arXiv subjects

Paolo Gondolo

Publications and source records attributed to Paolo Gondolo.

At least 19 recordsLinked to original sources

arXiv2026

A note on oscillons: non-negligible metric fluctuations during preheating

The dominant lattice approach to post-inflationary preheating and oscillon formation evolves an inhomogeneous inflaton field on a spatially homogeneous Friedmann-Lemaître-Robertson-Walker background, whose expansion is sourced by a volume-averaged energy density, while metric perturbations are neglected. We examine the consistency of this approximation with General Relativity. Using the linearized Einstein constraints, we show that the suppression of the Bardeen potential, characteristic of slow-roll inflation, disappears during preheating: once the first slow-roll parameter becomes of order unity, the metric and inflaton fluctuations enter at the same perturbative order. We further show that the scalar-field equation evolved in the fixed-FLRW prescription omits leading-order contributions generated by metric fluctuations. We then derive the proper-volume average of the ADM Hamiltonian constraint and show that it does not reduce to the Friedmann equation used in lattice simulations: the exact averaged constraint contains additional contributions from the spatial curvature, the variance of the local expansion, the shear, and metric corrections to the local energy density. Finally, we illustrate numerically, for Starobinsky and $α$-attractor models, that the metric contribution becomes comparable to the scalar-field contribution during the amplification stage. These results directly affect the standard lattice description of oscillon formation and motivate numerical relativity as a consistent framework for the nonlinear preheating problem.

astro-ph.CO↗

arXiv2026

Primordial black holes through preheating instabilities in $α$-attractor models

In this work, we explore the production of primordial black holes (PBHs) within the context of $α$-attractor inflationary models, focusing on the preheating phase following inflation. During this phase, self-resonance instabilities arise due to deviations of the inflationary potential from a quadratic form. PBH formation is analyzed using three criteria: (1) the perturbation must lie within the instability band, (2) its characteristic length must exceed the Jeans length, and (3) it must have sufficient time to collapse based on the estimations of massive scalar field spherical collapse in Einstein-de Sitter universe. Based on these criteria, we calculate the PBH mass fraction using the Press-Schechter (PS) and Khlopov-Polnarev (KP) formalisms. Our results show that the PS formalism tends to overestimate PBH abundance during preheating, as it neglects nonspherical effects. In contrast, the KP formalism yields more realistic predictions by incorporating such effects. We provide a detailed comparison with observational constraints from evaporating PBHs. Notably, the PS formalism is excluded by these constraints, which are based on Hawking radiation, while the KP formalism remains viable. These findings underscore the importance of accounting for nonspherical effects and accurate collapse dynamics in studies of PBH formation during preheating.

astro-ph.CO↗

arXiv2025

Scalar-Induced Gravitational Waves from self-resonant preheating in $α$-attractor models

After the inflationary phase, the universe enters the preheating phase, during which the inflaton field rolls down its potential and oscillates. When the potential significantly deviates from a parabolic shape at its minimum, these oscillations trigger an instability in the scalar perturbations, leading to their amplification. This phenomenon, known as self-resonance, has important implications in cosmology. Notably, since scalar perturbations couple to tensor perturbations at second order in the equations of motion, this amplification results in the production of Gravitational Waves (GWs), referred to as Scalar-Induced Gravitational Waves (SIGWs). In this study, we investigate the production of SIGWs during the preheating phase for a class of inflationary models known as $α$-attractors, characterized by a single parameter $α$. We focus on small values of this parameter, specifically $α\sim O(10^{-1} - 10^{-4})$, where the self-resonance effect is particularly pronounced. We obtain lower bounds on this parameter, $\log_{10}(α)>-3.54$ for the T-model and $\log_{10}(α)>-3.17$ for the E-model, based on the energy density of SIGWs constrained by Big Bang nucleosynthesis, which ultimately translates into lower bounds on the tensor-to-scalar ratio, $r>9.61\times10^{-7}$ for the T-model and $r>2.25\times10^{-6}$ for the E-model. Note that these bounds on $α$ and $r$ are derived within the linear framework of tensor fluctuations at the level of equations of motion, which nevertheless include scalar-scalar-tensor interactions with metric and matter fields. However, fully non-linear approaches, with all higher-order metric fluctuations, would be needed in the future to further validate these conclusions.

