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Rudnei O. Ramos

Publications and source records attributed to Rudnei O. Ramos.

At least 37 records · Page 2Linked to original sources

$α$-attractor potentials in loop quantum cosmology

We perform in this work an analysis of the background dynamics for $α$-attractor models in the context of loop quantum cosmology. Particular attention is given to the determination of the duration of the inflationary phase that is preceded by the quantum bounce in these models. From an analysis of the general predictions for these models, it is shown that we can be able to put constraints in the parameter $α$ of the potentials and also on the quantum model itself, especially the Barbero-Immirzi parameter. In particular, the constraints on the tensor-to-scalar ratio and spectral tilt of the cosmological perturbations limit the $α$ parameter of the potentials to values such that $α_{n=0} \lesssim 10$, $α_{n=1} \lesssim 17$ and $α_{n=2} \lesssim 67$, for the $α$ attractors T, E, and $n=2$ models, respectively. Using the constraints on the minimal amount of e-folds of expansion from the quantum bounce up to the end of inflation leads to the upper bounds for the Barbero-Immirzi parameter for the $α$-attractor models studied in this work: $γ_{n=0} \lesssim 51.2$, $γ_{n=1}\lesssim 63.4$ and $γ_{n=2} \lesssim 64.2$, which are obtained when fixing the parameter $α$ in the potential at the values saturating the upper bounds given above for each model.

gr-qc↗

Viability of warm inflation with standard model interactions

The minimal warm inflation scenario proposed in Ref. [1] -- featuring an axionlike inflaton coupled to Standard Model (SM) gluons via the standard interaction $ϕG \tilde G$ -- offers a compelling bridge between inflationary dynamics and SM particle content. While the model retains only the inflaton as a beyond-SM field, its original analysis relied on some approximate treatments of warm inflation's (WI) dynamics. Here, we revisit this scenario using WI2easy, a precision computational tool for WI dynamics [2], to rigorously evaluate the model's viability and full range of model's parameters compatible with the observational parameters. Overall, we find that the results of Ref. [1] hold, but with significant differences in the weak and strong dissipative regimes of WI.

hep-ph↗

Quantum geometric formulation of Brans-Dicke theory for Bianchi I spacetime

This paper investigates Bianchi I spacetimes within the Jordan frame of Brans-Dicke theory, incorporating the framework of effective loop quantum gravity. After developing general formulas, we analyze the robustness of classical singularity resolution due to quantum geometric effects using two common quantization schemes. We then compare the resulting physical properties. We find that both schemes replace classical singularities with regular quantum bounces. Notably, in contrast to similar studies based on general relativity, we find that all three directional scale factors of the Bianchi I spacetimes increase and after the quantum bounce they reach values similar to their initial values, leading to a merging with classical spacetimes in both schemes.

gr-qc↗

Thermal effects on warm chromoinflation

We explore a model of a pseudo-Nambu-Goldstone boson inflaton field coupled to a non-Abelian $SU(2)$ gauge field. This model naturally leads to a warm inflation scenario, where the inflationary dynamics is dominated by thermal dissipation. In this work, we consider a scenario where the inflaton, an axion-like field, is coupled to the $SU(2)$ gauge field, similar to chromoinflation models. Both the inflaton and the gauge field with a non-vanishing vacuum expectation value are coupled to a thermal radiation bath. We demonstrate that the presence of the thermal bath during warm chromoinflation induces a thermal plasma mass for the background gauge field. This thermal mass can significantly disrupt the dynamics of the background gauge field, thereby driving it to its trivial null solution.

hep-ph↗

Charged scalars at finite electric field and temperature in the optimized perturbation theory

We study the symmetry breaking and restoration behavior of a self-interacting charged scalar field theory under the influence of a constant electric field and finite temperature. Our study is performed in the context of the optimized perturbation theory. The dependence of the effective potential with constant electric fields is established by means of the bosonic propagators in the Schwinger proper-time method. Explicit analytical expressions for the electric and thermal contributions are found. Our results show a very weak decreasing behavior of the vacuum expectation value as a function of the electric field, which is strengthened by the temperature effect. A first-order phase transition that occurs at zero/weak electric fields changes to a second-order phase transition under strong electric fields. The critical temperature for the phase transition exhibited a very weak dependence on the electric field. Additionally, we computed the vacuum persistence probability rate for the interacting theory, finding a peak at the critical point. The maximum value of this rate at the critical point is found to be independent of the coupling constant but depended solely on the magnitude of the electric field.

hep-ph↗

Deconfinement and chiral phase transitions in quark matter with chiral imbalance

We study the thermodynamics of the Polyakov-Nambu-Jona-Lasinio model considering the effects of an effective chiral chemical potential. We offer a new parametrization of the Polyakov-loop potential depending on temperature and the chiral chemical potential which, when used together with a proper regularization scheme of vacuum contributions, predicts results consistent with those from lattice simulations.

hep-ph↗

Testing the equivalence between the planar Gross-Neveu and Thirring models at $N=1$

