arXiv subjects

Rudnei O. Ramos

Publications and source records attributed to Rudnei O. Ramos.

At least 55 records · Page 3Linked to original sources

arXiv2023

Axion effects in the stability of hybrid stars

We investigate the effects of including strong charge-parity (CP) violating effects through axion fields in the microscopic equation of state of massive hybrid neutron stars. We assume that their cores contain deconfined quark matter and include the effects of axions via an effective 't Hooft determinant interaction. The hadronic crusts are described using different approaches in order to make our results more general. We find that the presence of axions stabilizes massive hybrid neutron stars against gravitational collapse by weakening the deconfinement phase transition and bringing it to lower densities. This enables to reproduce hybrid neutron stars in agreement with modern astrophysical constraints.

hep-ph↗

arXiv2022

Gravitational Particle Production and the Validity of Effective Descriptions in Loop Quantum Cosmology

The effective approach in Loop Quantum Cosmology (LQC) has provided means to obtain predictions for observable quantities in LQC models. While an effective dynamics in LQC has been extensively considered in different scenarios, a robust demonstration of the validity of effective descriptions for the perturbative level still requires further attention. The consistency of the description adopted in most approaches requires the assumption of a test field approximation, which is limited to the cases in which the backreaction of the particles gravitationally produced can be safely neglected. Within the framework of LQC, some of the main approaches to quantize the linear perturbations are the dressed metric, the hybrid approaches and the closed/deformed algebra approach. Here, we analyze the consistency of the test field assumption in these frameworks by computing the energy density stored in the particles gravitationally produced compared to the background energy density. This analysis ultimately provides us with a consistency test of the effective descriptions of LQC.

gr-qc↗

arXiv2022

Quantum phase transitions in a bidimensional $O(N) \times {\mathbb{Z}_2}$ scalar field model

We analyze the possible quantum phase transition patterns occurring within the $O(N) \times {\mathbb{Z}_2}$ scalar multi-field model at vanishing temperatures in $(1+1)$-dimensions. The physical masses associated with the two coupled scalar sectors are evaluated using the loop approximation up to second order. We observe that in the strong coupling regime, the breaking $O(N) \times {\mathbb{Z}_2} \to O(N)$, which is allowed by the Mermin-Wagner-Hohenberg-Coleman theorem, can take place through a second-order phase transition. In order to satisfy this no-go theorem, the $O(N)$ sector must have a finite mass gap for all coupling values, such that conformality is never attained, in opposition to what happens in the simpler ${\mathbb{Z}_2}$ version. Our evaluations also show that the sign of the interaction between the two different fields alters the transition pattern in a significant way. These results may be relevant to describe the quantum phase transitions taking place in cold linear systems with competing order parameters. At the same time the super-renormalizable model proposed here can turn out to be useful as a prototype to test resummation techniques as well as non-perturbative methods.

hep-th↗

arXiv2021

Tilted Dirac cone effects and chiral symmetry breaking in a planar four-fermion model

We analyze the chiral symmetry breaking in a planar four-fermion model with non-null chemical potential, temperature and including the effect of the tilt of the Dirac cone. The system is modeled with a $(2 + 1)$-dimensional Gross-Neveu-like interaction model in the context of the generalized Weyl Hamiltonian and its phase structure is studied in the mean-field and large-$N$ approximations. Possible applications of the results obtained, e.g., in connection to graphene, are discussed. We also discuss the effect of an external magnetic field applied to the system, which can give rise to the appearance of the anomalous Hall effect and that is expected to arise in connection with two-dimensional Weyl and Dirac semimetals.

cond-mat.mes-hall↗

arXiv2021

Symmetry breaking patterns for two coupled complex scalar fields at finite temperature and in an external magnetic field

A model of two coupled complex scalar fields is studied at finite temperature and under an external magnetic field. The results are obtained in the context of the nonperturbative method of the optimized perturbation theory and contrasted with those obtained in perturbation theory and in the one-loop approximation. The emergence of phenomena related to inverse symmetry breaking and symmetry nonrestoration are analyzed.

