publications | Leonardo G. Trombetta

2026

IR side of bounds on theories with spontaneously broken Lorentz symmetry

Francesco Serra and Leonardo G. Trombetta

Phys. Rev. D, Mar 2026

Abs DOI arXiv

In nature, some UV features of dynamics are reflected in IR quantities. In fully relativistic theories, this connection can be probed through the analyticity properties of scattering amplitudes, allowing one to understand which IR theories respect the UV assumptions of quantum field theory. The ensuing analyticity bounds can usually be rephrased as the absence of faster-than-light propagation for low-energy excitations. While it is interesting to understand these relations and their IR characterization for theories that have less idealized properties, it is also more difficult to derive analyticity bounds in these cases. For theories that spontaneously break Lorentz symmetry, recent progress was made by considering correlators of conserved currents and their analyticity properties. In this work, we focus on such theories and work to close the gap from the IR side, finding a natural way to express the known analyticity bounds purely in terms of low-energy kinematical quantities. Our analysis shows that the bounds require gapped excitations to have a slower speed than the gapless ones, at least for momenta that are low with respect to the mass gap. These results suggest a way to interpret the UV/IR connection in more complex theories.

2025

Primary black-hole scalar charges and kinetic screening in \(K\)-essence-Gauss-Bonnet gravity

Guillermo Lara, Georg Trenkler, and Leonardo G. Trombetta

Dec 2025

Abs arXiv

Black holes beyond General Relativity may carry non-standard charges that impact their phenomenology. We study how the scalar charge that is induced by the scalar-Gauss-Bonnet coupling is affected by the presence of a nontrivial kinetic term \(K(X)\). We discuss the corresponding kinetic screening in the asymptotically flat, static solution first. We then turn to the case where self-accelerating cosmology is driven by \(K(X)\), finding that the time-dependence of the scalar field opens up the parameter space, turning the black-hole scalar charge from secondary to primary. We provide a stability analysis and a measure of the intensity of the kinetic screening from the quartic dispersion relation of the mixed scalar and gravitational modes.
Large black-hole scalar charges induced by cosmology in Horndeski theories

Eugeny Babichev, Gilles Esposito-Farèse, Ignacy Sawicki, and Leonardo G. Trombetta

Phys. Rev. D, Apr 2025

Abs DOI arXiv

The regularity of black-hole solutions, embedded in an expanding universe, is studied in a subclass of Horndeski theories, namely the sum of the simplest quadratic, cubic, and quintic actions. We find that in the presence of a time derivative of the scalar field, driven by the cosmological expansion, this regularity generically imposes large scalar charges for black holes, even when assuming strictly no direct coupling of matter to the scalar field. Such charges cause a significant accretion of the scalar field by the black holes, driving its local time derivative to a small value. This phenomenon, together with the Vainshtein screening typical of these theories, strongly suppresses observable scalar effects. We show that this full class of models is consistent with LIGO/Virgo detections of gravitational waves, but that the LISA mission should be able to constrain the coefficient of the quintic term at the \(10^{-30}\,\,\)level in a self-acceleration scenario, an improvement by 16 orders of magnitude with respect to what is imposed by the speed of gravitational waves.
The cosmic trimmer: Black-hole hair in scalar-Gauss-Bonnet gravity is altered by cosmology

Eugeny Babichev, Ignacy Sawicki, and Leonardo G. Trombetta

Phys. Rev. D, Feb 2025

Abs DOI arXiv

Static black holes in general relativity modified by a linear scalar coupling to the Gauss-Bonnet invariant always carry hair. We show that the same mechanism that creates the hair makes it incompatible with a cosmological horizon. Other scalar-tensor models do not have this problem when time dependence of the scalar provides a natural matching to cosmology. Scalar-Gauss-Bonnet gravity is particularly rigid, and such a scenario does not help. An extra operator makes the theory behave like the other models, and the cosmological horizon can be accommodated. The hair, however, is drastically altered.

