arXiv subjects

Mar Bastero-Gil

Publications and source records attributed to Mar Bastero-Gil.

At least 19 recordsLinked to original sources

arXiv2026

Not So Minimal Warm Inflation

An axion-like inflaton coupled to non-Abelian gauge bosons provides a compelling microphysical framework for warm inflation. Starting even from cold initial conditions, in these systems, sphaleron heating may generate thermal friction sufficient to sustain finite temperatures throughout the inflationary epoch. Insisting on shift-symmetric potentials, in this work we revisit the viability of these scenarios under the designation of Minimal Warm Inflation. We examine both observational constraints and model-building limitations on models with a hierarchy between the decay constants appearing in the friction rate and in the inflaton potential. We conclude that the popular clockwork mechanism cannot generate the required hierarchy; however, partial-wave unitarity bounds admit effective descriptions that remain consistent with observations.

hep-ph↗

arXiv2025

Classical constant electric fields and the Schwinger effect in de Sitter

We study constant classical electric fields and the Schwinger effect in de Sitter space, with potential implications for magnetogenesis and inflationary dark matter production. Treating the photon as a dynamical field, we show that sustaining a constant electric field in de Sitter requires a tachyonic photon mass of order the Hubble scale. This observation has physical implications, as it alters the infrared behaviour of the induced Schwinger current. Using an on-shell renormalization condition consistent with a tachyonic photon, we recompute the current for charged fermions and scalars, finding it to be finite and positive even in the massless limit of the charge carriers-contrary to earlier results predicting a puzzling negative IR divergence. For scalars, we include a non-minimal coupling to the Ricci curvature, enabling us to analyze the conformal limit, where the current closely matches that of charged fermions.

hep-ph↗

arXiv2025

Schwinger Current in de Sitter Space

We study classical background electric fields and the Schwinger effect in de Sitter space. We show that having a constant electric field in de Sitter requires the photon to have a tachyonic mass proportional to the Hubble scale. This has physical implications for the induced Schwinger current which affect its IR behaviour. To study this we recompute the Schwinger current in de Sitter space for charged fermions and minimally coupled scalars imposing a physically consistent renormalization condition. We find a finite and positive Schwinger current even in the massless limit. This is in contrast to previous calculations in the literature which found a negative IR divergence. We also obtain the first result of the Schwinger current for a non-minimally coupled scalar, including for a conformally coupled scalar which we find has very similar behaviour to the fermion current. Our results may have physical implications for both magnetogenesis and inflationary dark matter production.

hep-ph↗

arXiv2024

The neutron decay anomaly, neutron stars and dark matter

The discrepancies in different measurements of the lifetime of isolated neutrons could be resolved by considering an extra neutron decay channel into dark matter, with a branching ratio of the order of $O(1$\%). Although the decay channel into a dark fermion $\chi$ plus visible matter has been already experimentally excluded, a dark decay with either a scalar or dark photon remains still a possibility. In particular, a model with a fermion mass $m_\chi\approx 1$ GeV and a scalar $m_\phi \approx O(\rm{MeV})$ could provide not only the required branching ratio to explain the anomaly but also a good dark matter (DM) candidate with the right thermal abundance today. Although the interaction DM-neutron will affect the formation of neutron stars, the combined effect of the dark matter self-interactions mediated by the light scalar and an effective repulsive interaction with the neutrons induced by the scalar-Higgs coupling would allow heavy enough neutron stars. The combined constraints from neutron lifetime, dark matter abundance, neutron star and Higgs physics, and Big Bang Nucleosynthesis, restrict the light scalar mass to the range $2 m_e < m_\phi < 2 m_e + 0.0375$ MeV.

astro-ph.CO↗

arXiv2023

Schwinger dark matter production

Building on recently constructed inflationary vector dark matter production mechanisms as well as studies of magnetogenesis, we show that an inflationary dark Schwinger mechanism can generate the observed dark matter relic abundance for `dark electron' masses as light as $\sim 0.1$ eV and as heavy as $10^{12}$ GeV. The dark matter can interact very weakly via the exchange of light dark photons with a power spectrum which is peaked at very small scales, thus evading isocurvature constraints. This mechanism is viable even when (purely) gravitational particle production is negligible. Thus dark matter can be produced solely via the Schwinger effect during inflation including for light masses.