hep-ph↗

arXiv2021

Dirac-Born-Infeld warm inflation realization in the strong dissipation regime

We consider warm inflation with a Dirac-Born-Infeld (DBI) kinetic term in which both the nonequilibrium dissipative particle production and the sound speed parameter slow the motion of the inflaton field. We find that a low sound speed parameter removes, or at least strongly suppresses, the growing function appearing in the scalar of curvature power spectrum of warm inflation, which appears due to the temperature dependence in the dissipation coefficient. As a consequence of that, a low sound speed helps to push warm inflation into the strong dissipation regime, which is an attractive regime from a model building and phenomenological perspective. In turn, the strong dissipation regime of warm inflation softens the microscopic theoretical constraints on cold DBI inflation. The present findings, along with the recent results from swampland criteria, give a strong hint that warm inflation may consistently be embedded into string theory.

hep-th↗

arXiv2021

On the graceful exit problem in warm inflation

A seemingly simple question, how does warm inflation exit gracefully?, has a more complex answer than in a cold paradigm. It has been highlighted here that whether warm inflation exits gracefully depends on three independent choices: The form of the potential, the choice of the warm inflation model (i.e., on the form of its dissipative coefficient) and the regime, of weak or strong dissipation, characterizing the warm inflation dynamics. Generic conditions on slow-roll parameters and several constraints on the different model parameters required for warm inflation to exit gracefully are derived.

gr-qc↗

arXiv2021

Towards a reliable effective field theory of inflation

We present the first quantum field theory model of inflation that is renormalizable in the matter sector, with a super-Hubble inflaton mass and sub-Planckian field excursions, which is thus technically natural and consistent with a high-energy completion within a theory of quantum gravity. This is done in the framework of warm inflation, where we show, for the first time, that strong dissipation can fully sustain a slow-roll trajectory with slow-roll parameters larger than unity in a way that is both theoretically and observationally consistent. The inflaton field corresponds to the relative phase between two complex scalar fields that collectively break a U(1) gauge symmetry, and dissipates its energy into scalar degrees of freedom in the warm cosmic heat bath. A discrete interchange symmetry protects the inflaton mass from large thermal corrections. We further show that the dissipation coefficient decreases with temperature in certain parametric regimes, which prevents a large growth of thermal inflaton fluctuations. We find, in particular, a very good agreement with the Planck legacy data for a simple quadratic inflaton potential, predicting a low tensor-to-scalar ratio $r\lesssim 10^{-5}$.

hep-ph↗

arXiv2020

Runaway potentials in warm inflation satisfying the swampland conjectures

The runaway potentials, which do not possess any critical points, are viable potentials which befit the recently proposed de Sitter swampland conjecture very well. In this work, we embed such potentials in the warm inflation scenario motivated by quantum field theory models generating a dissipation coefficient with a dependence cubic in the temperature. It is demonstrated that such models are able to remain in tune with the present observations and they can also satisfy all the three Swampland conjectures, namely the Swampland Distance conjecture, the de Sitter conjecture and the Transplackian Censorship Conjecture, simultaneously. These features make such models viable from the point of view of effective field theory models in quantum gravity and string theory, away from the Swampland.

hep-th↗

arXiv2020

Warm bounce in loop quantum cosmology and the prediction for the duration of inflation