2024

Mergers of hairy black holes: Constraining topological couplings from entropy

Kabir Chakravarti, Amit Reza, and Leonardo G. Trombetta

Phys. Rev. D, Sep 2024

Abs DOI arXiv

Hairy black-holes are a unique prediction of certain theories that extend general relativity (GR) with a scalar field. The presence of scalar hair is reflected nontrivially in the entropy of the black hole along with any topological coupling that may be present in the action. Demanding that a system of two merging black holes obeys the global second law of thermodynamics imposes a bound on this topological coupling coefficient. In this work we study how this bound is pushed from its GR value by the presence of scalar hair by considering estimates of binary black-hole merger parameters through inference studies of both mock and real gravitational-wave (GW) events. Although the scalar charge may produce a statistically significant deviation of the change in entropy over the GR prediction, we find no evidence of this happening in the data from real GW events taken from GWTC-1. We also find the entropy change to be susceptible to biases arising out of GW inferences which ends up being two orders of magnitude larger, therefore overwhelming any change, if at all, induced by the scalar hair.
Five-point superluminality bounds

Francesco Serra and Leonardo G. Trombetta

JHEP, Jun 2024

Abs DOI arXiv

We investigate how the speed of propagation of physical excitations is encoded in the coefficients of five-point interactions. This leads to a superluminality bound on scalar five-point interactions, which we present here for the first time. To substantiate our result, we also consider the case of four-point interactions for which bounds from S-matrix sum rules exist and show that these are parametrically equivalent to the bounds obtained within our analysis. Finally, we extend the discussion to a class of higher-point interactions.

2023

Symmetric teleparallel Gauss-Bonnet gravity and its extensions

Juan Manuel Armaleo, Sebastian Bahamonde, Georg Trenkler, and Leonardo G. Trombetta

Phys. Rev. D, Nov 2023

Abs DOI arXiv

General teleparallel theories assume that curvature is vanishing in which case gravity can be solely represented by torsion and/or nonmetricity. Using differential form language, we express the Riemannian Gauss-Bonnet invariant concisely in terms of two general teleparallel Gauss-Bonnet invariants, a bulk and a boundary one. Both terms are boundary terms in four dimensions. We also find that the split is not unique and present two possible alternatives. In the absence of nonmetricity our expressions coincide with the well-known metric teleparallel Gauss-Bonnet invariants for one of the splits. Next, we focus on the description where only nonmetricity is present and show some examples in different spacetimes. We finish our discussion by formulating novel modified symmetric teleparallel theories constructed with our new scalars.
Constraining a stochastic variation of the gravitational coupling with binary systems

Francisco D. Mazzitelli and Leonardo G. Trombetta

Phys. Rev. D, Jun 2023

Abs DOI arXiv

We consider the effect of stochastic fluctuations of the gravitational coupling G on the evolution of binary systems. We work at an elementary level, in the Newtonian limit, and focus mainly on laser ranging. We show that, due to cumulative effects, observational data may be used to put bounds on the stochastic fluctuations. We also reanalyze previous results on the implications of stochastic fluctuations of G on cosmological models.
Stochastic Particle Creation: From the Dynamical Casimir Effect to Cosmology

Matı́as Mantiñan, Francisco D. Mazzitelli, and Leonardo G. Trombetta

Entropy, Jan 2023

Abs DOI arXiv

We study a stochastic version of the dynamical Casimir effect, computing the particle creation inside a cavity produced by a random motion of one of its walls. We first present a calculation perturbative in the amplitude of the motion. We compare the stochastic particle creation with the deterministic counterpart. Then, we go beyond the perturbative evaluation using a stochastic version of the multiple scale analysis, that takes into account stochastic parametric resonance. We stress the relevance of the coupling between the different modes induced by the stochastic motion. In the single-mode approximation, the equations are formally analogous to those that describe the stochastic particle creation in a cosmological context, that we rederive using multiple scale analysis.
Symmetric teleparallel Horndeski gravity

Sebastian Bahamonde, Georg Trenkler, Leonardo G. Trombetta, and Masahide Yamaguchi

Phys. Rev. D, May 2023

Abs DOI arXiv

Horndeski gravity is the most general scalar-tensor theory with one scalar field leading to second-order Euler-Lagrange field equations for the metric and scalar field, and it is based on Riemannian geometry. In this paper, we formulate an analog version of Horndeski gravity in a symmetric teleparallel geometry which assumes that both the curvature (general) and torsion are vanishing and gravity is only related to nonmetricity. Our setup requires that the Euler-Lagrange equations for not only metric and scalar field but also connection should be at most second order. We find that the theory can be always recast as a sum of the Riemannian-Horndeski theory and new terms that are purely teleparallel. Due to the nature of nonmetricity, there are many more possible ways of constructing second-order theories of gravity. In this regard, up to some assumptions, we find the most general k-essence extension of symmetric teleparallel Horndeski gravity. We also formulate a novel theory containing higher-order derivatives acting on nonmetricity while still respecting the second-order conditions, which can be recast as an extension of kinetic gravity braiding. We finish our study by presenting the FLRW cosmological equations for our model.

