arXiv subjects

Fabrizio Rompineve

Publications and source records attributed to Fabrizio Rompineve.

At least 19 recordsLinked to original sources

arXiv2026

Biased Domain Wall Networks and their Gravitational Waves

Cosmic Domain Wall networks are among the most interesting sources of a stochastic Gravitational Wave (GW) background from the early Universe. We present a thorough analysis of their annihilation, with a focus on scenarios where the collapse is induced by a population bias, whereby one of two degenerate vacua is initially preferred over the other. Our state-of-the-art $3+1$ lattice field theory simulations in the expanding Universe reveal that the network decays around the temperature $T_\text{ann}\sim T_s\,\mathcal{B}_s^{0.8}$, where $\mathcal{B}_s$ quantifies the preference for one vacuum over the other at the onset of the scaling regime at the temperature $T_s$. Furthermore, we obtain the spectrum of GWs from such networks, and provide a detailed comparison with the alternative potential bias annihilation mechanism that relies on a small explicit symmetry breaking in the potential. En passant, we update results on the evolution of these networks and on their GWs, and clarify existing disagreements in the recent literature. Our results sharpen the phenomenological viability of spontaneously broken discrete symmetries, and provide GW spectra that Pulsar Timing Arrays (PTAs) and ground-based interferometers (LIGO-Virgo-KAGRA) can readily use in their searches for a cosmological GW background.

astro-ph.CO↗

arXiv2025

The Spectrum of Gravitational Waves from Annihilating Domain Walls

Networks of cosmic domain walls can form in the early Universe as a consequence of the spontaneous breaking of discrete symmetries. We study the production of a cosmological background of gravitational waves (GWs) from such networks, when they annihilate due to a small explicit symmetry breaking term. Averaging over several 3+1-dimensional high-resolution lattice field simulations, we obtain a GW spectrum with the following characteristics: (1) a broad asymmetric peak, roughly located at frequency (at the time of emission) $f\sim 2 H_{\rm gw}$, where $H_{\rm gw}$ is the Hubble rate at the end of GW production, shortly after annihilation, (2) a doubly broken power law spectrum $\propto k^{-n}$, with initial slope $n \sim 0.5$ after the main peak and $n \sim 1.8$ at high $f$, while the low frequency region $f<f_p$ agrees with the causality behavior $\sim k^3$. Additionally, extending previous results, we find that GW production continues to be efficient until a value of the Hubble scale $H_{\text gw}$ that is roughly an order of magnitude smaller than the naive estimate $σH = ΔV$, where $σ$ is the wall tension and $ΔV$ the size of the symmetry breaking term, thereby leading to a $O(100)$ larger GW signal. We find such results to be robust when changing the shape of the scalar field potential or including a time-dependent symmetry breaking term. Our findings have important implications for GW searches, especially in light of the reported evidence for a stochastic GW background in Pulsar Timing Array data.

astro-ph.CO↗

arXiv2025

Stupendously Large Primordial Black Holes from the QCD axion

The inflationary diffusion of (pseudo-)scalar fields with discrete symmetries can seed the formation of a gas of closed domain walls after inflation, when the distance between degenerate minima in field space is not too far from the inflationary Hubble scale. Primordial black holes (PBHs) can then be formed once sufficiently heavy domain walls re-enter the Hubble sphere. In this scenario, inflation determines a distinctive PBH mass distribution that is rather flat and can thus lead to a sizable total abundance of PBHs, while avoiding some of the downsides of PBH formation from critical collapse. We show that generic QCD axion models, with decay constant close to the inflationary Hubble scale, can yield up to $1\%$ of the dark matter (DM) today in the form of PBHs, while being compatible with isocurvature constraints from Cosmic Microwave Background observations. This occurs for values of axion decay constants around $f_a\simeq 10^{8}~\text{GeV}$, that is the region targeted by axion helioscopes and partially constrained by astrophysical observations. The resulting PBHs have \textit{stupendously} large masses, above $10^{11}M_\odot$, and their existence can be probed by Large Scale Structure observations. Larger PBH abundances can be generated by axion-like particles. Alternatively, in scenarios where isocurvature constraints can be relaxed, we find that the totality of the DM can be produced by the QCD axion misalignment mechanism, accompanied by a ${\cal O}(10^{-3})$ DM fraction in PBHs of masses $(10^5-10^6)~M_\odot$. These can act as seeds for the formation of massive black holes at large redshifts, as suggested by recent JWST observations.