astro-ph.CO↗

arXiv2025

Gertsenshtein effect on the spacetime curved by background magnetic field with geometric optics

When electromagnetic (or gravitational) waves propagate in the presence of a background magnetic field, a portion of the waves converts into gravitational (or electromagnetic) waves. This phenomenon, known as the (inverse) Gertsenshtein effect, is typically analyzed in Minkowski spacetime, neglecting the spacetime curvature induced by the magnetic field itself. This paper investigates, for the first time, the influence of spacetime curvature on the (inverse) Gertsenshtein effect. To this end, we first determine the metric perturbation from Minkowski spacetime up to second order in the magnetic field strength, assuming cylindrical symmetry. We also discuss the ambiguities in the form of the metric perturbation arising from gauge freedom and boundary conditions. Using the geometric optics approximation, we then derive a set of coupled equations governing the propagation of electromagnetic and gravitational waves in the resulting curved spacetime. These equations are solved for two specific scenarios: a plane wave and a spherical wave. From the solutions, we compute the evolution of the wave amplitudes and the associated energy fluxes. Our analysis reveals that two competing effects govern the amplitude evolution: magnification due to the focusing of waves by spacetime curvature, and attenuation due to wave conversion via the Gertsenshtein effect. In the plane wave case, these effects precisely cancel, resulting in no net change in amplitude. In contrast, for the spherical wave, the Gertsenshtein effect dominates over focusing, leading to an overall reduction in amplitude.

gr-qc↗

arXiv2025

Revisiting primordial black holes formation from preheating instabilities: the case of Starobinsky inflation

In recent years, the formation of primordial black holes (PBH) in the early universe inflationary cosmology has garnered significant attention. One plausible scenario for primordial black hole (PBH) formation arises during the preheating stage following inflation. Notably, this scenario does not necessitate any ad-hoc fine-tuning of the scalar field potential. This paper focuses on the growth of primordial density perturbation and the consequent possibility of PBH formation in the preheating stage of the Starobinsky model for inflation. The typical mechanism for PBH formation during preheating is based on the collapse of primordial fluctuations that become super-horizon during inflation (type I) and re-enter the particle horizon in the different phases of cosmic expansion. In this work, we show that there exists a certain range of modes that remain in the sub-horizon (not exited) during inflation (type II modes) but evolve identically to type I modes if they fall into the instability band, leading to large density perturbation above the threshold and can potentially also contribute to the PBH formation. We detail the conditions determining the possible collapse of type I and/or type II modes whose wavelengths are larger than the Jeans length we derive from the effective sound speed of scalar field fluctuations. Since the preheating stage is an 'inflaton' (approximately) matter-dominated phase, we follow the framework of the critical collapse of fluctuations and compute the mass fraction using the well-known Press-Schechter and the Khlopov-Polnarev formalisms, and compare the two. Finally, we comment on the implications of our study for the investigations concerned with primordial accretion and consequent PBH contribution to the dark matter.

astro-ph.CO↗

arXiv2022

Slowly rotating gravastars

We solve Einstein's equations for slowly-rotating gravitational condensate stars (gravastars) up to second order in the rotation by expanding about the spherically symmetric gravastar with de Sitter interior and Schwarzschild exterior matched at their common horizon. Requiring that the perturbations are finite on the null surface reduces the exterior geometry to that of a Kerr black hole, implying that a slowly rotating gravastar cannot be distinguished from a Kerr black hole by any measurement or observation restricted to the macroscopic spacetime exterior to the horizon. We determine the interior solution, the surface stress tensor, and the Komar mass and angular momentum localized on the slowly rotating horizon surface. With the interior equation of state fixed at $p=-ρ$, finite junction conditions on the null horizon surface necessarily lead to an interior solution with a singular core, where the perturbative expansion breaks down. Comparison to other models and implications for more rapidly rotating gravastars are briefly discussed.