It is known that the Fierz identities predict that the Gross-Neveu and Thirring models should be equivalent when describing systems composed of a single fermionic flavor, $N=1$. Here, we consider the planar version of both models within the framework of the optimized perturbation theory at the two-loop level, in order to verify if the predicted equivalence emerges explicitly when different temperature and density regimes are considered. At vanishing densities, our results indicate that both models indeed describe exactly the same thermodynamics, provided that $N=1$. However, at finite chemical potentials we find that the $N=1$ Fierz equivalence no longer holds. After examining the relevant free energies, we have identified the contributions which lead to this puzzling discrepancy. Finally, we discuss different frameworks in which this (so far open) problem could be further understood and eventually circumvented.

hep-ph↗

Stability analysis of warm quintessential dark energy model

A dynamical system analysis is performed for a model of dissipative quintessential inflation realizing warm inflation at early primordial times and dissipative interations in the dark sector at late times. The construction makes use of a generalized exponential potential realizing both phases of accelerated expansion. A focus is given on the behavior of the dynamical system at late times and the analysis is exemplified by both analytical and numerical results. The results obtained demonstrate the viability of the model as a quintessential inflation model and in which stable solutions can be obtained.

gr-qc↗

First order phase transitions within Weyl type of materials at low temperatures

We analyze the possible dynamical chiral symmetry breaking patterns taking place within Weyl type of materials. Here, these systems are modeled by the (2+1)-dimensional Gross-Neveu model with a tilt in the Dirac cone. The optimized perturbation theory (OPT) is employed in order to evaluate the effective potential at finite temperatures and chemical potentials beyond the traditional large-$N$ limit. The nonperturbative finite-$N$ corrections generated by the OPT method and its associated variational procedure show that a first-order phase transition boundary, missed at large $N$, exists in the regime of low temperatures and large chemical potentials. This result, which represents our main finding, implies that one should hit a region of mixed phases when exploring the low-temperature range. The associated first order transition line, which starts at $T=0$, terminates at a tricritical point such that the transitions taking place at high $T$ are of the second kind. In particular, we discuss how the tilt in the Dirac cone affects the position of the tricritical point as well as the values of critical temperature and coexistence chemical potential among other quantities. Some experimental implications and predictions are also briefly discussed.

cond-mat.str-el↗

Bouncing and inflationary dynamics in quantum cosmology in the de Broglie-Bohm interpretation

The quantum cosmology of the flat Friedmann-Lema{î}tre-Robertson-Walker Universe, filled with a scalar field, is considered in the de Broglie-Bohm (dBB) interpretation framework. A stiff-matter quantum bounce solution is obtained. The bouncing and subsequent pre-inflationary and inflationary dynamics are studied in details. We consider some representative primordial inflation models as examples, for which analytical expressions characterizing the dynamical quantities can be explicitly derived. The dependence of the inflationary dynamics on the quantum bounce parameters is then analyzed. The parameters emerging from our description are constrained by requiring the produced dynamics to be in accordance with some key cosmological quantities. The constraining conditions are also illustrated through regions of parameter space in terms of the bounce quantities.

gr-qc↗

Minimal Preheating

An oscillating inflaton field induces small amplitude oscillations of the Hubble parameter at the end of inflation. These Hubble parameter induced oscillations, in turn, trigger parametric particle production of all light fields, even if they are not directly coupled to the inflaton. We here study the induced particle production for a light scalar field (e.g. the Standard Model Higgs field) after inflation as a consequence of this effect. Our analysis yields a model-independent lower bound on the efficiency of energy transfer from the inflaton condensate to particle excitations.

hep-ph↗

Finite size effects in the phase transition patterns of coupled scalar field systems

It is considered in this work the phase transition patterns for a coupled two-scalar field system model under the combined effects of finite sizes and temperature. The scalar fields are taken as propagating in a D=4 Euclidean space with the usual periodic compactification in the Euclidean time direction (with dimension given by the inverse of the temperature) and also under a compact dimension in the space direction, which is restricted to size L. In the latter case, a Dirichlet boundary condition is considered. Finite-size variation of the critical temperature for the cases of symmetry restoration and inverse symmetry breaking are studied. At fixed finite-temperature values, the variation of the inverse correlation lengths with the size L might display a behavior analogous to reentrant phase transitions. Possible applications of our results to physical systems of interest are also discussed.

hep-ph↗

Decay of ALP Condensates via Gravitation-Induced Resonance

Oscillating scalar field condensates induce small amplitude oscillations of the Hubble parameter which can induce a decay of the condensate due to a parametric resonance instability [1]. We show that this instability can lead to the decay of the coherence of the condensate of axion-like particle (ALP) fields during the radiation phase of standard cosmology for rather generic ALP parameter values, with possible implications for certain experiments aiming to search for ALP candidates. As an example, we study the application of this instability to the QCD axion. We also study the magnitude of the induced entropy fluctuations.

