arXiv subjects

Swayamsiddha Maharana

Publications and source records attributed to Swayamsiddha Maharana.

6 recordsLinked to original sources

arXiv2025

Physical black holes: spacetime and matter near trapping surfaces

Within a semiclassical framework, we investigate spherically symmetric solutions of the Einstein equations that (i) develop a trapped region within a finite time as measured by distant observers, and (ii) remain sufficiently regular at the apparent horizon. These requirements characterize the self-consistent approach, and it imposes specific constraints on the limiting form of the energy momentum tensor (EMT). Previous works argued that only certain EMT classes admit viable dynamical solutions, while discarding others (see, e.g., Terno 2020) based on reasoning that is not entirely robust. Here, we explicitly demonstrate that the discarded classes are indeed unphysical. In addition, we also introduce a new class of static solutions that are regular at the horizon and allow finite-time horizon crossing according to a distant observer.

gr-qc↗

arXiv2025

Padé metrics for black hole perturbations and light rings

Most distinguishing features of black holes and their mimickers are concentrated near the horizon. In contrast, astrophysical observations and theoretical considerations primarily constrain the far-field geometry. In this work we develop tools to effectively describe both, using the two-point Padé approximation to construct interpolating metrics connecting the near and far-field. We extend our previous work by computing the quasinormal modes of gravitational perturbations for static, spherically symmetric metrics that deviate from Schwarzschild spacetime. Even at the lowest order, this approach compares well with existing methods in both accuracy and applicability. Additionally, we show that the lowest-order interpolating metric reliably predicts light ring locations. It closely matches exact results, even when unsuitable for quasinormal frequency calculations.

gr-qc↗

arXiv2025

Horizon-bound objects: Kerr--Vaidya solutions

Kerr-Vaidya metrics are the simplest dynamical axially-symmetric solutions, all of which violate the null energy condition and thus are consistent with the formation of a trapped region in finite time according to distant observers. We examine different classes of Kerr-Vaidya metrics, and find two which possess spherically-symmetric counterparts that are compatible with the finite formation time of a trapped region. These solutions describe evaporating black holes and expanding white holes. We demonstrate a consistent description of accreting black holes based on the ingoing Kerr--Vaidya metric with increasing mass, and show that the model can be extended to cases where the angular momentum to mass ratio varies. For such metrics we describe conditions on their dynamical evolution required to maintain asymptotic flatness. Pathologies are also identified in the evaporating white hole geometry in the form of an intermediate singularity accessible by timelike observers. We also describe a generalization of the equivalence between Rindler and Schwarzschild horizons to Kerr--Vaidya black holes, and describe the relevant geometric constructions.

gr-qc↗

arXiv2024

The Hawking temperature of dynamical black holes via conformal transformations

In this second part of our two-series on extracting the Hawking temperature of dynamical black holes, we focus into spacetimes that are conformal transformations of static spacetimes. Our previous investigation builds upon the Unruh-Hawking analogy, which relates the spacetime of a uniformly accelerating observer to the near-horizon region of a black hole, to obtain the Hawking temperature. However, in this work, we explicitly compute the Bogoliubov coefficients associated with incoming and outgoing modes, which not only yields the temperature but also thermal spectrum of particles emitted by a black hole. For illustration, we take the simplest nontrivial example of the linear Vaidya spacetime, which is conformal to the static metric and using this property, we analytically solve the massless scalar field in its background. This allows the explicit computations of the Bogoliubov coefficients to study the particle production in this spacetime. We also derive an expression for the total mass of such dynamical spacetimes using the conformal Killing vector. We then perform differential variations of the mass formula to determine whether the laws of dynamical black hole mechanics correspond to the laws of thermodynamics.

gr-qc↗

arXiv2023

Models of cosmological black holes

We study various aspects of modeling astrophysical black holes using the recently introduced semiclassical formalism of physical black holes (PBHs). This approach is based on the minimal requirements of observability and regularity of the horizons. We demonstrate that PBHs do not directly couple to the cosmological background in the current epoch, and their equation of state renders them unsuitable for describing dark energy. Utilizing their properties for analysis of more exotic models, we present a consistent semiclassical scenario for a black-to-white hole bounce and identify obstacles to the transformation from a black hole horizon to a wormhole mouth.

gr-qc↗

arXiv2023

Semiclassical corrections to the photon orbits of a non-rotating black hole

In this brief article we discuss the corrections to the photon orbits of a non-rotating black hole due to semiclassical fluctuations of the metric. It is found that the photon orbit impact parameter differences with the critical impact parameter become of the order of the semiclassical fluctuations. We calculate the effect of the semi-classical fluctuations on the photon orbits and show that instead of circulating the black hole infinite number of times at the critical orbit, the photons bounce off the semiclassical geometry.

gr-qc↗