arXiv subjects

João G. Rosa

Publications and source records attributed to João G. Rosa.

At least 19 recordsLinked to original sources

arXiv2026

Stimulated capture of axion dark matter around primordial black holes

We study the stimulated capture of axion and axion-like particles around primordial black holes, mediated by quartic self-interactions. We identify the parametric regions where Bose-enhanced growth overcomes cosmological dilution and derive analytic criteria for efficient formation and long-term survival of the hydrogenic ground-state cloud, confronting them with current bounds on axion-photon couplings. We discuss observational implications of these PBH+cloud systems, including resonant axion-to-photon conversion in neutron star magnetospheres and finite-size microlensing distortions. In the former case, the resonance condition selects a conversion shell outside the neutron star surface, and we show that the large axion occupation number of the cloud can compensate for the small conversion probability. The resulting transient line flux densities can exceed nominal VLA and ALMA sensitivities in regions where the Galactic encounter rate is observationally relevant. Independently, when the cloud size becomes comparable to or larger than the Einstein radius, finite-size effects can modify the standard point-lens microlensing light curve.

astro-ph.CO↗

arXiv2026

Lukewarm inflation, primordial black holes and gravitational waves

We show that a secondary period of warm inflation, at temperatures parametrically below the GUT scale, may not only dilute any unwanted thermal relics formed after reheating but also significantly enhance the power spectrum of primordial curvature perturbations on small scales. The latter is nearly scale-invariant over an exponentially large range of comoving scales, resulting in a stochastic gravitational wave background with a nearly constant energy density over a broad range of frequencies and an associated population of sub-solar mass primordial black holes (PBHs). This scenario could, in particular, explain the NANOGrav signal if it persists at much higher frequencies with a comparable magnitude, up to a sharp cut-off, $f_{max}$. The associated PBH mass distribution may exhibit either a peak at a mass $M_*\propto f_{max}^{-2}$ or a broad plateaux above this mass threshold, depending on the spectral tilt. Moreover, in this scenario the NANOGrav signal is compatible with a significant fraction of dark matter in PBHs in the asteroid- and planetary-mass ranges.

hep-ph↗

arXiv2026

Lukewarm inflation and the QCD axion

We show that a late period of warm inflation, at temperatures just above the QCD scale, can dilute the abundance of the QCD axion produced by the misalignment mechanism, thus avoiding the need to fine-tune the initial misalignment angle in scenarios with an axion decay constant close to the GUT scale. We develop a concrete realization of this lukewarm inflation stage involving right-handed neutrinos in 0.1 - 1 GeV mass range and an associated scalar sector, showing that coherent axion oscillations are damped during this period. In this scenario, a previously generated large baryon asymmetry may also be diluted to yield the observed value, and lukewarm inflation is followed by an early-matter era dominated by the lightest right-handed neutrino, before it decays into Standard Model states. This model can be tested via changes to the number of relativistic species and the induced small-scale enhancements of the primordial curvature power spectrum, leading to primordial black holes and scalar-induced gravitational waves.

hep-ph↗

arXiv2026

Hawking emission of massive vector fields by Kerr black holes

We compute, for the first time, the Hawking emission spectrum of massive vector (Proca) fields by spinning Kerr black holes, determining the associated greybody factors and the resulting mass and spin loss functions. We show, in particular, that the scalar (longitudinal) polarization of the Proca field has a spectrum approaching that of a free scalar field in the massless limit (in which it becomes a pure gauge mode), although we find substantial differences for finite mass. The contribution of the two vector (transverse) polarization modes coincides, as expected, with the one obtained by Page for the Maxwell field in the massless limit. The black hole's evaporation rate is dominated by the scalar mode for slowly spinning black holes and by the two vector modes as the black hole approaches extremality. As for other fields, we find that Proca Hawking emission is Boltzmann-suppressed for Hawking temperatures $T_H\lesssim |μ-Ω_H|$, where $μ$ is the field mass and $Ω_H$ is the angular velocity of the black hole's horizon. This implies that highly spinning black holes can efficiently emit massive vector fields at temperatures parametrically below the field's mass. Finally, we also find that superradiant emission is more pronounced for massive vector fields, with a maximum amplification factor of $\simeq 7\%$ (compared to $\simeq 4\%$ for massless photons).

gr-qc↗

arXiv2026

String Axiverse Enhancement of Superradiant Dark Matter Production

We study the effects of string axion emission on dark matter production by light primordial black holes (PBHs), through both evaporation and superradiance. We show, in particular, that the Hawking emission of $\mathcal{O}(100-10^5)$ light axion species predicted in realistic string theory constructions can significantly enhance the efficiency of superradiance, given the associated increase in the PBH spin. The string axiverse thus significantly expands the parametric regions (dark matter mass and PBH mass and spin) for which a sizeable fraction of dark matter may presently be in the form of "micro-boson stars": the self-gravitating remnants of superradiant dark matter clouds. Conversely, for too large a number of axion species PBHs evaporate too quickly for superradiant clouds to attain their maximum mass. Finally, assuming that all dark matter is produced by PBHs (through both superradiance and Hawking emission), we show that the axions emitted during PBH evaporation give an immeasurably small contribution to the relativistic degrees of freedom at recombination.

