gravitational theory. We further consider the Kerr-Newman black hole in general relativity and analyse, using the quantum tunnelling approach, the conditions under which isentropic absorption may become allowed. Broader implications for the second law and extremality bounds are discussed.

gr-qc↗

arXiv2025

Harvesting Fermionic Field Entanglement in Schwarzschild Spacetime

We explore entanglement harvesting using two Unruh-DeWitt (UDW) detectors linearly coupled to the scalar density of a massless spin-1/2 field in 1+1 Schwarzschild spacetime. We consider different vacua, including the Boulware, Hartle-Hawking-Israel (HHI), and Unruh vacua, and investigate various configurations of detector trajectories. We find that the transition rate of the static UDW detector exhibits the expected Planckian behavior in the HHI state, while the Unruh state leads to the Helmholtz free energy density of a fermionic thermal bath. We demonstrate that the near-horizon entanglement properties for static detectors in the HHI state have similar behaviour to those in Minkowski vacua for uniformly accelerated detectors in Rindler spacetime. We further consider a different interaction Hamiltonian which breaks local Lorentz symmetry and find that the transition rate of the static detector still exhibits Planckian behavior in the HHI state, while in the Unruh state, it leads to the Helmholtz free energy density of a bosonic or fermionic thermal bath corresponding to the static or conformal 2-bein in interaction, respectively. We observe that the anti-Hawking effect enhances the entanglement between the two detectors while the gravitational redshift and Hawking radiation decrease it. In particular, due to the presence of the anti-Hawking effect, the mutual information and concurrence near the event horizon can be non-zero even for static detectors with static 2-bein, which is in contrast with the case of the scalar field. Conclusions are discussed.

gr-qc↗

arXiv2025

Wigner distributions in Rindler spacetime and nonvacuum Minkowski states

In the 1970s, Fulling, Davies, and Unruh demonstrated that the vacuum state perceived by an inertial observer in Minkowski space appears as a thermal bath to a uniformly accelerated observer. We explore the transformation of the Wigner distribution of a real scalar field from an inertial to a Rindler frame, utilizing both Minkowski and Unruh modes. We present a general expression for the reduced Wigner distribution for a specific set of massless scalar field configurations, and validate it against known distributions within this set. This includes arbitrary Gaussian states of Unruh-Minkowski modes, the Minkowski vacuum state, the Rindler vacuum, and the thermal bath of Unruh particles. Additionally, we analyze several other distributions, such as a uniform momentum distribution, a slight deviation from the Minkowski vacuum, and a distribution with a Fermionic component in the Rindler frame. The conclusions are discussed.

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↗

arXiv2023

Black hole shadow and acceleration bounds for spherically symmetric spacetimes

We explore an interesting connection between black hole shadow parameters and the acceleration bounds for radial linear uniformly accelerated (LUA) trajectories in static spherically symmetric black hole spacetime geometries of the Schwarzschild type. For an incoming radial LUA trajectory to escape back to infinity, there exists a bound on its magnitude of acceleration and the distance of closest approach from the event horizon of the black hole. We calculate these bounds and the shadow parameters, namely the photon sphere radius and the shadow radius, explicitly for specific black hole solutions in $d$-dimensional Einstein's theory of gravity, in pure Lovelock theory of gravity and in the $\mathcal{F}(R)$ theory of gravity. We find that for a particular boundary data, the photon sphere radius $r_{ph}$ is equal to the bound on radius of closest approach $r_b$ of the incoming radial LUA trajectory while the shadow radius $r_{sh}$ is equal to the inverse magnitude of the acceleration bound $|a|_b$ for the LUA trajectory to turn back to infinity. Using the effective potential technique, we further show that the same relations are valid in any theory of gravity for static spherically symmetric black hole geometries of the Schwarzschild type. Investigating the trajectories in a more general class of static spherically symmetric black hole spacetimes, we find that the two relations are valid separately for two different choices of boundary data.