astro-ph.CO↗

arXiv2025

Reduced Hubble Tension in Dark Radiation Models after DESI 2024

We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos $N_\text{eff}$, as $ΔN_\text{eff}\equiv N_\text{eff}-3.044\geq 0$, in light of the recent measurements of Baryon Acoustic Oscillations (BAO) by the DESI collaboration. We analyze one-parameter extensions of the $Λ$CDM model where dark radiation (DR) is free streaming or behaves as a perfect fluid, due to self-interactions. We report a significant relaxation of upper bounds on $ΔN_\text{eff}$, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting $ΔN_\text{eff}\leq 0.39$ ($95\%$ C.L.) for free streaming DR, and a very mild preference for fluid DR, $ΔN_\text{eff} = 0.221^{+0.088}_{-0.18}$ ($\leq 0.46$, $95\%$ C.L.). Applying constraints from primordial element abundances leads to slightly tighter constraints on $ΔN_\text{eff}$, but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH$_0$ES determination of $H_0$ to be less than $3σ$ and as low as $2σ$, and for free-streaming DR the tension is below $3σ$ if production occurs after BBN. This lesser degree of tension motivates a combination with SH$_0$ES in these cases, resulting in a $4.4σ-5σ$ evidence for dark radiation with $ΔN_\text{eff}\simeq 0.6$ and large improvements in $χ^2$ over $Λ$CDM, $-18\lesssim Δχ^2\lesssim -25$. Upcoming data releases by DESI and other CMB and LSS surveys will decisively confirm or disfavour this conclusion.

astro-ph.CO↗

arXiv2024

Primordial gravitational wave backgrounds from phase transitions with next generation ground based detectors

Third generation ground-based gravitational wave (GW) detectors, such as Einstein Telescope and Cosmic Explorer, will operate in the $(\text{few}-10^4)$ Hz frequency band, with a boost in sensitivity providing an unprecedented reach into primordial cosmology. Working concurrently with pulsar timing arrays in the nHz band, and LISA in the mHz band, these 3G detectors will be powerful probes of beyond the standard model particle physics on scales $T\gtrsim 10^{7}$GeV. Here we focus on their ability to probe phase transitions (PTs) in the early universe. We first overview the landscape of detectors across frequencies, discuss the relevance of astrophysical foregrounds, and provide convenient and up-to-date power-law integrated sensitivity curves for these detectors. We then present the constraints expected from GW observations on first order PTs and on topological defects (strings and domain walls), which may be formed when a symmetry is broken irrespective of the order of the phase transition. These constraints can then be applied to specific models leading to first order PTs and/or topological defects. In particular we discuss the implications for axion models, which solve the strong CP problem by introducing a spontaneously broken Peccei-Quinn (PQ) symmetry. For post-inflationary breaking, the PQ scale must lie in the $10^{8}-10^{11}$ GeV range, and so the signal from a first order PQ PT falls within reach of ground based 3G detectors. A scan in parameter space of signal-to-noise ratio in a representative model reveals their large potential to probe the nature of the PQ transition. Additionally, in heavy axion type models domain walls form, which can lead to a detectable GW background. We discuss their spectrum and summarise the expected constraints on these models from 3G detectors, together with SKA and LISA.

astro-ph.CO↗

arXiv2024

Footprints of the QCD Crossover on Cosmological Gravitational Waves at Pulsar Timing Arrays

