Minimal coupling in presence of non-metricity and torsion

We deal with the question of what it means to define a minimal coupling prescription in presence of torsion and/or non-metricity, carefully explaining while the naive substitution \partial\to\na introduces extra couplings between the matter fields and the connection that can be regarded as non-minimal in presence of torsion and/or non-metricity. We will also investigate whether minimal coupling prescriptions at the level of the action (MCPL) or at the level of field equations (MCPF) lead to different dynamics. To that end, we will first write the Euler-Lagrange equations for matter fields in terms of the covariant derivatives of a general non-Riemannian space, and derivate the form of the associated Noether currents and charges. Then we will see that if the minimal coupling prescriptions is applied as we discuss, for spin 0 and 1 fields the results of MCPL and MCPF are equivalent, while for spin 1/2 fields there is a difference if one applies the MCPF or the MCPL, since the former leads to charge violation.Comment: 18 pages, 1 figure, matching the content in the published version in EPJ

A diffeomorphism invariant family of metric-affine actions for loop cosmologies

In loop quantum cosmology (LQC) the big bang singularity is generically resolved by a big bounce. This feature holds even when modified quantization prescriptions of the Hamiltonian constraint are used such as in mLQC-I and mLQC-II. While the later describes an effective description qualitatively similar to that of standard LQC, the former describes an asymmetric evolution with an emergent Planckian de-Sitter pre-bounce phase even in the absence of a potential. We consider the potential relation of these canonically quantized non-singular models with effective actions based on a geometric description. We find a 3-parameter family of metric-affine $f(\mathcal{R})$ theories which accurately approximate the effective dynamics of LQC and mLQC-II in all regimes and mLQC-I in the post-bounce phase. Two of the parameters are fixed by enforcing equivalence at the bounce, and the background evolution of the relevant observables can be fitted with only one free parameter. It is seen that the non-perturbative effects of these loop cosmologies are universally encoded by a logarithmic correction that only depends on the bounce curvature of the model. In addition, we find that the best fit value of the free parameter can be very approximately written in terms of fundamental parameters of the underlying quantum description for the three models. The values of the best fits can be written in terms of the bounce density in a simple manner, and the values for each model are related to one another by a proportionality relation involving only the Barbero-Immirzi parameter.Comment: 19 pages, 4 figures and 3 table

Comment on "Einstein-Gauss-Bonnet Gravity in Four-Dimensional Spacetime"

We argue that several statements in Phys. Rev. Lett. 124, 081301 (2020) are not correct.Comment: 2 pages, accepted as a Comment in Physical Review Letter

Inconsistencies in four-dimensional Einstein-Gauss-Bonnet gravity

We attempt to clarify several aspects concerning the recently presented four-dimensional Einstein-Gauss-Bonnet gravity. We argue that the limiting procedure outlined in [Phys. Rev. Lett. 124, 081301 (2020)] generally involves ill-defined terms in the four dimensional field equations. Potential ways to circumvent this issue are discussed, alongside some remarks regarding specific solutions of the theory. We prove that, although linear perturbations are well behaved around maximally symmetric backgrounds, the equations for second-order perturbations are ill-defined even around a Minkowskian background. Additionally, we perform a detailed analysis of the spherically symmetric solutions, and find that the central curvature singularity can be reached within a finite proper time.Comment: 8 pages, 2 figures, and a supplementary Mathematica notebook; version accepted for publication in Chinese Physics