gr-qc↗

arXiv2021

The Life and Science of Thanu Padmanabhan

Thanu Padmanabhan was a renowned Indian theoretical physicist known for his research in general relativity, cosmology, and quantum gravity. In an extraordinary career spanning forty-two years, he published more than three hundred research articles, wrote ten highly successful technical and popular books, and mentored nearly thirty graduate students and post-doctoral fellows. He is best known for his deep work investigating gravitation as an emergent thermodynamic phenomenon. He was an outstanding teacher, and an indefatigable populariser of science, who travelled very widely to motivate and inspire young students. Paddy, as he was affectionately known, was also a close friend to his students and collaborators, treating them as part of his extended academic family. On September 17, 2021 Paddy passed away very unexpectedly, at the age of sixty-four and at the height of his research career, while serving as a Distinguished Professor at the Inter-University Centre for Astronomy and Astrophysics, Pune. His untimely demise has come as a shock to his family and friends and colleagues. In this article, several of them have come together to pay their tributes and share their fond memories of Paddy.

physics.hist-ph↗

arXiv2021

Can a localised quantum system see soft photons?

We ask whether soft photons, defined by asymptotic charges, can have consequences for the outcome of localised quantum processes. We consider a spatially localised two-state system, at rest in flat spacetime, coupled to a $U(1)$ gauge invariant charged scalar field. We find that the system's de-excitation rate does depend on the soft charges that correspond to the radial component of the electric field dressing at the asymptotic infinity; the excitation rate, by contrast, remains zero, regardless of the soft charges. Some implications are discussed.

hep-th↗

arXiv2020

Generation of CMB and Cosmological constant via bulk viscosity

A simple model of uniformly expanding, homogeneous Universe with a bulk viscosity is studied wherein the inflationary density decays due to viscous dissipation during the expansion phase of the Universe. The model is shown to generate the Cosmic Microwave Background radiation (CMB). We also demonstrate that, at late times, the inflationary density asymptotically approaches a small finite constant value.

gr-qc↗

arXiv2020

The role of Unruh effect in Bremsstrahlung

An equivalence is demonstrated, by an explicit first order quantum calculation, between the Minkowski photon emission rate in the inertial frame for an accelerating charge moving on a Rindler trajectory with additional transverse drift motion and the combined Rindler photon emission and absorption rate of the same charge in the Rindler frame in the presence of the Davies Unruh thermal bath. The equivalence also extends, for the Bremsstrahlung emitted by the same charge as calculated using the machinery of classical electrodynamics. The equivalence is shown to also hold for the case of accelerating charges moving on a Rindler trajectory with additional arbitrary transverse motion. Our results generalise those of Higuchi et. al. (1992) and of Cozzella et. al. (2017) for accelerated trajectories with circular transverse motion. Related issues and experimental implications are discussed.

gr-qc↗

arXiv2019

Directional dependence of the Unruh effect for spatially extended detectors

We analyse the response of a spatially extended direction-dependent local quantum system, a detector, moving on the Rindler trajectory of uniform linear acceleration in Minkowski spacetime, and coupled linearly to a quantum scalar field. We consider two spatial profiles: (i) a profile defined in the Fermi-Walker frame of an arbitrarily-accelerating trajectory, generalising the isotropic Lorentz-function profile introduced by Schlicht to include directional dependence; and (ii) a profile defined only for a Rindler trajectory, utilising the associated frame, and confined to a Rindler wedge, but again allowing arbitrary directional dependence. For (i), we find that the transition rate on a Rindler trajectory is non-thermal, and dependent on the direction, but thermality is restored in the low and high frequency regimes, with a direction-dependent temperature, and also in the regime of high acceleration compared with the detector's inverse size. For (ii), the transition rate is isotropic, and thermal in the usual Unruh temperature. We attribute the non-thermality and anisotropy found in (i) to the leaking of the Lorentz-function profile past the Rindler horizon.

gr-qc↗