arXiv subjects

Pau Beltrán-Palau

Publications and source records attributed to Pau Beltrán-Palau.

10 recordsLinked to original sources

arXiv2026

AGN hosting jets. II: Deriving fundamental information from semi-analytical models and the LoTSS sample

Galactic activity has been shown to play a fundamental role in galaxy and cluster evolution throughout the history of the Universe. In the last decades, a number of works have tackled the determination of radio luminosity functions and their evolution, with the aim to understand and quantify this role. In this work we propose a different approach, based on the application of a semi-analytical model to produce mock populations of radio loud active galaxies throughout several orders of magnitude in power, and the comparison with observational samples. The aim of this work is to test the ability of our semi-analytical model to reproduce luminosity and size distributions of large numbers of sources from the radio-loud active galactic nuclei sample from the LoTSS survey. We apply our semi-analytical model for the evolution of radio galaxies and generate hundreds of thousands of sources applying random selection of parameters within probability distributions for each of them. Our results show that the model is able to reproduce the main features of the observed sample, and identifies jet mass-load included in our model and activity periods as critical parameters that should be addressed in more detail in future work. These initial results allow us to establish a maximum activity period ranging from 10 to 200 Myr within an active time distribution probability p(t_max) \propto 1/t_{max}. We also reproduce the radio luminosity function for the z<0.6 population, and the jet power spectrum from 10^{34.5} W to 10^{38} W. Our results show that mass-load is a fundamental parameter in the determination of radio source luminosities with distance, mainly in the low-power regime. Our model predicts that at least 3-4 x 10^{5} radio galaxies must exist in the LoTSS field in order to obtain the 17045 observed ones. This population is vastly dominated by low power sources.

astro-ph.CO↗

arXiv2025

AGN hosting jets. I: A semi-analytical model for the evolution of radio galaxies

Three-dimensional simulations of relativistic jets are a useful tool to understand the evolution of jets and radio galaxies in detail. However, computationally demanding as they are, their use is limited to a relatively small number of representative cases. When comparing to the distribution of large samples of objects in the luminosity-distance plane (P-D plane), the most efficient approach is to use analytical or semi-analytical models that reproduce the evolution of the main parameters governing the dynamics and radio luminosity of the sources. Our aim is to build a semi-analytical model that allows us to produce mock samples of radio galaxies to be compared with real populations and use this approach to constrain the general properties of active galaxies with jets in a cosmological context. In this work, we present a new model for the evolution of radio galaxies based on the resolution of ordinary differential equations and inspired both by previous experience on numerical simulations of jets across several orders of magnitude in power, and by observational evidence. Our results show remarkable agreement between the results given by the semi-analytical model and those obtained by both 2D and 3D relativistic hydrodynamics simulations of jets ranging from 1.e35 W to 1.e39 W. From the derived trajectories of powerful radio galaxies through the P-D diagram (powers greater than 1.e36 W), our model agrees with typical lifetimes of galactic activity of < 500 Myr. We also compare our results with previous models in the literature. In a follow-up paper, we use this model to generate mock populations of radio galaxies at low redshifts and compare them to the LoTSS sample.

astro-ph.CO↗

arXiv2023

Hadamard and boundary conditions for the Big Bang quantum vacuum

General relativity predicts final-type singularities inside black holes, as well as a cosmological initial-type singularity. Cosmic censorship protects external observers from black hole singularities, while Penrose's Weyl curvature hypothesis protects the smoothness of the initial (Big Bang) singularity. We discuss a simple realization of the Weyl curvature hypothesis by assuming a very early radiation-dominated universe and analytically extending the expansion factor to negative values of conformal time. We impose time-reversal conditions at the Big Bang to characterize a natural set of preferred vacuum states for quantized matter fields. We implement the prescription of States of Low Energy constructed around the Big Bang obtaining Hadamard states. We also explore the physical implications of these vacua for cosmological dark matter production.

gr-qc↗

arXiv2023

Quantum corrections to the Schwarzschild metric from vacuum polarization

We explore static and spherically symmetric solutions of the 4-dimensional semiclassical Einstein's equations using the quantum vacuum polarization of a conformal field as a source. These solutions may be of interest for \black{the study of} exotic, compact objects (ECOs). The full backreaction problem is addressed by solving the semiclassical Tolman-Oppenheimer-Volkoff (TOV) equations making use of effective equations of state inspired \black{by} the trace anomaly and an extra simplifying and reasonable assumption. We combine analytical and numerical techniques to solve the resulting differential equations, both perturbatively and nonperturbatively in $\hbar$. In all cases the solution is similar to the Schwarzschild metric up to the vicinity of the classical horizon $r=2M$. However, at $r=2M + \varepsilon$, with $\varepsilon\sim O(\sqrt{\hbar})$, we find a coordinate singularity. In \black{the} case of matching with a static star, this leads to an upper bound in the compactness, and sets a constraint on the family of stable ECOs. We also study the corrections that the quantum-vacuum polarization induces on the propagation of waves, and discuss the implications. For the pure vacuum case, we can further extend the solution by using appropriate coordinates until we reach another singular point, where this time a null curvature singularity arises and prevents extending beyond. This picture qualitatively agrees with the results obtained in the effective two-dimensional approach, and reinforces the latter as a reasonable method.

