Research

I study black holes through the lens of semiclassical gravity, based on quantum field theory in curved spacetime. In particular, I study how quantum vacuum fields behave both inside the event horizon and in the exterior during formation and evaporation. Much of my work develops concrete computations of renormalized quantum observables (such as energy fluxes) and uses them to understand how quantum effects feed back on the geometry through semiclassical backreaction - clarifying questions like the fate of the inner (Cauchy) horizon and how Hawking radiation builds up during collapse.

This page offers an overview of the questions I've worked on so far, and the directions I'm seeking to pursue next.

My PhD concluding seminar, "Quantum Effects inside Black Holes," delivered at the Technion in March 2024 (PhD advisor: Prof. Amos Ori)

Quantum effects inside black holes

Astrophysical black holes are known to be rotating. Within classical General Relativity, the simplest spacetime solution describing a rotating black hole - the Kerr spacetime - reveals an inner (Cauchy) horizon which may act as a traversable passage that could, in principle, lead to another external universe. But does this picture survive once quantum effects are taken into account?

Answering this - and related questions about the internal structure of realistic black holes - requires understanding how quantum energy fluxes shape the interior black hole geometry. For decades it was widely anticipated, but not conclusively demonstrated, that semiclassical effects might diverge at the Cauchy horizon of a four-dimensional black hole, potentially transforming the classical interior and eliminating any notion of traversability. Clarifying this issue requires explicit computations of key renormalized quantum observables inside black hole - which remained an outstanding technical and conceptual challenge for many years.

Penrose diagram of a black hole formed in collapse. As customary, time runs upward and space horizontally; null rays propagate at 45° (gold arrows), with ingoing rays pointing left and outgoing rays right. The collapsing star’s surface is the yellow curve, with its interior shaded, and the dashed vertical line marks its regular center. The black line labeled ∞ denotes future null infinity. The event horizon (EH) forms the outer black-hole boundary, while the red line labeled CH is the Cauchy horizon, i.e., the boundary of the maximal Cauchy development for an initial slice such as the light-blue hypersurface. Beyond the CH, the arrow indicates a hypothetical extension to other spacetime regions. The question marks serve a dual purpose: firstly, indicating the region where predictability is lost, and secondly, highlighting the uncertain traversability of the CH. The dashed worldline and spaceship illustrate an infalling traveler embarking from the initial surface, headed toward the CH to confront the associated question marks.

Motivated by this, our work has focused on developing and carrying out first-of-their-kind quantitative computations of renormalized quantum energy fluxes in black-hole interiors, focusing on the Cauchy horizon of four-dimensional spinning (Kerr) and charged (Reissner-Nordström) black holes in physically motivated vacuum states (including the Unruh state, appropriate for a black hole formed by collapse and evaporating via Hawking radiation). Our results demonstrated - firmer than ever - that vacuum quantum effects induce a curvature singularity at the Cauchy horizon. Moreover, when taken on a fixed background (i.e., before including evaporation), the resulting semiclassical singularity is generally stronger than the one induced by classical perturbations, suggesting that quantum effects can dominate the backreaction near the Cauchy horizon.

A crucial additional output of these computations is the sign of the quantum energy fluxes, which dictates the nature of tidal deformation and backreaction on the geometry through the semiclassical Einstein equation. In particular, we find that the flux may be positive or negative depending on the black hole parameters (and, in the spinning case, on the polar angle), providing key input for understanding the interior’s dynamical fate.