Theoretical and Observational Aspects in Metric-Affine Gravity

En esta tesis se tratan varios aspectos teóricos y fenomenológicos de las teorías de gravedad métrico-afin. En la introducción se presentan las herramientas necesarias para comprender el marco y entender algunas sutilezas relacionadas con la prescripción de acoplamiento mínimo entre geometría y materia en presencia de torsión y nometricidad. La parte central de la tesis está dedicada a estudiar la estructura de teorías de la gravedad basada en el tensor de Ricci (RBG), que serán de uso posterior para comprender las propiedades genéricas de las teorías métrico afines. Empezamos analizando la estructura de las ecuaciones de campo RBG y aspectos no triviales de su espacio de soluciones. Luego analizamos los espectros de abrosción de algunas soluciones esféricamente simétricas. Finalmente mostramos que, si la simetría proyectiva en estas teorías se rompe explícitamente, entonces surgen grados de libertad inestables de tipo fantasma, argumentando que esta será una característica genérica de las teorías de la gravedad métrica-afín. Concluimos estra parte analizando teorías métrico-afines desde la perspectiva de teorías de campo efectivas (EFT), mostrando cómo la no metricidad toma una forma particular en las teorías genéricas donde el tensor Ricci simétrizado aparece en la acción más allá del término de Einstein-Hilbert. Esto genera interacciones efectivas observables, que usamos para imponer restricciones estrictas a estas teorías. En la tercera parte de la tesis presentamos una miscelanea de trabajos que no están tan relacionados con la estructura de las teorías RBG. Primero nosotros encontrar una familia de teorías f (R) métrico-afines que imitan la dinámica de los modelos de cosmología cuántica de bucles a nivel de background. Luego estudiamos un modelo para la ruptura espontánea de la simetría de Lorentz en el enfoque métrico-afín. En el siguiente capítulo generalizamos una definición invariante conforme de tiempo propio dada por Perlick al caso con no metricidad general. Finalmente, presentamos argumentos que muestran que la teoría D->4 EGB propuesta recientemente no está bien definida en su forma original. Terminamos con unas breves conclusiones.In this thesis we deal with several theoretical and phenomenological apsects of metric-affine theories of gravity. Concretely, we first give a broad introduction to the necessary tools to understand the framework and elaborate on some subtleties of the minimal coupling prescription between geometry and matter in presence of torsion and nonmetricity. Then we dedicate the central part of the thesis to study the structure of Ricci Based gravity (RBG) theories, which will be of later use to understand generic properties of metric-affine theories. We begin by analysing the structure of the RBG field equations and nontrivial aspects of their solution space. We then analyse the abrosption spectra of some spherically symmetric solutions. Then, we show that, if the projective symmetry in these theories is explicitly broken, then there arise ghost degrees of freedom, and we argue that this will be a generic feature of metric-affine gravity theories. Having done this, we analyse metricafine theories through the EFT lens, showing how the nonmetricity tkes a particular form in generic theories where the symmetrised Ricci tensor appears in the action beyond the Einstein-Hilbert term. This sources effective interactions that we use to place tight constraints to these theories. In the third part of the thesis we present a miscelanea of works which are not so related to the structure of RBG theories. First we find a family of metric-affine f(R) theories that mimicks the dynamics of Loop Cosmology models at the background level. Then we study a model for spontaneous breaking of Lorentz symmetry, namely the bumblebee model, in the metric-affine approach. In the following chapter we generalise a conformal invariant definition of proper time given by Perlick to the case with general nonmetricity. Finally, we present arguments that show that the recently proposed D$EGB theory is not well defined in its original form. We finish with a brief outlook

Born-Infeld Gravity: Constraints from Light-by-Light Scattering and an Effective Field Theory Perspective

By using a novel technique that establishes a correspondence between general relativity and metric-affine theories based on the Ricci tensor, we are able to set stringent constraints on the free parameter of Born-Infeld gravity from the ones recently obtained for Born-Infeld electrodynamics by using light-by-light scattering data from ATLAS. We also discuss how these gravity theories plus matter fit within an effective field theory framework.Comment: 7 page

Entanglement from superradiance and rotating quantum fluids of light

The amplification of radiation by superradiance is a universal phenomenon observed in numerous physical systems. We demonstrate that superradiant scattering generates entanglement for different input states, including coherent states, thereby revealing the inherently quantum nature of this phenomenon. To put these concepts to the test, we propose a novel approach to create horizonless ergoregions, which are nonetheless dynamically stable thanks to the dissipative dynamics of a polaritonic fluid of light. We numerically simulate the system to demonstrate the creation of a stable ergoregion, and experimentally realize a comparable configuration. Subsequently, we investigate rotational superradiance within this system, with a primary focus on entanglement generation and the possibilities for its enhancement using current techniques. Our methods permit the investigation of quantum emission by rotational superradiance by controlling the input state at will.Comment: 13 pages with 10 figures + 9 pages (references + appendices with an extra figure and a table with numerical data

Observable traces of non-metricity: New constraints on metric-affine gravity

Relaxing the Riemannian condition to incorporate geometric quantities such as torsion and non-metricity may allow to explore new physics associated with defects in a hypothetical space–time microstructure. Here we show that non-metricity produces observable effects in quantum fields in the form of 4-fermion contact interactions, thereby allowing us to constrain the scale of non-metricity to be greater than 1 TeV by using results on Bahbah scattering. Our analysis is carried out in the framework of a wide class of theories of gravity in the metric-affine approach. The bound obtained represents an improvement of several orders of magnitude to previous experimental constraints