2022

Causality constraints on black holes beyond GR

Francesco Serra, Javi Serra, Enrico Trincherini, and Leonardo G. Trombetta

JHEP, Aug 2022

Abs DOI arXiv

We derive causality constraints on the simplest scalar-tensor theories in which black holes differ from what General Relativity predicts, a scalar coupled to the Gauss-Bonnet or the Chern-Simons terms. Demanding that time advances are unobservable within the regime of validity of these effective field theories, we find their cutoff must be parametrically of the same size as the inverse Schwarzschild radius of the black holes for which the non-standard effects are of order one. For astrophysical black holes within the range of current gravitational wave detectors, this means a cutoff length of the order of kilometers. We further explore the leading additional higher-dimensional operators potentially associated with the scale of UV completion and discuss their phenomenological implications for gravitational wave science.

2021

Scalar-tensor cosmologies without screening

Johannes Noller, Luca Santoni, Enrico Trincherini, and Leonardo G. Trombetta

JCAP, Jan 2021

Abs DOI arXiv

Scalar-tensor theories are frequently only consistent with fifth force constraints in the presence of a screening mechanism, namely in order to suppress an otherwise unacceptably large coupling between the scalar and ordinary matter. Here we investigate precisely which subsets of Horndeski theories do not give rise to and/or require such a screening mechanism. We investigate these subsets in detail, deriving their form and discussing how they are restricted upon imposing additional bounds from the speed of gravitational waves, solar system tests and cosmological observables. Finally, we also identify what subsets of scalar-tensor theories precisely recover the predictions of standard (linearised) ΛCDM cosmologies in the quasi-static limit.

2020

Hairy Black-holes in Shift-symmetric Theories

Paolo Creminelli, Nicolás Loayza, Francesco Serra, Enrico Trincherini, and Leonardo G. Trombetta

JHEP, Aug 2020

Abs DOI arXiv

Scalar hair of black holes in theories with a shift symmetry are constrained by the no-hair theorem of Hui and Nicolis, assuming spherical symmetry, time-independence of the scalar field and asymptotic flatness. The most studied counterexample is a linear coupling of the scalar with the Gauss-Bonnet invariant. However, in this case the norm of the shift-symmetry current \(J^2\,\,\)diverges at the horizon casting doubts on whether the solution is physically sound. We show that this is not an issue since \(J^2\,\,\)is not a scalar quantity, since \(J^μ\,\,\)is not a diffinvariant current in the presence of Gauss-Bonnet. The same theory can be written in Horndeski form with a non-analytic function \(G_5 ∼\log X\). In this case the shift-symmetry current is diff-invariant, but contains powers of \(X\,\,\)in the denominator, so that its divergence at the horizon is again immaterial. We confirm that other hairy solutions in the presence of non-analytic Horndeski functions are pathological, featuring divergences of physical quantities as soon as one departs from time-independence and spherical symmetry. We generalise the no-hair theorem to Beyond Horndeski and DHOST theories, showing that the coupling with Gauss-Bonnet is necessary to have hair.
Black Hole Ringdown as a Probe for Dark Energy

Johannes Noller, Luca Santoni, Enrico Trincherini, and Leonardo G. Trombetta

Phys. Rev. D, Apr 2020

Abs DOI arXiv

Under the assumption that a dynamical scalar field is responsible for the current acceleration of the Universe, we explore the possibility of probing its physics in black hole merger processes with gravitational wave interferometers. Remaining agnostic about the microscopic physics, we use an effective field theory approach to describe the scalar dynamics. We investigate the case in which some of the higher derivative operators, that are highly suppressed on cosmological scales, instead become important on typical distances for black holes. If a coupling to the Gauss-Bonnet operator is one of them, a non-trivial background profile for the scalar field can be sourced in the surroundings of the black hole, resulting in a potentially large amount of "hair". In turn, this can induce sizeable modifications to the spacetime geometry or a mixing between the scalar and the gravitational perturbations. Both effects will ultimately translate into a modification of the quasi-normal mode spectrum in a way that is also sensitive to other operators besides the one sourcing the scalar background. The presence of deviations from the predictions of general relativity in the observed spectrum can therefore serve as a window onto dark energy physics.