gr-qc↗

arXiv2022

Surface Stress Tensor and Junction Conditions on a Rotating Null Horizon

The general form of the surface stress tensor of an infinitesimally thin shell located on a rotating null horizon is derived, when different interior and exterior geometries are joined there. Although the induced metric on the surface must be the same approached from either side, the first derivatives of the metric need not be. Such discontinuities lead to a Dirac $δ$-distribution in the Einstein tensor localized on the horizon. For a general stationary axisymmetric geometry the surface stress tensor can be expressed in terms of two geometric invariants that characterize the surface, namely the discontinuities $[κ]$ and $[\cal J]$ of the surface gravity $κ$ and angular momentum density $\cal J$. The Komar energy and angular momentum are given in coordinates adapted to the Killing symmetries, and the surface contributions to each determined in terms of $[κ]$ and $[\cal J]$. Guided by these, a simple modification of the original Israel junction conditions is verified directly from the Einstein tensor density to give the correct finite result for the surface stress, when the normal $\boldsymbol n$ to the surface is allowed to tend continuously to a null vector. The relation to Israel's original junction conditions, which fail on null surfaces, is given. The modified junction conditions are suitable to the matching of a rotating `black hole' exterior to any interior geometry joined at the Kerr null horizon surface, even when the surface normal is itself discontinuous and the Barrabès-Israel formalism is also inapplicable. This joining on a rotating null horizon is purely of the matter shell type and does not contain a propagating gravitational shock wave.

gr-qc↗

arXiv2021

Physical interpretation of Newman-Janis rotating systems. I. A unique family of Kerr-Schild systems

The Newman-Janis algorithm and its generalizations can be used mathematically to generate rotating solutions from nonrotating spherically-symmetric solutions within general relativity. The energy-momentum tensors of these solutions may or may not represent the same physical system, in the sense of both being a perfect fluid, or an electromagnetic field, or a $Λ$-term, and so on. In a series of two papers, we compare the structure of the eigenvalues and eigenvectors of the rotating and nonrotating energy-momentum tensors (their Segre types) and look for the existence of equations of state relating the energy density and the principal pressures. Part I covers Kerr-Schild systems, Part II more general systems. We find that there is a unique family of stationary axisymmetric Kerr-Schild systems that obey the same equation of state in both the rotating and nonrotating configurations. This family includes the Kerr and Kerr-Newman black holes, as well as rotating spacetimes whose mass function in the nonrotating limit contains a constrained superposition of a cloud of strings term, a Reissner-Nordstrom term, a cosmological constant term, and a Schwarzschild term. We describe the common equation of state relating energy density and pressure in this family of spacetimes and discuss some of its properties.

gr-qc↗

arXiv2021

Physical interpretation of Newman-Janis rotating systems. II. General systems

Drake and Szekeres have extended the Newman-Janis algorithm to produce stationary axisymmetric spacetimes from general static spherically symmetric solutions of the Einstein equations. The algorithm mathematically generates an energy-momentum tensor for the rotating solution, but the rotating and nonrotating system may or may not represent the same physical system, in the sense of both being a perfect fluid, or an electromagnetic field, or a $Λ$-term, and so on. In Part I (arxiv:2104.02255), we compared the structure of the eigenvalues and eigenvectors of the rotating and nonrotating energy-momentum tensors (their Segre types) and looked for the existence of equations of state relating the rotating energy density and principal pressures for Kerr-Schild systems. Here we extend our analysis to general static spherically symmetric systems obtained according to the Drake-Szekeres generalization of the Newman-Janis algorithm. We find that these rotating systems can have almost all Segre types except [31] and [(31)]. Moreover, the Segre type of the spacetime can change severely in passing from the nonrotating to the rotating configurations, for example to $[11Z\bar{Z}]$ from seed systems which were initially [(111,1)]. We also find conditions dictating how many equations of state may exist in a Drake-Szekeres system.

gr-qc↗

arXiv2021

Reionization in the Light of Dark Stars

We investigate the effect of Dark Stars (DSs) on the reionization history of the Universe, and the interplay between them and feedback due to Lyman-Werner (LW) radiation in reducing the Cosmic Microwave Background (CMB) optical depth to a value within the $τ= 0.054 \pm 0.007$ range measured by Planck. We use a semi-analytic approach to evaluate reionization histories and CMB optical depths, which includes Population II (Pop II) stars in atomic cooling halos and Pop III stars in minihalos with LW feedback, preceded by a DS phase. We show that while LW feedback by itself can reduce the integrated optical depth to the last scattering surface to $\sim 0.05$ only if the Pop III star formation efficiency is less than $\sim 0.2\%$, the inclusion of a population of DSs can naturally lead to the measured CMB optical depth for much larger Pop III star formation efficiencies $\gtrsim 1\%$.