Selected papers in this program

Quantum fluxes at the inner horizon of a spherical charged black hole (PRL, 2020): First computations of renormalized quantum energy fluxes at the inner horizon of a black hole, establishing the semiclassical picture in a spherical setting.
Quantum fluxes at the inner horizon of a near-extremal spherical charged black hole (PRD, 2021): Focuses on the near-extremal regime, which lends itself to analytical treatment. Quantifies how key Cauchy horizon observables, as well as the Hawking outflux, scale as the extremal limit is approached, and derives low-frequency scattering coefficients outside the black both (both sub-extremal and extremal).
Two-point function of a quantum scalar field in the interior region of a Kerr black hole (PRD, 2022): Develops the core renormalization ingredient for the Kerr interior by constructing the interior Hadamard two-point function in the Unruh state in terms of numerically tractable ingredients.
Quantum fluxes at the inner horizon of a spinning black hole (PRL, 2022): First computations of renormalized quantum energy fluxes at the Kerr inner horizon, both on and off the axis of rotation - advancing the problem beyond spherical symmetry to a more physically realistic setting.
Computation of ⟨Φ²⟩ and quantum fluxes at the polar interior of a spinning black hole (PRD, 2025): Develops and implements t-splitting renormalization methods in the Kerr interior along the axis of rotation, computing ⟨Φ²⟩ and quantum energy fluxes in the Unruh state throughout the Kerr interior and with a focus on the Cauchy-horizon vicinity.

Research roadmap and connections between some published works (figure taken from my thesis). Schematic overview of the five manuscripts above and how they relate to one another. Works on Reissner–Nordström (RN) black holes are shown in green, and works on Kerr black holes in blue. Colored markers indicate the dominant methodology: orange denotes primarily analytical work, purple denotes primarily computational/numerical work, and paired markers indicate a mix of both. Arrows and brief annotations summarize the main logical links between works. [IH stands for the inner horizon; HTPF stands for the Hadamard two-point function]

Ongoing projects in this program explore:

Backreaction: how the computed observables feed into the spacetime geometry via the semiclassical Einstein equation.
Fluctuations: stress-tensor fluctuations and what they imply for the validity of the semiclassical description near the Cauchy horizon.
Extensions: generalizations to additional fields and broader settings.

The origin of the energy in black hole evaporation

Black holes are known to emit particles that carry energy across spacetime (Hawking radiation), but the origin of that radiated energy has long been debated. Some proposals suggested it might be released directly from the collapsing matter, potentially as a sudden burst that could disrupt the collapse and even prevent a black hole from forming. In the absence of concrete computations, the question remained: where does the energy carried away by Hawking radiation come from? (As Chen, Unruh, Wu and Yeom put it in 2017, “The origin of the energy in black hole evaporation has been a longstanding issue, which is still not entirely settled.”)

In a 2025 Physical Review Letters paper, titled semiclassical outflux emerging from a collapsing shell, we address this question through an explicit computation in a four-dimensional collapse model (see below). We compute the renormalized quantum energy outflux emerging from a collapsing shell and find that it builds up gradually in the region surrounding the forming black hole - rather than propagating in a conserved manner starting from the shell's surface. This offers a concrete resolution to a long-standing issue, and corroborates the more conventional picture of Hawking radiation as a gradual, steady process that cannot interfere with black-hole formation in gravitational collapse.

A semi-popular account of this work is available here.

Collapse model and the build-up of Hawking outflux. We consider an idealized four dimensional collapse: a thin, lightlike spherical shell separating an interior Minkowski region from an exterior Schwarzschild region (left: spatial picture at fixed time; right: spacetime diagram; the shell is shown in red). We compute the renormalized outgoing quantum energy flux on the shell’s outer surface as a function of the shell's shrinking radius r_0. In particular, as the shell approaches its Schwarzschild radius, the outflux vanishes at the rate required for horizon regularity. To visualize where the Hawking flux "comes form," we consider the evolution of the quantum outflux along a late-time outgoing ray (yellow). This geodesic emerges from the shell at very close to the Schwarzschild radius, where the quantum outflux is negligibly small. As the ray propagates outward, the outflux evolves gradually in the strong-field region outside the forming horizon, and it asymptotes to the standard Hawking value (denoted F_H) at future lightlike infinity. In other words, the outgoing radiation starts from zero at (extremely close to) the event horizon, and gradually develops over a fairly broad strong-field region.

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