arXiv subjects

Ilija Rakic

Publications and source records attributed to Ilija Rakic.

4 recordsLinked to original sources

arXiv2025

The evaporation of near-extremal black holes through charged particle emission

We compute the quantum rate for massless charged scalar emission by a near-extremal Reissner-Nordström black hole using Schwarzian theory as the effective description of the black hole. This is compared to the semi-classical Hawking rate which we also compute near extremality. We classify black holes into small and large, each with a unique spectrum. For small black holes, at energies below a particular quantum scale, the emission is captured by the quantum rate, which gives different predictions from the semi-classical. Furthermore, depending on how the energy compares to another scale associated with the phenomenon of superradiance, the radiation either comes out as mostly non-superradiant or mostly superradiant. For non-superradiant emission, it is found that the quantum rate is suppressed compared to the semi-classical in the same way as recently observed for neutral radiation. For superradiant emission, a novel behavior is observed, the quantum rate is enhanced compared to the semi-classical. For large black holes, we argue that the quantum rate always reduces to the semi-classical. In the limit of very large black holes, from our semi-classical rate, we recover the Gibbons result of Schwinger-like suppression. This gives a unified story of near-extremal charged emission rates, both quantum and semi-classical, which covers all sizes of black hole and all energy regimes. We use these rates to discuss the evaporation history of each type of black hole, from when it starts very near extremality, until it has left this regime. Finally, for the near-extremal Kerr black hole, we argue that the quantum rate always reduces to the semi-classical with the superradiant modes dominating. This rate is computed for spin $0,1,2$. Our analysis emphasizes the AdS$_2$ structure of the Reissner-Nordström and Kerr near-horizon regions, which enables a completely parallel treatment of the two.

hep-th↗

arXiv2025

Looking at extremal black holes from very far away

Near-extremal black holes are subject to large quantum effects, which modify their low-temperature thermodynamic behavior. Hitherto, these quantum effects were analyzed by separating the geometry into the near-horizon region and its exterior. It is desirable to understand and reproduce such corrections from the full higher-dimensional asymptotically flat or AdS geometry's perspective. We address this question in this article and fill this gap. Specifically, we find off-shell eigenmodes of the quadratic fluctuation operator of the Euclidean gravitational dynamics, with eigenvalues that vanish linearly with temperature. We illustrate this for BTZ and neutral black holes with hyperbolic horizons in AdS in Einstein-Hilbert theory, and for the charged black holes in Einstein-Maxwell theory. The linear scaling with Matsubara frequency, which is a distinctive feature of the modes, together with the fact that their wavefunctions localize close to the horizon as we approach extremality, identifies them as responsible for the aforementioned quantum effects. We provide a contour prescription to deal with the sign indefiniteness of the Euclidean Einstein-Maxwell action, which we derive to aid our analysis. We also resolve a technical puzzle regarding modes associated with rotational isometries in stationary black hole spacetimes.

hep-th↗

arXiv2025

One-loop determinants in AdS$_3$ supergravity with extended supersymmetry

We re-examine the computation of the one-loop partition function of Type II supergravity theory compactified on $AdS_3 \times \mathbf{S}^3 \times X$, where $X$ can be $K_3$, $T^4$ and $\mathbf{S}^3 \times \mathbf{S}^1$. These backgrounds preserve eight supercharges (four left and four right moving) and are known to be holographically dual to either small or large $N=(4,4)$ superconformal field theories. By extending well-established heat kernel techniques, we evaluate the one-loop determinants associated to these supergravity backgrounds. The resulting expressions are then used to reconstruct the characters of short multiplets of the corresponding dual boundary theories. A distinctive aspect of our analysis is the emergence of contributions from multiple gravitational saddle points in the path integral, reflecting the spectral flow structure present in the boundary theory.

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arXiv2024

Thermodynamics of the near-extremal Kerr spacetime

We examine the thermodynamics of a near-extremal Kerr black hole, and demonstrate that the geometry behaves as an ordinary quantum system with a vanishingly small degeneracy at low temperatures. This is in contrast with the classical analysis, which instead predicts a macroscopic entropy for the extremal Kerr black hole. Our results follow from a careful analysis of the gravitational path integral. Specifically, the low temperature canonical partition function behaves as $Z \sim \, T^\frac{3}{2}\, e^{S_0+ c \log S_0}$, with $S_0$ the classical degeneracy and $c$ a numerical coefficient we compute. This is in line with the general expectations for non-supersymmetric near-extremal black hole thermodynamics, as has been clarified in the recent past, although cases without spherical symmetry have not yet been fully analyzed until now. We also point out some curious features relating to the rotational zero modes of the near-extremal Kerr black hole background that affects the coefficient $c$. This raises a puzzle when considering similar black holes in string theory. Our results generalize to other rotating black holes, as we briefly exemplify.

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