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Sudip Karan

Publications and source records attributed to Sudip Karan.

9 recordsLinked to original sources
arXiv2026

New-born strings are tensionless

We report a physical origin of tensionless strings, obtained by formulating string dynamics in finite-lifetime settings for the first time. Departing from the conventional paradigm of eternal strings, we construct an inertial worldsheet confined to a finite region of two-dimensional Minkowski spacetime, known as a causal diamond. This reveals a striking result: tensionless strings arise only at the moment of their birth. The ultra-shrinking limit of the diamond worldsheet realizes this birth configuration, thereby uncovering a new tensionless string phase characterized by a global, ultra-local Carrollian structure.

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arXiv2025

Logarithmic correction to the entropy of a Kerr-Newman family of black holes in $U(1)^2$-charged STU supergravity models

The leading quantum-gravitational correction to the black hole entropy is known to be a universal logarithmic term. In this study, we investigate the logarithmic corrections for the black holes in the STU supergravity models, which are a bosonic truncation into a specific class of $U(1)^2$-charged Einstein-Maxwell-dilaton theory. We demonstrate how the entire Kerr-Newman-AdS and Kerr-Newman family of black holes can be recovered within the gauged and ungauged STU supergravity models as special embedding choices in 4D. Logarithmic corrections are computed using two distinct Euclidean quantum gravity setups for extremal and non-extremal limits of all embedded rotating, static, charged, and neutral black holes. Our calculations employ the on-shell heat kernel method based Seeley-DeWitt expansion computations. Notably, all the AdS$_4$ results exhibit a confirmed non-topological nature as compared to the flat counterparts, offering a natural and more comprehensive ``infrared window into the microstates'' of black holes.

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arXiv2024

A time-like window into tensionless worldsheets

Rindler worldsheets are well known to transition into a tensionless regime in the infinite acceleration limit, acquiring an emergent Carrollian structure. In this work, we uncover an entirely new manifestation of such tensionless dynamics by constructing a time-evolving Milne worldsheet within the future (expanding) and past (contracting) regions of the background target spacetime. We show that as the Milne worldsheet approaches its null horizons, it reveals a deep and previously unexplored interplay between tensionless string dynamics and an emergent Carrollian structure, governed by an ultra-high frequency limit in time evolution. Our results demonstrate that non-inertial string dynamics can emerge in two structurally independent yet complementary settings--governed either by worldsheet acceleration (Rindler) or by the frequency of its intrinsic time evolution (Milne). Remarkably, we uncover a non-trivial duality-like correspondence between Rindler and Milne worldsheets near their respective null horizons, both exhibiting identical tensionless physics despite their distinct causal and dynamical origins. This discovery positions the Milne construction as a genuine time-like counterpart to the ultra-relativistic (Carrollian) or infinite-boost limit, offering new foundational insights into the deep structure of non-inertial string theory.

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arXiv2024

Seeley-DeWitt Coefficients in $\mathcal{N}=2$ Einstein-Maxwell Supergravity Theory and Logarithmic Corrections to $\mathcal{N}=2$ Extremal Black Hole Entropy

We investigate the heat kernel method for one-loop effective action following the Seeley-DeWitt expansion technique of heat kernel with Seeley-DeWitt coefficients. We also review a general approach of computing the Seeley-DeWitt coefficients in terms of background or geometric invariants. We, then consider the Einstein-Maxwell theory embedded in minimal $\mathcal{N}=2$ supergravity in four dimensions and compute the first three Seeley-DeWitt coefficients of the kinetic operator of the bosonic and the fermionic fields in an arbitrary background field configuration. We find the applications of these results in the computation of logarithmic corrections to Bekenstein-Hawking entropy of the extremal Kerr-Newman, Kerr and Reissner-Nordstrom black holes in minimal $\mathcal{N}=2$ Einstein-Maxwell supergravity theory following the quantum entropy function formalism.