hep-ph↗

arXiv2025

From scalar clouds around evaporating black holes to boson star

We study, for the first time, the evolution of a scalar cloud bound to an evaporating black hole. Our simulations of the associated Schrödinger-Poisson system for non-relativistic and spherically symmetric clouds reveal that a scalar cloud may (partially) survive as a self-gravitating boson star if the black hole evaporates adiabatically until its mass becomes less than one half of the cloud's mass. This yields a novel mechanism for boson star formation and shows that, as previously conjectured, bosonic dark matter production by light primordial black holes may result in micro-boson stars with very large occupation numbers, greatly enhancing their potential detectability even for very weakly interacting dark matter particles.

gr-qc↗

arXiv2025

Dark neutron stars from a heavy dark sector

We study the formation and properties of dark neutron stars in a scenario where dark matter is made up of (heavy) dark baryons in a sequestered copy of the MSSM. This scenario naturally explains the coincidence of baryonic and dark matter abundances without the need for tuning particle masses. In particular, the supersymmetry breaking scales in the visible and dark sectors may differ by up to 10-11 orders of magnitude. We argue that dark neutrons should be the lightest dark baryons, but that dark protons may be cosmologically long lived. This allows a small fraction of dark matter to remain ionized until the first halos start to form, providing cooling mechanisms that foster the gravitational collapse and fragmentation of sub-halo structures, ultimately resulting in dark neutron star and black hole formation. For a wide range of model parameters, we find dark neutron stars with generally smaller mass and radius than ordinary visible sector neutron stars. We also discuss their potential detectability, particularly through gravitational microlensing and dark magnetic dipole radiation at radio frequencies through photon-dark photon kinetic mixing.

hep-ph↗

arXiv2025

Constraints on Dark Matter Structures around Gaia Black Holes

We demonstrate that Gaia's detection of stars on wide orbits around black holes opens a new observational window on dark matter structures -- such as scalar clouds and dark matter spikes -- predicted in a range of theoretical scenarios. Using precise radial velocity measurements of these systems, we derive state-of-the-art constraints on dark matter density profiles and particle masses in previously unexplored regions of parameter space. We also test the black hole hypothesis against the alternative of a boson star composed of light scalar fields.

astro-ph.CO↗

arXiv2025

Preheated inflation

We propose a new mechanism of non-thermal particle production during inflation based on a narrow parametric resonance, akin to the dynamics of post-inflationary preheating. The mechanism is based on the production of scalar particles with a mass that is an oscillating function of the slowly-rolling inflaton field. This is achieved in a scenario for the collective spontaneous breaking of a U(1) gauge symmetry that, while originally proposed in the context of warm inflation, leads to non-equilibrium particle production sustaining a (sub-dominant) non-thermal radiation bath throughout inflation. We show that this may leave an observational imprint, namely oscillatory features in the primordial curvature power spectrum alongside a (mild) resonant enhancement of its amplitude, as well as secondary gravitational waves that can be probed with future CMB experiments.

hep-ph↗

arXiv2024

The inflation trilogy and primordial black holes

We propose an inflation scenario with three independent stages of cold, warm and thermal inflation, respectively, driven by different scalar fields, motivated by the large number of such fields predicted in most extensions of the Standard Model. We show, in particular, that the intermediate period of warm inflation naturally leads to large density fluctuations on small scales, which can lead to primordial black hole formation in the mass window where they may account for all dark matter. This type of scenario yields a distinctive primordial black hole mass function with a mass gap, with the final period of thermal inflation diluting the abundance of very light black holes.

hep-ph↗

arXiv2024

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↗

arXiv2024

Superradiant Darwinism: survival of the lightest axion

We study the dynamical evolution of superradiant instabilities of rotating black holes for multiple axion fields with comparable masses, motivated by string theory constructions, which typically exhibit a large number of light axions, with a broad range of masses. We show, in particular, that even though superradiant clouds for the heavier axion species grow faster, they are eventually reabsorbed by the black hole as the latter amplifies the lighter axion field(s), analogously to the dynamics of different species competing for the same resources in an ecosystem. We also incorporate in our study the effects of accretion and gravitational wave emission. We further demonstrate that the existence of multiple axion species with comparable masses may have a substantial impact on the stochastic gravitational wave background produced by axion clouds around black hole binary merger remnants, which could be probed with planned detectors.