Pulsar Timing Arrays (PTAs) have reported evidence for a stochastic gravitational wave (GW) background at nHz frequencies, possibly originating in the early Universe. We show that the spectral shape of the low-frequency (causality) tail of GW signals sourced at temperatures around $T\gtrsim 1$ GeV is distinctively affected by confinement of strong interactions (QCD), due to the corresponding sharp decrease in the number of relativistic species. Bayesian analyses in the NANOGrav 15 years and the previous International PTA datasets reveal a significant improvement in the fit with respect to cubic power-law spectra, previously employed for the causality tail. This suggests that the inclusion of Standard Model effects on GWs can have a potentially decisive impact on model selection.

astro-ph.CO↗

arXiv2024

Collapsing Domain Wall Networks: Impact on Pulsar Timing Arrays and Primordial Black Holes

Unstable domain wall (DW) networks in the early universe are cosmologically viable and can emit a large amount of gravitational waves (GW) before annihilating. As such, they provide an interpretation for the recent signal reported by Pulsar Timing Array (PTA) collaborations. A related important question is whether such a scenario also leads to significant production of Primordial Black Holes (PBH). We investigate both GW and PBH production using 3D numerical simulations in an expanding background, with box sizes up to $N=3240$, including the annihilation phase. We find that: i) the network decays exponentially, i.e. the false vacuum volume drops as $\sim \exp(-η^3)$, with $η$ the conformal time; ii) the GW spectrum is larger than traditional estimates by more than one order of magnitude, due to a delay between DW annihilation and the sourcing of GWs. We then present a novel semi-analytical method to estimate the PBH abundances: rare false vacuum pockets of super-Hubble size collapse to PBHs if their energy density becomes comparable to the background when they cross the Hubble scale. Smaller (but more abundant) pockets will instead collapse only if they are close to spherical. This introduces very large uncertainties in the final PBH abundance. The first phenomenological implication is that the DW interpretation of the PTA signal is compatible with observational constraints on PBHs, within the uncertainties. Second, in a different parameter region, the dark matter can be entirely in the form of asteroid-mass PBHs from the DW collapse. Remarkably, this would also lead to a GW background in the observable range of LIGO-Virgo-KAGRA and future interferometers, such as LISA and Einstein Telescope.

astro-ph.CO↗

arXiv2023

Search for scalar induced gravitational waves in the International Pulsar Timing Array Data Release 2 and NANOgrav 12.5 years datasets

We perform a Bayesian search in the latest Pulsar Timing Array (PTA) datasets for a stochastic gravitational wave (GW) background sourced by curvature perturbations at scales $10^5~\text{Mpc}^{-1}\lesssim k\lesssim 10^8~\text{Mpc}^{-1}$. These re-enter the Hubble horizon at temperatures around and below the QCD crossover phase transition in the early Universe. We include a stochastic background of astrophysical origin in our search and properly account for constraints on the curvature power spectrum from the overproduction of primordial black holes (PBHs). We find that the International PTA Data Release 2 significantly favors the astrophysical model for its reported common-spectrum process, over the curvature-induced background. On the other hand, the two interpretations fit the NANOgrav 12.5 years dataset equally well. We then set new upper limits on the amplitude of the curvature power spectrum at small scales. These are independent from, and competitive with, indirect astrophysical bounds from the abundance of PBH dark matter. Upcoming PTA data releases will provide the strongest probe of the curvature power spectrum around the QCD epoch.

astro-ph.CO↗

arXiv2023

Dark Sectors with Mass Thresholds Face Cosmological Datasets

Interacting dark sectors may undergo changes in the number of their relativistic species during the early universe, due to a mass threshold $m$ (similar to changes in the Standard Model bath), and in doing so affect the cosmic history. When such changes occur close to recombination, i.e., for $m\sim (0.1-10)~\text{eV}$, the stringent bound on the effective number of neutrino species, $N_{\text{eff}}$, can be relaxed and the value of the Hubble expansion rate $H_0$ inferred from Cosmic Microwave Background (CMB) observations raised. We search for such sectors (with and without mass thresholds) in the latest cosmological datasets, including the full-shape (FS) of BOSS DR12 galaxy power spectrum. We perform a detailed analysis, accounting for the choice of prior boundaries and additionally exploring the possible effects of dark sector interactions with (a fraction of) the dark matter. We find $ΔN_{\text{eff}}\leq 0.55\, (0.46)$ at 95% C.L. with (without) a mass threshold. While a significantly larger Hubble rate is achieved in this scenario, $H_0=69.01^{+0.66}_{-1.1}$, the overall fit to CMB+FS data does not provide a compelling advantage over the $Λ$CDM model. Furthermore, we find that dark matter interactions with the dark sector do not significantly improve the (matter fluctuations) $S_8$ tension with respect to the $Λ$CDM model. Our work provides model-independent constraints on (decoupled) dark sectors with mass thresholds around the eV scale.