2019

To the sphere and back again: de Sitter infrared correlators at NTLO in 1/N

Diana López Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

JHEP, Aug 2019

Abs DOI arXiv

We analyze the infrared behavior of the two and four-point functions for the massless O(N) model in Lorentzian de Sitter spacetime, using the 1/N expansion. Our approach is based in the study of the Schwinger-Dyson equations on the sphere (Euclidean de Sitter space), using the fact that the infrared behavior in Lorentzian spacetime is determined by the pole structure of the Euclidean correlation functions. We compute the two-point function up to the NTLO in 1/N , and show that in the infrared it behaves as the superposition of two massive free propagators with effective masses of the same order, but not equal to, the dynamical mass \(m_{dyn}\). We compare our results with those obtained using other approaches, and find that they are equivalent but retrieved in a considerably simpler way. We also discuss the infrared behavior of the equal-times four-point functions.

2018

Long distance behavior of O(N)-model correlators in de Sitter space and the resummation of secular terms

Diana López Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

JHEP, Oct 2018

Abs DOI arXiv

We analyze the long distance behavior of the two-point functions for an interacting scalar O(N) model in de Sitter spacetime. Following our previous work, this behavior is analyzed by analytic continuation of the Euclidean correlators, which are computed by treating the homogeneous zero mode exactly and using a partial resummation of the interactions between the zero and the non-zero modes. We focus on massless fields and present an alternative derivation of our method, which involves a double expansion in 1/N and the coupling constant of the theory. This derivation is simpler than the previous one and can be directly extended for fields with negative squared-mass. We extend our previous results by computing the long wavelength limit of the two-point functions at next-to-leading order in 1/N and at leading order in the coupling constant, which involves a further resummation of Feynman diagrams (needed when the two-point functions are analitically continued). We prove that, after this extra resummation, the two-point functions vanish in the long distance limit.
Behind Horndeski: structurally robust higher derivative EFTs

Luca Santoni, Enrico Trincherini, and Leonardo G. Trombetta

JHEP, Aug 2018

Abs DOI arXiv

Higher derivative scalar interactions can give rise to interesting cosmological scenarios. We present a complete classification of such operators that can yield sizeable effects without introducing ghosts and, at the same time, define an effective field theory robust under the inclusion of quantum corrections. A set of rules to power count consistently the coefficients of the resulting Lagrangian is provided by the presence of an approximate global symmetry. The interactions that we derive in this way contain a subset of the so-called Horndeski and beyond Horndeski theories. Our construction therefore provides a structurally robust context to study their phenomenology. Applications to dark energy/modified gravity and geodesically complete cosmologies are briefly discussed.
Comment on “How the huge energy of quantum vacuum gravitates to drive the slow accelerating expansion of the Universe”

Francisco D. Mazzitelli and Leonardo G. Trombetta

Phys. Rev. D, Mar 2018

Abs DOI arXiv

In a recent paper [Q. Wang, Z. Zhu, and W. G. Unruh, Phys. Rev. D 95, 103504 (2017)PRVDAQ2470-001010.1103/PhysRevD.95.103504] it was argued that, due to the fluctuations around its mean value, vacuum energy gravitates differently from what was previously assumed. As a consequence, the Universe would accelerate with a small Hubble expansion rate, solving the cosmological constant and dark energy problems. We point out here that the results depend on the type of cutoff used to evaluate the vacuum energy. In particular, they are not valid when one uses a covariant cutoff such that the zero-point energy density is positive definite.

2017

Nonlocal effective actions in semiclassical gravity: thermal effects in stationary geometries

M. Elı́as, F. D. Mazzitelli, and L. G. Trombetta

Phys. Rev. D, Nov 2017

Abs DOI arXiv

We compute the gravitational effective action by integrating out quantum matter fields in a weak gravitational field, using the Schwinger-Keldysh (in-in) formalism. We pay particular attention to the role of the initial quantum state in the structure of the nonlocal terms in the effective action, with an eye to nonlinear completions of the theory that may be relevant in astrophysics and cosmology. In this first paper we consider a quantum scalar field in thermal equilibrium, in a stationary gravitational field. We obtain a covariant expression for the nonlocal effective action, which can be expressed in terms of the curvature tensor, the four-velocity of the thermal bath, and the local Tolman temperature. We discuss the connection between the results for ultrastatic and static metrics through conformal transformations, and the main features of the thermal corrections to the semiclassical Einstein equations.