hep-ph↗

Superconducting phase transition in planar fermionic models with Dirac cone tilting

The chiral and superconducting gaps are studied in the context of a planar fermion model with four-fermion interactions. The effect of the tilt of the Dirac cone on both gaps is shown and discussed. Our results point to two different behaviors exhibited by planar fermionic systems. We show that there is a threshold value $\tilde{t}^*$ for the effective tilt parameter such that when $|{\bf \tilde{t}}| < \tilde{t}^*$, the superconducting phase persists for negative values of the superconducting coupling constant. For positive values of the superconducting coupling constant, the induction of a superconducting gap by a chemical potential exists and which is similar to the one seen in graphene-like systems. For $|{\bf \tilde{t}}| > \tilde{t}^*$ and a negative superconducting coupling constant, the superconducting phase can be present, but it is restricted to a smaller area in the phase portrait. Our analysis also shows that when $|{\bf \tilde{t}}| > \tilde{t}^*$ and for positive values for the superconducting coupling constant, the induction of a superconducting gap in the presence of a chemical potential is ruled out. In this case, the increase of the chemical potential works in favor of the manifestation of a metallic phase.

cond-mat.str-el↗

Recent developments in warm inflation

Warm inflation, its different particle physics model implementations and the implications of dissipative particle production for its cosmology are reviewed. First, we briefly present the background dynamics of warm inflation and contrast it with the cold inflation picture. An exposition of the space of parameters for different well-motivated potentials, which are ruled out, or severely constrained in the cold inflation scenario, but not necessarily in warm inflation, is provided. Next, the quantum field theory aspects in realizing explicit microscopic models for warm inflation are given. This includes the derivation of dissipation coefficients relevant in warm inflation for different particle field theory models. The dynamics of cosmological perturbations in warm inflation are then described. The general expression for the curvature scalar power spectrum is shown. We then discuss in details the relevant regimes of warm inflation, the weak and strong dissipative regimes. We also discuss the results predicted in these regimes of warm inflation and how they are confronted with the observational data. We explain how the dissipative dynamics in warm inflation can address several long-standing issues related to (post-) inflationary cosmology. This includes recent discussions concerning the so-called swampland criteria and how warm inflation can belong to the landscape of string theory.

hep-ph↗

Constraining the Barbero-Immirzi parameter from the duration of inflation in loop quantum cosmology

We revisit the predictions for the duration of the inflationary phase after the bounce in loop quantum cosmology. We present our analysis for different classes of inflationary potentials that include the monomial power-law chaotic type of potentials, the Starobinsky and the Higgs-like symmetry breaking potential with different values for the vacuum expectation value. Our setup can easily be extended to other forms of primordial potentials than the ones we have considered. Independently on the details of the contracting phase, if the dynamics starts sufficiently in the far past, the kinetic energy will come to dominate at the bounce, uniquely determining the amplitude of the inflaton at this moment. This will be the initial condition for the further evolution that will provide us with results for the number of e-folds from the bounce to the beginning of the accelerated inflationary regime and the subsequent duration of inflation. We also discuss under which conditions each model considered could lead to observable signatures on the spectrum of the cosmic microwave background or else be excluded for not predicting a sufficient amount of accelerated expansion. A first analysis is performed considering the standard value for the Barbero-Immirzi parameter obtained from black hole entropy calculations. In a second analysis, we consider the possibility of varying the value of this parameter, which is motivated by the fact that the Barbero-Immirzi parameter can be considered a free parameter of the underlying quantum theory in the context of loop quantum gravity. From this analysis, we obtain a lower limit for this parameter by requiring the minimum amount of inflationary expansion that makes the model consistent with the CMB observations. [abridged]

gr-qc↗

Phase transition patterns for coupled complex scalar fields at finite temperature and density

The phase transition patterns displayed by a model of two coupled complex scalar fields are studied at finite temperature and chemical potential. Possible phenomena like symmetry persistence and inverse symmetry breaking at high temperatures are analyzed. The effect of finite density is also considered and studied in combination with the thermal effects. The nonperturbative optimized perturbation theory method is considered and the results contrasted with perturbation theory. Applications of the results obtained are considered in the context of an effective model for condensation of kaons at high densities, which is of importance in the understanding of the color-flavor locked phase of quantum chromodynamics.

hep-ph↗

Running and running of the running of the scalar spectral index in warm inflation

Next generation of cosmological observations are expected to improve the measurements of several quantities connected to the primordial inflation in the early Universe. These quantities include for example improved measurements for the spectral index of the scalar curvature of the primordial power spectrum and to also bring a better understanding on the scaling dependence of the primordial spectrum. This includes the running of the tilt and, possibly, also the running of the running. In this paper, we investigate the possibility of generating large runnings in the context of warm inflation. Useful analytical expressions for the runnings are derived in the context of warm inflation in the strong dissipation regime. The results are compared to and discussed for some well motivated primordial inflaton potentials that have recently been of interest in the literature.

astro-ph.CO↗