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arXiv2023

Logarithmic correction to black hole entropy in universal low-energy string theory models

We calculate the logarithmic correction to the entropy of asymptotically flat and AdS black holes (rotating, non-rotating, charged, and uncharged) embedded in Einstein-Maxwell-dilaton (EMD) theories with $U(1)$-charged. The leading quantum gravitational corrections are achieved in both extremal and non-extremal limits of black hole temperature by designing a common Euclidean gravity setup that evaluates the "logarithmic term" from one-loop effective actions via heat kernel method-based calculations. EMD theories are universal building blocks of compactified string theory or supergravity models in 4D. For a concrete example, we generalize the entire setup and calculate logarithmic corrections for black holes in $U(1)^2$-charged EMD models intersecting with $\mathcal{N}=4$ ungauged and gauged bosonic supergravity. In contrast to flat backgrounds, all the AdS$_4$ results are found to be non-topological, providing a wider "infrared window" into the microscopic degrees of freedom of black holes in string theory.

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arXiv2021

Generalized Einstein-Maxwell theory: Seeley-DeWitt coefficients and logarithmic corrections to the entropy of extremal and non-extremal black holes

We present a consolidated manual of Euclidean gravity approaches for finding the logarithmic corrections to the entropy of the full Kerr-Newman family of black holes in both extremal and non-extremal limits. Seeley-DeWitt coefficients for the quadratic fluctuations of a concern gravity theory appear to be the key ingredients in this manual. Following the manual, we calculate the first three Seeley-DeWitt coefficients and logarithmic corrections to the entropy of extremal and non-extremal black holes in a generalized Einstein-Maxwell theory minimally-coupled to additional massless scalar, vector, spin-1/2 Dirac and spin-3/2 Rarita-Schwinger fields. We finally employ the Seeley-DeWitt data to reproduce the logarithmic entropy corrections for extremal black holes in all $\mathcal{N} \geq 2$ Einstein-Maxwell supergravity via an alternative local supersymmetrization method.

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arXiv2021

Logarithmic corrections to black hole entropy in matter coupled $\mathcal{N} \geq 1$ Einstein-Maxwell supergravity

We calculate the first three Seeley-DeWitt coefficients for fluctuation of the massless fields of an $\mathcal{N}=2$ Einstein-Maxwell supergravity theory (EMSGT) distributed into different multiplets in $d=4$ space-time dimensions. By utilizing the Seeley-DeWitt data in the quantum entropy function formalism, we then obtain the logarithmic correction contribution of individual multiplets to the entropy of extremal Kerr-Newman family of black holes. Our results allow us to find the logarithmic entropy corrections for the extremal black holes in a fully matter coupled $\mathcal{N}=2,d=4$ EMSGT, in a particular class of $\mathcal{N}=1,d=4$ EMSGT as consistent decomposition of $\mathcal{N}=2$ multiplets ($\mathcal{N}=2 \to \mathcal{N}=1$) and in $\mathcal{N} \geq 3,d=4$ EMSGTs by decomposing them into $\mathcal{N}=2$ multiplets ($\mathcal{N} \geq 3 \to \mathcal{N}=2$). For completeness, we also obtain logarithmic entropy correction results for the non-extremal Kerr-Newman black holes in the matter coupled $\mathcal{N} \geq 1,d=4$ EMSGTs by employing the same Seeley-DeWitt data into a different Euclidean gravity approach developed in arXiv:1205.0971.

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arXiv2021

Logarithmic correction to the entropy of extremal black holes in $\mathcal{N}=1$ Einstein-Maxwell supergravity

We study one-loop covariant effective action of \say{non-minimally coupled} $\mathcal{N}=1$, $d=4$ Einstein-Maxwell supergravity theory by heat kernel tool. By fluctuating the fields around the classical background, we study the functional determinant of Laplacian differential operator following Seeley-DeWitt technique of heat kernel expansion in proper time. We then compute the Seeley-DeWitt coefficients obtained through the expansion. A particular Seeley-DeWitt coefficient is used for determining the logarithmic correction to Bekenstein-Hawking entropy of extremal black holes using quantum entropy function formalism. We thus determine the logarithmic correction to the entropy of Kerr-Newman, Kerr and Reissner-Nordström black holes in {\say{non-minimally coupled}} $\mathcal{N}=1$, $d=4$ Einstein-Maxwell supergravity theory.

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