We study and estimate probabilistic predictions for the duration of the preinflationary and slow-roll phases after the bounce in loop quantum cosmology, determining how the presence of radiation in the prebounce phase affects these results. We present our analysis for different classes of inflationary potentials that include the monomial power-law chaotic type of potentials, namely, for the quadratic, quartic and sextic potentials and also for a Higgs-like symmetry breaking potential, considering different values for the vacuum expectation value in the latter case. We obtain the probability density function for the number of inflationary e-folds and for other relevant quantities for each model and produce probabilistic results drawn from these distributions. This study allows us to discuss under which conditions each model could eventually lead to observable signatures on the spectrum of the cosmic microwave background, or, else, be also excluded for not predicting a suffcient amount of accelerated expansion. The effect of radiation on the predictions for each model is explicitly quantified. The obtained results indicate that the number of inflationary e-folds in loop quantum cosmology is not a priori an arbitrary number, but can in principle be a predictable quantity, even though the results are dependent on the model and on the amount of radiation in the Universe prior to the start of the inflationary regime.

gr-qc↗

arXiv2020

Gravitational Particle Production in Loop Quantum Cosmology

We investigate the gravitational particle production in the bounce phase of Loop Quantum Cosmology (LQC). We perform both analytical and numerical analysis of the particle production process in a LQC scenario with Bunch-Davies vacuum initial condition in the contracting phase. We obtain that if we extend the validity of the dressed metric approach beyond the limit of small backreaction in which it is well justified, this process would lead to a radiation dominated phase in the pre-inflationary phase of LQC. Our results indicate that the test field approximation, which is required in the truncation scheme used in the dressed metric approach, might not be a valid assumption in a LQC scenario with such initial conditions.

gr-qc↗

arXiv2020

Projected Proca Field Theory: a One-Loop Study

The recent discovery of two-dimensional Dirac materials, such as graphene and transition-metaldichalcogenides, has raised questions about the treatment of hybrid systems, in which electrons moving in a two-dimensional plane interact via virtual photons from the three-dimensional space. In this case, a projected non-local theory, known as Pseudo-QED, or reduced QED, has shown to provide a correct framework for describing the interactions displayed by these systems. In a related situation, in planar materials exhibiting a superconducting phase, the electromagnetic field has a typical exponential decay that is interpreted as the photons having an effective mass, as a consequence of the Anderson-Higgs mechanism. Here, we use an analogous projection to that used to obtain the pseudo-QED to derive a Pseudo-Proca equivalent model. In terms of this model, we unveil the main effects of attributing a mass to the photons and to the quasi-relativistic electrons. The one-loop radiative corrections to the electron mass, to the photon and to the electron-photon vertex are computed. We calculate the quantum corrections to the electron g-factor and show that it smoothly goes to zero in the limit when the photon mass is much larger than the electron mass. In addition, we correct the results obtained for graphene within Pseudo-QED in the limit when the photon mass vanishes.

hep-th↗

arXiv2020

Strengthening the de Sitter swampland conjecture in warm inflation

The "de Sitter constraint" on the space of effective scalar field theories consistent with superstring theory provides a lower bound on the slope of the potential of a scalar field which dominates the evolution of the Universe, e.g., a hypothetical inflaton field. Whereas models of single scalar field inflation with a canonically normalized field do not obey this constraint, it has been claimed recently in the literature that models of warm inflation can be made compatible with it in the case of large dissipation. The de Sitter constraint is known to be derived from entropy considerations. Since warm inflation necessary involves entropy production, it becomes necessary to determine how this entropy production will affect the constraints imposed by the swampland conditions. Here, we generalize these entropy considerations to the case of warm inflation and show that the condition on the slope of the potential remains essentially unchanged and is, hence, robust even in the warm inflation dynamics. We are then able to conclude that models of warm inflation indeed can be made consistent with the "swampland" criteria.

hep-th↗

arXiv2020

Warm brane inflation with an exponential potential: A consistent realization away from the swampland

