Contributor info: Dr Eleni-Alexandra Kontou

Jackson R. Fliss, Ben Freivogel, Eleni-Alexandra Kontou, Diego Pardo Santos

SciPost Phys. 19, 087 (2025) · published 7 October 2025 | Toggle abstract · pdf

Smeared null energy has been shown to be bounded from below for free minimally coupled quantum field theories. This is not the case for conformally coupled free bosonic theories where states of unbounded null energy can be constructed by increasing the particle number. Little is known for interacting conformal field theories (CFTs) in dimensions larger than two. In this work we consider states that are superpositions of scalar primary operators or the stress-energy tensor itself in large $N$ CFTs. Within the large $N$ approximation we present arguments that the negative smeared null energy of such states scales at worst as the central charge of the theory, $C_T$. This provides evidence for a general bound for CFTs in $d$-dimensions proportional to the central charge.

Jackson R. Fliss, Ben Freivogel, Eleni-Alexandra Kontou, Diego Pardo Santos

SciPost Phys. 16, 119 (2024) · published 2 May 2024 | Toggle abstract · pdf

The non-minimal coupling of scalar fields to gravity has been claimed to violate energy conditions, leading to exotic phenomena such as traversable wormholes, even in classical theories. In this work we adopt the view that the non-minimal coupling can be viewed as part of an effective field theory (EFT) in which the field value is controlled by the theory's cutoff. Under this assumption, the average null energy condition, whose violation is necessary to allow traversable wormholes, is obeyed both classically and in the context of quantum field theory. In addition, we establish a type of "smeared" null energy condition in the non-minimally coupled theory, showing that the null energy averaged over a region of spacetime obeys a state dependent bound, in that it depends on the allowed field range. We finally motivate our EFT assumption by considering when the gravity plus matter path integral remains semi-classically controlled.

Jackson R. Fliss, Ben Freivogel, Eleni-Alexandra Kontou

SciPost Phys. 14, 024 (2023) · published 27 February 2023 | Toggle abstract · pdf

The null energy condition (NEC), an important assumption of the Penrose singularity theorem, is violated by quantum fields. The natural generalization of the NEC in quantum field theory, the renormalized null energy averaged over a finite null segment, is known to be unbounded from below. Here, we propose an alternative, the double smeared null energy condition (DSNEC), stating that the null energy smeared over two null directions has a finite lower bound. We rigorously derive DSNEC from general worldvolume bounds for free quantum fields in Minkowski spacetime. Our method allows for future systematic inclusion of curvature corrections. As a further application of the techniques we develop, we prove additional lower bounds on the expectation values of various operators such as conserved higher spin currents. DSNEC provides a natural starting point for proving singularity theorems in semi-classical gravity.

Ben Freivogel, Eleni-Alexandra Kontou, Dimitrios Krommydas

SciPost Phys. 13, 001 (2022) · published 15 July 2022 | Toggle abstract · pdf

The classic singularity theorems of General Relativity rely on energy conditions that can be violated in semiclassical gravity. Here, we provide motivation for an energy condition obeyed by semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. We then prove a semiclassical singularity theorem using SNEC as an assumption. This theorem extends the Penrose theorem to semiclassical gravity. We also apply our bound to evaporating black holes and the traversable wormhole of Maldacena-Milekhin-Popov, and comment on the relationship of our results to other proposed semiclassical singularity theorems.