hep-ph↗

arXiv2023

Evaporating Kerr black holes as probes of new physics

In the string axiverse scenario, primordial black holes (PBHs) can sustain non-negligible spin parameters as they evaporate. We show that tracking both the mass and spin evolution of a PBH in its final hour can yield a purely gravitational probe of new physics beyond the TeV scale, allowing one to determine the number of new scalars, fermions, vector bosons, and spin-3/2 particles. Furthermore, we propose a multi-messenger approach to accurately measure the mass and spin of a PBH from its Hawking photon and neutrino primary emission spectra, which is independent of putative interactions between the new degrees of freedom and the Standard Model particles, as well as from the Earth-PBH distance.

hep-ph↗

arXiv2023

Determining the spin of light primordial black holes with Hawking radiation II: high spin regime

We propose a method to determine the mass and spin of primordial black holes based on measuring the energy and emission rate at the dipolar and quadrupolar peaks in the primary photon Hawking spectrum, applicable for dimensionless spin parameters $\tilde{a}\gtrsim 0.6$. In particular, we show that the ratio between the energies of the two peaks is only a function of the black hole spin, while the ratio between their emission rates depends also on the line-of-sight inclination. The black hole mass and distance from the Earth may then be inferred from the absolute values of the peak energies and emission rates. This method is relevant for primordial black holes born with large spin parameters that are presently still in the early stages of their evaporation process.

gr-qc↗

arXiv2023

Shrinking the Warm Little Inflaton

We show that warm inflation can be successfully realized in the high temperature regime through dissipative interactions between the inflaton and a single fermionic degree of freedom, provided that the latter's mass is an oscillatory function of the inflaton field value. We demonstrate, in particular, that despite the consequent large amplitude oscillations of the eta slow-roll parameter, their effect is, on average, sufficiently suppressed to allow for a slow-roll trajectory. In addition, we demonstrate that, even though this also induces a parametric resonance that amplifies inflaton perturbations, this has a negligible effect on CMB scales in the relevant parametric range. Hence, the "Warm Little Inflaton" scenario can be realized with one less fermionic degree of freedom and no need of imposing an additional discrete interchange symmetry.

hep-ph↗

arXiv2023

Primordial black hole superradiance and evaporation in the string axiverse

In the string axiverse scenario, light primordial black holes may spin up due to the Hawking emission of a large number of light (sub-MeV) axions. We show that this may trigger superradiant instabilities associated with a heavier axion during the black holes' evolution, and study the coupled dynamics of superradiance and evaporation. We find, in particular, that the present black hole mass-spin distribution should follow the superradiance threshold condition for black hole masses below the value at which the superradiant cloud forms, for a given heavy axion mass. Furthermore, we show that the decay of the heavy axions within the superradiant cloud into photon pairs may lead to a distinctive line in the black hole's emission spectrum, superimposed on its electromagnetic Hawking emission.

hep-ph↗

arXiv2023

Superradiant axion clouds around asteroid-mass primordial black holes

We analyze the dynamics and observational signatures of axion clouds formed via the superradiant instability around primordial black holes, focusing on the mass range $10^{14}-10^{18}$ kg where the latter may account for all the dark matter. We take into account the leading effects of axion self-interactions, showing that, even though these limit the number of axions produced within each cloud, a large number of superradiant axions become free of the black hole's gravitational potential and accumulate in the intergalactic medium or even in the host galaxy, depending on their escape velocity. This means that primordial black hole dark matter may lead to a sizeable astrophysical population of non-relativistic axions, with masses ranging from 0.1 eV to 1 MeV, depending on the primordial black hole mass and spin. We then show that if such axions couple to photons their contribution to the galactic and extragalactic background flux, mainly in the X-ray and gamma-ray band of the spectrum, is already beyond current observational limits for a large range of parameters that are, therefore, excluded. We finish by showing the prospects of the Athena X-ray telescope to further probe this co-existence of primordial black holes and axions.

hep-ph↗

arXiv2022

Comparable dark matter and baryon abundances with a heavy dark sector

We propose a scenario that explains the comparable abundances of dark matter (DM) and baryons without any coincidence in the corresponding particle masses. Here, DM corresponds to heavy "dark baryons" in a hidden MSSM-like dark sector, where the supersymmetry breaking scale can be several orders of magnitude larger than in the visible sector. In both sectors a baryon asymmetry is generated via the Affleck-Dine mechanism, and the smaller dark baryon-to-entropy ratio partially compensates the larger dark baryon masses to give similar densities in the two sectors. The large mass hierarchy also naturally results in an asymmetric reheating of these sequestered sectors. Moreover, this scenario predicts uncorrelated DM and baryon isocurvature perturbations.

hep-ph↗