arXiv subjects

Lucas de Souza

Publications and source records attributed to Lucas de Souza.

6 recordsLinked to original sources

arXiv2026

Running Love Numbers of Charged Black Holes

Loops of virtual particles from the vacuum of quantum field theory (QFT) render black holes tidally deformable. We compute the static tidal response of unspinning charged black holes at arbitrary radius, using the perturbative formalism developed in 2501.18684. Since the gravitational and electromagnetic tidal responses mix, we generalize the notion of Love numbers to Love matrices. We derive the coupled equations of motion for the metric and electromagnetic fluctuations around purely electric and magnetic backgrounds. For large charged black holes, which are described by the Effective Field Theory (EFT) of gravity, we compute the full set of Love matrices induced by an arbitrary tower of $F^{2n}$ operators. We find that, although quantum corrections break electromagnetic duality, the Love matrices in electric and magnetic backgrounds are related by a $Z_2$ symmetry under electric-magnetic exchange. Going beyond EFT, we compute the Love matrices of small magnetic black holes. We show that the running of the Love matrices is governed by the running of the $U(1)$ gauge coupling, and we derive the correspondence between Love and $U(1)$ beta functions for arbitrary harmonics. The overall picture that emerges is that the QFT-induced tidal response of magnetic black holes saturates in the strong-field regime. These results imply that nearly-extremal magnetic black holes charged under an Abelian dark sector could be probed by gravitational-wave observations.

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arXiv2025

On The Black Hole Weak Gravity Conjecture and Extremality in the Strong-Field Regime

We point out that the Weak Gravity Conjecture (WGC) implies that sufficiently small extremal black holes are necessarily in the strong-field regime of electrodynamics, and therefore probe the UV completion of the Maxwell sector. To investigate the WGC bounds arising from these small extremal black holes, we revisit black hole decay in generic field theories in asymptotically flat space. We derive a necessary and a sufficient condition for any black hole to decay, the latter amounting to a bound on the growth of charge relative to mass. We apply these conditions to extremal black holes derived in various UV completions of the Maxwell sector. We find that the Euler-Heisenberg and DBI effective actions satisfy the sufficient condition for decay, while the ModMax model fails the necessary one, rendering it incompatible with the WGC. Using the decay conditions, we show that the black hole WGC implies positivity of the $U(1)$ gauge coupling beta function. This provides an independent argument that classically stable (embedded-Abelian) colored black holes cannot exist. We also show that the black hole WGC constrains conformal hidden sector models, and is always satisfied in their AdS dual realizations.

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arXiv2025

Vortex Fractional Fermion Number through Heat Kernel methods and Edge States

Computing the vacuum expectation of fermion number operator on a soliton background is often challenging. A recent proposal in arXiv:2305.13606 simplifies this task by considering the soliton in a bounded region and relating the $η$ invariant, and thus the fermion number, to a specific heat kernel coefficient and to contributions from the edge states. We test this method in a system of charged fermions living on an Abrikosov-Nielsen-Olesen (ANO) vortex background. We show that the resulting $η$ invariant does not depend on boundary conditions (within a certain class), thereby supporting the validity of the method. Our analysis reveals a nontrivial feature for the fermionic spectrum in the vortex-induced Higgs phase. As a by-product, we also find that for a vortex living on a disk, the edge states carry fractional charge.

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arXiv2025

Gravity-Induced Photon Interactions and Infrared Consistency in any Dimensions

We compute the four-photon ($F^4$) operators generated by loops of charged particles of spin $0$, $\frac{1}{2}$, $1$ in the presence of gravity and in any spacetime dimension $d$. To this end, we expand the one-loop effective action via the heat kernel coefficients, which capture both the gravity-induced renormalization of the $F^4$ operators and the low-energy Einstein-Maxwell effective field theory (EFT) produced by massive charged particles. We set positivity bounds on the $F^4$ operators using standard arguments from extremal black holes (for $d\geq 4$) and from infrared (IR) consistency of four-photon scattering (for $d\geq 3$). We find that both approaches yield nearly equivalent results, even though in the amplitudes we discard the graviton $t$-channel pole and use the vanishing of the Gauss-Bonnet term at quadratic order for any $d$. The positivity bounds constrain the charge-to-mass ratio of the heavy particles. If the Planckian $F^4$ operators are sufficiently small or negative, such bounds produce a version of the $d$-dimensional Weak Gravity Conjecture (WGC) in most, but not all, dimensions. In the special case of $d=6$, the gravity-induced beta functions of $F^4$ operators from charged particles of any spin are positive, leading to WGC-like bounds with a logarithmic enhancement. In $d=9,10$, the WGC fails to guarantee extremal black hole decay in the infrared EFT, thereby requiring the existence of sufficiently large Planckian $F^4$ operators.

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arXiv2025

Running Love Numbers and the Effective Field Theory of Gravity

Massive states produce higher derivative corrections to Einstein gravity in the infrared, which are encoded into operators of the Effective Field Theory (EFT) of gravity. These EFT operators modify the geometry and affect the tidal properties of black holes, either neutral or charged. A thorough analysis of the perturbative tidal deformation problem leads us to introduce a tidal Green function, which we use to derive two universal formulae that efficiently provide the constant and running Love numbers induced by the EFT. We apply these formulae to determine the tidal response of EFT-corrected non-spinning black holes induced by vector and tensor fields, reproducing existing results where available and deriving new ones. We find that neutral black hole Love numbers run classically for $l\geq 3$ while charged ones run for $l\geq2$. Insights from the Frobenius method and from EFT principles confirm that the Love number renormalization flow is a well-defined physical effect. We find that extremal black holes can have Love numbers much larger than neutral ones, up to ${\cal O}(1)$ within the EFT validity regime, and that the EFT cutoff corresponds to the exponential suppression of the Schwinger effect. We discuss the possibility of probing an Abelian dark sector through gravitational waves, considering a scenario in which dark-charged extremal black holes exist in the present-day Universe.

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arXiv2023

Edge states and the $η$ invariant

We propose a relation between the $η$ invariant on a manifold with boundary, the $η$ invariants of edge states, and the $η$ invariant in an infinite volume limit. With the example of planar fermions with bag and chiral bag boundary conditions we show that this relation holds whenever edge states are sufficiently well-localized near the boundary. As a by-product we show that the spectrum of edge modes for chiral bag boundary conditions is linear but bounded.

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