hep-ph↗

arXiv2023

Non-thermal moduli production during preheating in $α$-attractor inflation models

Production of gravitationally coupled light moduli fields must be suppressed in the early universe, so that its decay products do not alter Big Bang Nucleosynthesis (BBN) predictions for light elements. On the other hand, the moduli quanta can be copiously produced non-thermally during preheating after the end of inflation. In this work, we study the production of moduli in the $α$-attractor inflationary model through parametric resonances. For our case, where the inflationary potential at its minimum is quartic, the inflaton field self-resonates, and subsequently induces large production of moduli particles. We find that this production is suppressed for small values of $α$. Combining semi-analytical estimation and numerical lattice simulations, we infer the parametric dependence on $α$ and learn that $α$ needs to be $\lesssim 10^{-8}\,m_{\rm Pl}^2$ to be consistent with BBN. This in turn predicts an upper bound on the energy scale of inflation and on the reheating temperature.

astro-ph.CO↗

arXiv2023

Super heavy dark matter from inflationary Schwinger production

We consider a simple setup with a dark sector containing dark electrons charged under an abelian $U(1)_D$ gauge symmetry. We show that if the massless dark photon associated to the $U(1)_D$ is produced during inflation in such a way as to form a classical dark electric field, then dark electron-positron pairs are also produced close to the end of inflation via the Schwinger effect even if they are very massive. For large enough dark electric force, dark electrons with masses larger than the Hubble scale can be produced which are non-relativistic at production and throughout their cosmic evolution. They can account for the dark matter abundance today for masses in the range $\sim$ 100 GeV to $10^{17}$ GeV and up to six orders of magnitude larger than the Hubble scale at the end of inflation where purely gravitational production is exponentially suppressed. We examine the regime where the dark electrons do not thermalize with the dark photons throughout their cosmic history and assume negligible kinetic mixing with the visible $U(1)$ so they remain decoupled from the Standard Model thermal bath as well. Thus the final dark matter relic abundance is determined only by the initial inflationary Schwinger production and redshifting after reheating.

hep-ph↗

arXiv2023

Parity violating gravitational waves at the end of inflation

Inflaton-vector interactions of the type $ϕF\tilde{F}$ have provided interesting phenomenology to tackle some of current problems in cosmology, namely the vectors could constitute the dark matter component. It could also lead to possible signatures imprinted in a gravitational wave spectrum. Through this coupling, a rolling inflaton induces an exponential production of the transverse polarizations of the vector field, having a maximum at the end of inflation when the inflaton field velocity is at its maximum. These gauge particles, already parity asymmetric, will source the tensor components of the metric perturbations, leading to the production of parity violating gravitational waves. In this work we examine the vector particle production with an attempt to mimic its backreaction effects on the inflation evolution in the weak coupling regime. Furthermore, we fully integrate the gauge particle amplitudes spectrum during this production epoch, studying the behavior until the end of reheating. Finally, we calculate the gravitational wave spectrum solely relying on the vector mode WKB expansion in its regime of validity.

gr-qc↗

arXiv2023

New solar X-ray constraints on keV Axion-Like Particles

The decay of Axion-Like Particles (ALPs) trapped in the solar gravitational field would contribute to the observed solar X-ray flux, hence constraining ALP models. We improve by one order of magnitude the existing limits in the parameter space $(g_{aγγ}, m)$ by considering ALPs production via photon coalescence. For $g_{ae} \neq 0$, we demonstrate that trapped ALPs can be Compton-absorbed while crossing the Sun, resulting in two regimes in the exclusion limits, with a transition triggered by $g_{ae}$. Out of the transitional region, the solar X-ray constraints on ALPs are exclusively governed by $g_{aγγ}$.