2016

Massless Interacting Scalar Fields in de Sitter space

Diana López Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

EPJ Web Conf., Oct 2016

Abs DOI arXiv

We present a method to compute the two-point functions for an O(N) scalar field model in de Sitter spacetime, avoiding the well known infrared problems for massless fields. The method is based on an exact treatment of the Euclidean zero modes and a perturbative one of the nonzero modes, and involves a partial resummation of the leading secular terms. This resummation, crucial to obtain a decay of the correlation functions, is implemented along with a double expansion in an effective coupling constant sqrtlambda and in 1/N. The results reduce to those known in the leading infrared approximation and coincide with the ones obtained directly in Lorentzian de Sitter spacetime in the large N limit. The new method allows for a systematic calculation of higher order corrections both in \(\sqrtλ\,\,\)and in 1/N.
O(N) model in Euclidean de Sitter space: beyond the leading infrared approximation

Diana López Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

JHEP, Sep 2016

Abs DOI arXiv

We consider an O(N) scalar field model with quartic interaction in d-dimensional Euclidean de Sitter space. In order to avoid the problems of the standard perturbative calculations for light and massless fields, we generalize to the O(N) theory a systematic method introduced previously for a single field, which treats the zero modes exactly and the nonzero modes perturbatively. We compute the two-point functions taking into account not only the leading infrared contribution, coming from the self-interaction of the zero modes, but also corrections due to the interaction of the ultraviolet modes. For the model defined in the corresponding Lorentzian de Sitter spacetime, we obtain the two-point functions by analytical continuation. We point out that a partial resummation of the leading secular terms (which necessarily involves nonzero modes) is required to obtain a decay at large distances for massless fields. We implement this resummation along with a systematic double expansion in an effective coupling constant \(\sqrtλ\,\,\) and in 1/N . We explicitly perform the calculation up to the next-to-next-to-leading order in \(\sqrtλ\,\,\) and up to next-to-leading order in 1/N. The results reduce to those known in the leading infrared approximation. We also show that they coincide with the ones obtained directly in Lorentzian de Sitter spacetime in the large N limit, provided the same renormalization scheme is used.

2014

Hartree approximation in curved spacetimes revisited II: The semiclassical Einstein equations and de Sitter self-consistent solutions

Diana L. López Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

Phys. Rev. D, Apr 2014

Abs DOI arXiv

We consider the semiclassical Einstein equations (SEE) in the presence of a quantum scalar field with self-interaction \(λφ^4\). Working in the Hartree truncation of the two-particle irreducible effective action, we compute the vacuum expectation value of the energy-momentum tensor of the scalar field, which acts as a source of the SEE. We obtain the renormalized SEE by implementing a consistent renormalization procedure. We apply our results to find self-consistent de Sitter solutions to the SEE in situations with or without spontaneous breaking of the \(Z_2\)-symmetry.
Hartree approximation in curved spacetimes revisited: The effective potential in de Sitter spacetime

Diana L. Lopez Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

Phys. Rev. D, Jan 2014

Abs DOI arXiv

We consider a quantum scalar field with a \(λφ^4\,\,\)interaction in curved spacetimes. The quantum effects are taken into account nonperturbatively using the Hartree approximation to the two-particle irreducible effective action. Although this approximation has been considered in many previous works, we reconsider it using a consistent nonperturbative renormalization procedure, which we extend to general curved spacetimes. We obtain the renormalized equations for the mean field and for the propagator of the fluctuations, showing explicitly their independence on the arbitrary scale introduced by the regularization scheme. We apply our results to the particular case of de Sitter spacetime and discuss spontaneous symmetry breaking. The results depend strongly on the renormalization procedure.

2012

Lifshitz scalar fields: one loop renormalization in curved backgrounds

Diana L. Lopez Nacir, Francisco D. Mazzitelli, and Leonardo G. Trombetta

Phys. Rev. D, Jan 2012

Abs DOI arXiv

We consider an interacting Lifshitz field with z=3 in a curved spacetime. We analyze the renormalizability of the theory for interactions of the form \(λφ^n\), with arbitrary even n. We compute the running of the coupling constants both in the ultraviolet and infrared regimes. We show that the Lorentz violating terms generate couplings to the spacetime metric that are not invariant under general coordinate transformations. These couplings are not suppressed by the scale of Lorentz violation and therefore survive at low energies. We point out that in these theories, unless the effective mass of the field is many orders of magnitude below the scale of Lorentz violation, the coupling to the four dimensional Ricci scalar \(ξ^{(4)} R φ^2\,\,\)does not receive large quantum corrections \(ξ≫1\).