gr-qc↗

arXiv2022

Renormalization and a non-adiabatic vacuum choice in a radiation-dominated universe

Vacuum and particles can be naturally defined in the adiabatic regime of an expanding universe. In general, however, there is no preferred choice of a vacuum state, unless the spacetime background possesses special symmetries. In the absence of symmetries the standard viewpoint is to construct distinguished adiabatic states permitting renormalizability of the stress-energy tensor. We study a special non-adiabatic vacuum for a massive scalar field in a radiation-dominated universe defined by imposing early-times conformal symmetry. We show that this state is consistent with renormalization, despite its ultraviolet behaviour is not the conventional one due to the emergence of oscillatory terms.

gr-qc↗

arXiv2021

Note on the pragmatic mode-sum regularization method: translational-splitting in a cosmological background

The point-splitting renormalization method offers a prescription to calculate finite expectation values of quadratic operators constructed from quantum fields in a general curved spacetime. It has been recently shown by Levi and Ori that when the background metric possesses an isometry, like stationary or spherically symmetric black holes, the method can be upgraded into a pragmatic procedure of renormalization that produces efficient numerical calculations. In this note we show that when the background enjoys three-dimensional spatial symmetries, like homogeneous expanding universes, the above pragmatic regularization technique reduces to the well established adiabatic regularization method.

gr-qc↗

arXiv2020

Adiabatic regularization for Dirac fields in time-varying electric backgrounds

The adiabatic regularization method was originally proposed by Parker and Fulling to renormalize the energy-momentum tensor of scalar fields in expanding universes. It can be extended to renormalize the electric current induced by quantized scalar fields in a time-varying electric background. This can be done in a way consistent with gravity if the vector potential is considered as a variable of adiabatic order one. Assuming this, we further extend the method to deal with Dirac fields in four spacetime dimensions. This requires a self-consistent ansatz for the adiabatic expansion, in presence of a prescribed time-dependent electric field, which is different from the conventional expansion used for scalar fields. Our proposal is consistent, in the massless limit, with the conformal anomaly. We also provide evidence that our proposed adiabatic expansion for the fermionic modes parallels the Schwinger-DeWitt adiabatic expansion of the two-point function. We give the renormalized expression of the electric current and analyze, using numerical and analytical tools, the pair production induced by a Sauter-type electric pulse. We also analyze the scaling properties of the current for a large field strength.

hep-th↗

arXiv2019

Breaking of adiabatic invariance in the creation of particles by electromagnetic fields

Particles are spontaneously created from the vacuum by time-varying gravitational or electromagnetic backgrounds. It has been proven that the particle number operator in an expanding universe is an adiabatic invariant. In this paper we show that, in some special cases, the expected adiabatic invariance of the particle number fails in presence of electromagnetic backgrounds. In order to do this, we consider as a prototype a Sauter-type electric pulse. Furthermore, we also show a close relation between the breaking of the adiabatic invariance and the emergence of the axial anomaly.

hep-th↗

arXiv2019

The hyperbolic Einstein-Rosen bridge

Using systematically isothermal coordinates we show that there exist three different maximal extensions of the original Einstein-Rosen bridge. One of them, the hyperbolic Einstein-Rosen bridge, has two-dimensional sections diffeomorphic to the covering space of an hyperboloid of revolution, a singularity satisfying the cosmic censorship and a bridge generated by light-like geodesics that can be traversed by time-like curves. The collapse process that might produce this object is an interesting open problem.

gr-qc↗

arXiv2019

Translational anomaly of chiral fermions in two dimensions

It is well known that a quantized two-dimensional Weyl fermion coupled to gravity spoils general covariance and breaks the covariant conservation of the energy-momentum tensor. In this brief article, we point out that the quantum conservation of the momentum can also fail in flat spacetime, provided the Weyl fermion is coupled to a time-varying homogeneous electric field. This signals a quantum anomaly of the space-translation symmetry, which has not been highlighted in the literature so far.

hep-th↗