Using the in-out formalism, we derive the exact one-loop QED effective actions for spinor field in a uniform electric field in two-dimensional global (anti-)de Sitter (A)dS_2 spacetime. The one-loop effective action probed by a scalar or spinor field is determined by the scattering matrix relating the out-vacuum to the in-vacuum, which is in turn fixed by the Bogoliubov coefficients of the corresponding Klein-Gordon or Dirac equation in the presence of both a gauge field and curved spacetime. Remarkably, the vacuum persistence amplitude -- twice the imaginary part of the one-loop effective action -- is related, via the Bogoliubov relations, to the mean number of particle-antiparticle pairs spontaneously produced by the background fields. The Bogoliubov coefficients or mean number of pair-production for charged scalar and spinor fields in global (A)dS_2 lead to QED effective actions expressed in terms of both proper-time integrals and Hurwitz zeta functions. These effective actions reveal a strong interplay between the electric field and spacetime curvature and correctly reproduce the limiting cases of pure (A)dS_2 spacetime and a uniform electric field in Minkowski space. In this talk, we also further discuss the physical implications of the resulting QED effective actions in (A)dS_2.

Based on: https://arxiv.org/abs/2603.05826

Speaker webpage: https://sites.google.com/view/cristian-andres-rivera-medina/info

This seminar will move from conventional cosmological challenges to a systematic understanding of inflationary reconstruction in terms of the number of e-folds (N). After reviewing the limitations of the standard hot Big Bang scenario and the role of inflation in addressing these problems, we introduce the reconstruction mechanism at the level of background dynamics, initially within the framework of General Relativity and subsequently extending the discussion to Rastall gravity.

Within a general formalism, we reconstruct inflationary models by expressing the scalar spectral index n_s as a function of N, along with a parameterization of the Rastall sector. This methodology facilitates the derivation of the effective inflationary potential directly in terms of cosmological observables and the underlying gravitational dynamics.

As a representative example, we examine the behavior of the attractor n_s(N) - 1 \propto 1/N, which encompasses a broad class of phenomenologically viable inflationary models. Using these data, we reconstruct the corresponding inflationary dynamics and derive the scalar potential V(φ) within Rastall gravity. The resulting framework is analyzed in comparison with the limit of General Relativity.

The presentation is aimed at a broad scientific audience, highlighting a clear conceptual progression from fundamental cosmological problems to reconstruction techniques, while maintaining a rigorous and self-contained treatment of the formalism.

CV:

Professor Carlos Ríos Morales holds a B.Sc. in Physics from the Universidad de Concepción (Chile), an M.Sc. in Physical Sciences from the Universidad del Bío-Bío (Chile), and a Ph.D. in Physical Sciences jointly awarded by the Pontificia Universidad Católica de Valparaíso and the Universidad Técnica Federico Santa María  (Chile). His research focuses on early-universe cosmology, including pre-inflationary scenarios in emergent universe models, anomalies in the cosmic microwave background, and the theoretical reconstruction of the inflationary phase and reheating, often within frameworks beyond the standard cosmological model. He has also worked on interactions within the dark sector, particularly between dark energy and dark matter.

In addition to his research, he has contributed to the development of teaching methodologies in university-level physics. He currently serves as Director of the Department of Basic Sciences and Head of Research within the same department at the Universidad Católica del Norte (Chile). He is also a member of the university’s Research Advisory Board.

Las transiciones de fase cosmológicas de primer orden pudieron haber desempeñado un papel central en la formación del contenido de partículas del universo temprano. Cuando burbujas de una nueva fase del vacío se nucleann, expanden y colisionan, la energía liberada puede ser tan alta que consiga generar ondas gravitacionales y, bajo condiciones adecuadas, producir nuevas partículas. En particular, paredes de burbuja altamente energéticas podrían proporcionar un mecanismo no térmico para generar materia oscura.

En esta charla presentaré una visión introductoria de este escenario, enfatizando cómo la producción de partículas, la abundancia de materia oscura y las señales estocásticas de ondas gravitacionales pueden surgir de un mismo evento cosmológico. El objetivo es mostrar cómo futuras observaciones de ondas gravitacionales podrían abrir una ventana indirecta hacia el origen de la materia oscura.

We investigate the relativistic structure of compact stars within the modified gravity framework $f(R,T,L_m)=R+\alpha T L_m$, which introduces a non-minimal coupling between matter and curvature. The modified Tolman-Oppenheimer-Volkoff equations are obtained and solved for neutron stars, quark stars and white dwarfs using realistic equations of state, considering two standard choices for the matter Lagrangian density, $L_m=p$ and $L_m=-\rho$. We show that the coupling parameter $\alpha$ significantly affects the mass-radius relations of such stellar systems. In particular, for white dwarfs, the gravity model allows stable super-Chandrasekhar configurations at high central densities, while for neutron stars it leads to noticeable deviations depending on the choice of $L_m$. The general relativity limit is recovered for $\alpha \to 0$. Furthermore, we confront our models with observational data, including neutron star measurements and white dwarf constraints, to place bounds on $\alpha$. These findings demonstrate that $f(R,T,L_m)$ gravity provides a viable and testable framework for describing the internal structure of compact stars beyond general relativity. 

Based on: https://arxiv.org/abs/2402.13360 , https://arxiv.org/abs/2507.18745 

CV:

 Juan Z. Pretel obtained his PhD from the Federal University of Rio de Janeiro (UFRJ) and has experience in gravitation and astrophysics, working mainly on the following topics: General relativity, modified gravity theories, dark energy models, radial oscillations and stellar stability, anisotropy and electric charge in compact stars, gravitational collapse, relativistic hydrodynamics and thermodynamics of non-perfect fluids. He primarily studies the microphysical and global properties of compact stars under specific and well-motivated modifications of standard Einstein gravity. He also compares his theoretical findings with gravitational wave detections and electromagnetic observations in order to provide a testing ground for modified gravity theories in astrophysical settings. He is currently a postdoctoral researcher at the Brazilian Center for Physics Research (CBPF).

Speaker webpage: https://www.webofscience.com/wos/author/record/HZI-2238-2023  https://orcid.org/0000-0003-0883-3851 

La integrabilidad en teoría de campos y cuerdas es la propiedad que nos dice que el sistema o configuración puede ser resuelto. En esta charla voy a explicar una técnica para generar backgrounds integrables a partir de deformaciones de  backgrounds conocidos. Esto permite, mediante la correspondencia AdS/CFT, generar teorías duales que sean integrables. La familia de deformaciones abarca inclusive puntos de partida no integrables.