hep-ph↗

arXiv2023

Thermal curvature perturbations in thermal inflation

We compute the power spectrum of super-horizon curvature perturbations generated during a late period of thermal inflation, taking into account fluctuation-dissipation effects resulting from the scalar flaton field's interactions with the ambient radiation bath. We find that, at the onset of thermal inflation, the flaton field may reach an equilibrium with the radiation bath even for relatively small coupling constants, maintaining a spectrum of thermal fluctuations until the critical temperature $T_c$, below which thermal effects stop holding the field at the false potential minimum. This enhances the field variance compared to purely quantum fluctuations, therefore increasing the average energy density during thermal inflation and damping the induced curvature perturbations. In particular, we find that this inhibits the later formation of primordial black holes, at least on scales that leave the horizon for $T>T_c$. The larger thermal field variance also reduces the duration of a period of fast-roll inflation below $T_c$, as the field rolls to the true potential minimum, which should also affect the generation of (large) curvature perturbations on even smaller scales.

hep-ph↗

arXiv2021

Gravity Waves and Primordial Black Holes in Scalar Warm Little Inflation

In warm inflation, dissipation due to the interactions of the inflaton field to other light degrees of freedom leads naturally to the enhancement of the primordial spectrum during the last 10-20 efolds of inflation. We study this effect in a variant of the Warm Little Inflaton model, where the inflaton couples to light scalars, with a quartic chaotic potential. These large fluctuations on re-entry will form light, evaporating Primordial Black Holes, with masses lighter than $10^6$ g. But at the same time they will act as a source for the tensors at second order. The enhancement is maximal near the end of inflation, which result in a spectral density of Gravitational Waves (GW) peaked at frequencies $f \sim O(10^3-10^5)$ Hz today, and with an amplitude $Ω_{GW} \sim 10^{-10}-10^{-8}$. Although the frequency range is outside the reach of present and planned GW detectors, it might be reached in future high-frequency gravitational waves detectors, designed to search for cosmological stochastic GW backgrounds above MHz frequencies.

hep-ph↗

arXiv2021

Solar axions in large extra dimensions

The axion could be used as a probe for extra dimensions. In large extra dimensions, besides the QCD axion one obtains an infinite tower of massive Kaluza-Klein (KK) states. We describe the processes of KK axions production in the Sun via the axion-photon coupling, $g_{aγγ}$, and we derive the number density of KK axions that get trapped into the solar gravitational field and then accumulate over cosmic times. The large multiplicity of states, as well as their masses in the keV-range, deeply alter the phenomenology of the axion. This scenario leads us to propose the presence of KK axions as an interpretation of the non-thermal distribution of the solar X-rays. In this work, we dedicate special attention on the astrophysical and cosmological bounds that apply on the model. In particular, we show how the KK axions may escape the EBL limit that constrains standard ALPs in the same mass range. Present searches for KK axions make use of the decay channel, $a\rightarrowγγ$, for which we revise the event rate; our value lies orders of magnitude below the rate usually quoted in the literature. This major conclusion stems from recent measurements of the luminosity of the quiet Sun which acts as an irreducible limit. The revised model remains a viable and an attractive explanation for multiple astrophysical observations, and we propose several approaches to search for solar KK axions in the near future.

hep-ph↗

arXiv2021

Dark photon dark matter from a rolling inflaton

We study in detail a recently proposed mechanism for producing non-thermal dark photon dark matter at the end of inflation in the mass range $\mu\,{\rm eV} \lesssim m \lesssim 10\,{\rm TeV}$. A tachyonic instability induced by a rolling inflaton leads to the coherent production of dark (abelian) gauge bosons with a peak in the power spectrum corresponding to the Hubble scale at the end of inflation. As the Universe expands after inflation the dark photons redshift and, at some point in their cosmic evolution, they obtain a mass. We focus in particular on the case where the dark photons are relativistic at the time their mass is generated and examine the associated cosmic evolution to compute the relic abundance today. We also examine the late time power spectrum demonstrating explicitly that it preserves the peak generated at the end of inflation. We show that the peak corresponds to small physical scales today, $\ell_{\rm today} \sim {\rm cm} - 100\,{\rm km}$, with large density fluctuations at $\ell_{\rm today}$ leading to a clumpy nature for the dark photon dark matter. We also discuss potential phenomenology and future directions, briefly commenting on the non-relativistic case.