astro-ph.CO↗

arXiv2021

The phenomenology of nuclear scattering for a WIMP of arbitrary spin

We provide a first systematic and quantitative discussion of the phenomenology of the non-relativistic effective Hamiltonian describing the nuclear scattering process for a Weakly Interacting Massive Particle (WIMP) of arbitrary spin $j_χ$. To this aim we obtain constraints from a representative sample of present direct detection experiments assuming the WIMP-nucleus scattering process to be driven by each one of the 44 effective couplings that arise for $j_χ\le$ 2. We find that a high value of the multipolarity $s\le 2 j_χ$ of the coupling, related to the power of the momentum transfer $q$ appearing in the scattering amplitude, leads to a suppression of the expected rates and pushes the expected differential spectra to large recoil energies $E_R$. For $s\le$ 4 the effective scales probed by direct detection experiments can be suppressed by up to 5 orders of magnitude compared to the case of a standard spin-independent interaction. For operators with large $s$ the expected differential spectra can be pushed to recoil energies in the MeV range, with the largest part of the signal concentrated at $E_R\gtrsim$ 100 keV and a peculiar structure of peaks and minima arising when both the nuclear target and the WIMP are heavy. As a consequence the present bounds on the effective operators can be significantly improved by extending the recoil energy intervals to higher recoil energies. Our analysis assumes effective interaction operators that are irreducible under the rotation group. Such operators drive the interactions of high-multipole dark matter candidates, i.e. states that possess only the highest multipole allowed by their spin. As a consequence our analysis represents also the first phenomenological study of the direct detection of quadrupolar, octupolar, and hexadecapolar dark matter.

hep-ph↗

arXiv2020

The Effects of Primordial Black Holes on Dark Matter Models

We investigate the effects of producing dark matter by Hawking evaporation of primordial black holes (PBHs) in scenarios that may have a second well-motivated dark matter production mechanism, such as freeze-out, freeze-in, or gravitational production. We show that the interplay between PBHs and the alternative sources of dark matter can give rise to model-independent modifications to the required dark matter abundance from each production mechanism, which in turn affect the prospects for dark matter detection. In particular, we demonstrate that for the freeze-out mechanism, accounting for evaporation of PBHs after freeze-out demands a larger annihilation cross section of dark matter particles than its canonical value for a thermal dark matter. For mechanisms lacking thermalization due to a feeble coupling to the thermal bath, we show that the PBH contribution to the dark matter abundance leads to the requirement of an even feebler coupling. Moreover, we show that when a large initial abundance of PBHs causes an early matter-dominated epoch, PBH evaporation alone cannot explain the whole abundance of dark matter today. In this case, an additional production mechanism is required, in contrast to the case when PBHs are formed and evaporate during a radiation-dominated epoch.

hep-ph↗

arXiv2020

The effective theory of nuclear scattering for a WIMP of arbitrary spin

We introduce a systematic approach to characterize the most general non-relativistic WIMP-nucleus interaction allowed by Galilean invariance for a WIMP of arbitrary spin $j_χ$ in the approximation of one-nucleon currents. Five nucleon currents arise from the nonrelativistic limit of the free nucleon Dirac bilinears. Our procedure consists in (1) organizing the WIMP currents according to the rank of the $2 j_χ+1$ irreducible operator products of up to $2 j_χ$ WIMP spin vectors, and (2) coupling each of the WIMP currents to each of the five nucleon currents. The transferred momentum $q$ appears to a power fixed by rotational invariance. For a WIMP of spin $j_χ$ we find a basis of 4+20$j_χ$ independent operators that exhaust all the possible operators that drive elastic WIMP-nucleus scattering in the approximation of one-nucleon currents. By comparing our operator basis, which is complete, to the operators already introduced in the literature we show that some of the latter for $j_χ=1$ were not independent and some were missing. We provide explicit formulas for the squared scattering amplitudes in terms of the nuclear response functions, which are available in the literature for most of the targets used in WIMP direct detection experiments.