It has very recently been realized that coupling branes to higher dimensional quantum gravity theories and considering the consistency of what lives on the branes, one is able to understand whether such theories can belong either to the swampland or to the landscape. In this regard, in the present work, we study a warm inflation model embedded in the Randall-Sundrum braneworld scenario. It is explicitly shown that this model belongs to the landscape by supporting a strong dissipative regime with an inflaton steep exponential potential. The presence of extra dimension effects from the braneworld allow achieving this strong dissipative regime, which is shown to be both theoretically and observationally consistent. In fact, such strong dissipation effects, which decrease towards the end of inflation, together with the extra dimension effect, allow the present realization to simultaneously satisfy all previous restrictions imposed on such a model and to evade the recently proposed swampland conjectures. The present implementation of this model, in terms of an exponential potential for the scalar field, makes it also a possible candidate for describing the late-time Universe in the context of a dissipative quintessential inflation model and we discuss this possibility in the Conclusions.

gr-qc↗

arXiv2019

Unified early and late Universe cosmology through dissipative effects in steep quintessential inflation potential models

By making use of a class of steep exponential type of potentials, which has been recently used to describe quintessential inflation, we show how a unified picture for both inflation, dark energy and dark matter can emerge entirely through dissipative effects. Dissipation provides a way for extending the applicability of a larger class of these potentials in the sense of leading to a consistent early Universe inflationary picture and producing observables in agreement with the Planck legacy data. Likewise, dissipative effects lead to dark matter production with consistent abundances and, towards the recent time of the Universe, drives the potential energy of the scalar quintessential field to dominate again, essentially mimicking a cosmological constant by today, with all cosmological parameters consistent with the observations. Both early and late Universes are connected and with no kination period in between.

astro-ph.CO↗

arXiv2019

Quantum chromodynamics axion in a hot and magnetized medium

It is analyzed the effects of a hot and magnetized medium on the axion mass, self-coupling and topological susceptibility when in presence of an anisotropic external magnetic field along the $z$-direction, within the Nambu--Jona-Lasinio effective model for quantum chromodynamics. The effects of both Magnetic Catalysis and Inverse Magnetic Catalysis are explicitly taken into account through appropriate matching of parameters with those from lattice Monte-Carlo numerical simulations. It is also analyzed the dependence of the results with respect to different model parametrizations in the context of the Nambu--Jona-Lasinio model.

hep-ph↗

arXiv2019

Observational Constraints on Warm Inflation in Loop Quantum Cosmology

By incorporating quantum aspects of gravity, Loop Quantum Cosmology (LQC) provides a self-consistent extension of the inflationary scenario, allowing for modifications in the primordial inflationary power spectrum with respect to the standard General Relativity one. We investigate such modifications and explore the constraints imposed by the Cosmic Microwave Background (CMB) Planck Collaboration data on the Warm Inflation (WI) scenario in the LQC context. We obtain useful relations between the dissipative parameter of WI and the bounce scale parameter of LQC. We also find that the number of required e-folds of expansion from the bounce instant till the moment the observable scales crossed the Hubble radius during inflation can be smaller in WI than in CI. In particular, we find that this depends on how large is the dissipation in WI, with the amount of required e-folds decreasing with the increasing of the dissipation value. Furthermore, by performing a Monte Carlo Markov Chain analysis for the considered WI models, we find good agreement of the model with the data. This shows that the WI models studied here can explain the current observations also in the context of LQC.

astro-ph.CO↗

arXiv2019

Formulating the Kramers problem in field theory

The escape problem is defined in the context of quantum field theory. The escape rate is explicitly derived for a scalar field governed by fluctuation-dissipation dynamics, through generalizing the standard Kramers problem. In the presence of thermal fluctuations, there is a nonvanishing probability for a classical background field, initially located at a minimum of its potential in a homogeneous configuration, to escape from the well. The simple and well-known related problem of the escape of a classical point particle due to random forces is first reviewed. We then discuss the difficulties associated with a well-defined formulation of an escape rate for a scalar field and how these can be overcome. A definition of the Kramers problem for a scalar field and a method to obtain the rate are provided. Finally, we discuss some of the potential applications of our results, which can range from condensed matter systems, i.e., nonrelativistic fields, to applications in high-energy physics, like for cosmological phase transitions.

hep-ph↗