hep-ph↗

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

Supersymmetric $\nu$-Inflaton Dark Matter

We present the supersymmetric extension of the unified model for inflation and Dark Matter studied in Ref. arXiv:1811.02302. The scenario is based on the incomplete decay of the inflaton field into right-handed (s)neutrino pairs. By imposing a discrete interchange symmetry on the inflaton and the right-handed (s)neutrinos, one can ensure the stability of the inflaton field at the global minimum today, while still allowing it to partially decay and reheat the Universe after inflation. Compatibility of inflationary predictions, BBN bounds and obtaining the right DM abundance for the inflaton Dark Matter candidate typically requires large values of its coupling to the neutrino sector, and we use supersymmetry to protect the inflaton from potentially dangerous large radiative corrections which may spoil the required flatness of its potential. In addition, the inflaton will decay now predominatly into sneutrinos during reheating, which in turn give rise both to the thermal bath made of Standard Model particles, and inflaton particles. We have performed a through analyses of the reheating process following the evolution of all the partners involved, identifying the different regimes in the parameter space for the final Dark Matter candidate. This as usual can be a WIMP-like inflaton particle or an oscillating condensate, but we find a novel regime for a FIMP-like candidate.

hep-ph↗

arXiv2019

Reexamination of the warm inflation curvature perturbations spectrum

The two approaches to compute perturbations in warm inflation are examined. It is shown that both approaches lead to different expressions for the amplitude of the primordial spectrum, with a difference between them of $Υ/(4H)$ at leading order, where $Υ$ is the dissipation coefficient. In terms of observables, this discrepancy can lead to the spectral index differing by up to order $\mathcal{O}(10^{-3})$, which is within precision demands for current CMB data. Thus, it is important to resolve this ambiguity to have reliable predictions from warm inflation. For this we prove the extent of this discrepancies by deriving a formula for the spectral index and the tensor-to-scalar ratio in each approach. In doing so, we find disparities to be more noticeable in a regime where dissipation is comparable with the expansion rate, which is a very important regime from a phenomenological point of view. To determine the extent of the discrepancy, several cases are examined, including quadratic, quartic and hybrid potentials with quadratic and $T-$dependent dissipative coefficients. The origin of the discrepancy is found to be due to the approximation performed in one of the methods, which underestimates the variation of the momentum perturbation with expansion. Once this is corrected, both approaches are then in agreement.

astro-ph.CO↗

arXiv2019

Warm inflation within a supersymmetric distributed mass model

We study the dynamics and observational predictions of warm inflation within a supersymmetric distributed mass model. This dissipative mechanism is well described by the interactions between the inflaton and a tower of chiral multiplets with a mass gap, such that different bosonic and fermionic fields become light as the inflaton scans the tower during inflation. We examine inflation for various mass distributions, analyzing in detail the dynamics and observational predictions. We show, in particular, that warm inflation can be consistently realized in this scenario for a broad parametric range and in excellent agreement with the Planck legacy data. Distributed mass models can be viewed as realizations of the landscape property of string theory, with the mass distributions coming from the underlying spectra of the theory, which themselves would be affected by the vacuum of the theory. We discuss the recently proposed swampland criteria for inflation models on the landscape and analyze the conditions under which they can be met within the distributed mass warm inflation scenario. We demonstrate mass distribution models with a range of consistency with the swampland criteria including cases in excellent consistency.

hep-ph↗

arXiv2019

Vector dark matter production at the end of inflation

It has been shown that the longitudinal mode of a massive vector boson can be produced by inflationary fluctuations and account for the dark matter content of the Universe. In this work we examine the possibility of instead producing the transverse mode via the coupling $ϕF \tilde F$ between the inflaton and the vector field strength. Such a coupling leads to a tachyonic instability and exponential production of one transverse polarization of the vector field, reaching its maximum near the end of inflation. At production the mass is negligible and the vectors add up coherently to form a dark electromagnetic field. As the Universe expands, the energy density of the dark electromagnetic field then redshifts like radiation until its wavelength stretches to beyond its Compton wavelength. After this point the vectors become non-relativistic and their energy density redshifts like matter. We show that these polarized transverse vectors can account for the observed dark matter relic density in the mass range $μ$eV to hundreds of GeV. We also find that the tachyonic production mechanism of the transverse mode can accommodate larger vector masses and lower Hubble scales of inflation compared to the production mechanism for the longitudinal mode via inflationary fluctuations.

hep-ph↗