hep-ph↗

arXiv2020

A curious general relativistic sphere

We present a particular solution to the Einstein field equations that could arise as a magnetic monopole in the strong field limit of the Born-Infeld electrodynamics. We solve its geodesics exactly and show that they can mimic the gravitational lensing of a singular isothermal sphere, but with matter and radiation following the same trajectories. This solution has previously been called "cloud of strings" or "string hedgehog," and has appeared as the limit of a global O(3) monopole.

gr-qc↗

arXiv2019

Formation of dark energy stars

Dark energy stars are finite size astrophysical objects with an interior equation of state typical of dark energy. Examples are self-gravitating false vacuum bubbles, vacuum non-singular black holes, and gravastars. We present a time-dependent solution of Einstein's field equations that describes the collapse of a spherical system from an initial state of positive pressure to a final state with a dark energy core. Our solution has no singularities, no event horizons, and does not violate the weak or null energy conditions.

gr-qc↗

arXiv2019

An exact time-dependent interior Schwarzschild solution

We present a time-dependent uniform-density interior Schwarzschild solution, an exact solution to the Einstein field equations. Our solution describes the collapse (or the time-reversed expansion) of an object from an infinite radius to an intermediate radius of 9/8 of the Schwarzschild radius, at which time a curvature singularity appears at the origin, and then continues beyond the singularity to a gravastar with radius equal to the Schwarzschild radius.

gr-qc↗

arXiv2018

Anapole Dark Matter after DAMA/LIBRA-phase2

We re-examine the case of anapole dark matter as an explanation for the DAMA annual modulation in light of the DAMA/LIBRA-phase2 results and improved upper limits from other DM searches. If the WIMP velocity distribution is assumed to be a Maxwellian, anapole dark matter is unable to provide an explanation of the DAMA modulation compatible with the other searches. Nevertheless, anapole dark matter provides a better fit to the DAMA-phase2 modulation data than an isoscalar spin-independent interaction, due to its magnetic coupling with sodium targets. A halo-independent analysis shows that explaining the DAMA modulation above 2 keVee in terms of anapole dark matter is basically impossible in face of the other null results, while the DAMA/LIBRA-phase2 modulation measurements below 2 keVee are marginally allowed. We conclude that in light of current measurements, anapole dark matter does not seem to be a viable explanation for the totality of the DAMA modulation.

hep-ph↗

arXiv2018

DarkSUSY 6 : An Advanced Tool to Compute Dark Matter Properties Numerically

The nature of dark matter remains one of the key science questions. Weakly Interacting Massive Particles (WIMPs) are among the best motivated particle physics candidates, allowing to explain the measured dark matter density by employing standard big-bang thermodynamics. We introduce here a radically new version of the widely used DarkSUSY package, which allows to compute the properties of such dark matter particles numerically. With DarkSUSY 6 one can accurately predict a large variety of astrophysical signals from dark matter, such as direct detection rates in low-background counting experiments and indirect detection signals through antiprotons, antideuterons, gamma rays and positrons from the Galactic halo, or high-energy neutrinos from the center of the Earth or of the Sun. For thermally produced dark matter like WIMPs, high-precision tools are provided for the computation of the relic density in the Universe today, as well as for the size of the smallest dark matter protohalos. Furthermore, the code allows to calculate dark matter self-interaction rates, which may affect the distribution of dark matter at small cosmological scales. Compared to earlier versions, DarkSUSY 6 introduces many significant physics improvements and extensions. The most fundamental new feature of this release, however, is that the code has been completely re-organized and brought into a highly modular and flexible shape. Switching between different pre-implemented dark matter candidates has thus become straight-forward, just as adding new - WIMP or non-WIMP - particle models or replacing any given functionality in a fully user-specified way. In this article, we describe the physics behind the computer package, along with the main structure and philosophy of this major revision of DarkSUSY. A detailed manual is provided together with the public release at www.darksusy.org.

hep-ph↗