astro-ph.CO↗

arXiv2023

Gravitational Waves from Domain Walls in Pulsar Timing Array Datasets

We present a model-independent search for the gravitational wave background from cosmic domain walls (DWs) in the NANOGrav 12.5 years dataset and International PTA Data Release 2. DWs that annihilate at temperatures $\sim 20-50~\text{MeV}$ with tensions $\sim (40-100~\text{TeV})^3$ provide as good a fit to both datasets as the astrophysical background from supermassive black hole mergers. DWs may decay into the Standard Model (SM) or a dark sector. In the latter case we predict an abundance $ΔN_{\text{eff}}$ of dark radiation well within the reach of upcoming CMB surveys. Complementary signatures at colliders and laboratories can arise if couplings to the SM are present. As an example, we discuss heavy axion scenarios, where DW annihilation may interestingly be induced by QCD confinement.

astro-ph.CO↗

arXiv2022

Improved hot dark matter bound on the QCD axion

We strengthen the cosmological bound on the axion mass, by solving the momentum-dependent Boltzmann equations for axion-pion scatterings and by using a phenomenological production rate derived from pion-pion scattering data, overcoming the breakdown of chiral perturbation theory. Using present cosmological datasets we obtain $m_a\leq 0.24~\text{eV}$. To further improve the bound and exploit the reach of upcoming cosmological surveys, reliable non-perturbative calculations above the QCD crossover are needed.

hep-ph↗

arXiv2022

High Quality QCD Axion at Gravitational Wave Observatories

The axion solution to the strong CP problem is delicately sensitive to Peccei-Quinn breaking contributions that are misaligned with respect to QCD instantons. Heavy QCD axion models are appealing because they avoid this so-called "quality problem". We show that generic realizations of this framework can be probed by the LIGO-Virgo-KAGRA interferometers, through the stochastic gravitational wave (GW) signal sourced by the long-lived axionic string-domain wall network, and by upcoming measurements of the neutron and proton Electric Dipole Moments. Additionally, we provide predictions for searches at future GW observatories, which will further explore the parameter space of heavy QCD axion models.

hep-ph↗

arXiv2022

Dark Sector to Restore Cosmological Concordance

We develop a new phenomenological model that addresses current tensions between observations of the early and late Universe. Our scenario features: (i) a decaying dark energy fluid (DDE), which undergoes a transition at $z \sim 5,000$, to raise today's value of the Hubble parameter -- addressing the $H_0$ tension, and (ii) an ultra-light axion (ULA), which starts oscillating at $z\gtrsim 10^4$, to suppress the matter power spectrum -- addressing the $S_8$ tension. Our Markov Chain Monte Carlo analyses show that such a Dark Sector model fits a combination of Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations, and Large Scale Structure (LSS) data slightly better than the $Λ$CDM model, while importantly reducing both the $H_0$ and $S_8$ tensions with late universe probes ($\lesssim 3σ$). Combined with measurements from cosmic shear surveys, we find that the discrepancy on $S_8$ is reduced to the $1.4σ$ level, and the value of $H_0$ is further raised. Adding local supernovae measurements, we find that the $H_0$ and $S_8$ tensions are reduced to the $1.4σ$ and $1.2σ$ level respectively, with a significant improvement $Δχ^2\simeq -18$ compared to the $Λ$CDM model. With this complete dataset, the DDE and ULA are detected at $\simeq 4σ$ and $\simeq2σ$, respectively. We discuss a possible particle physics realization of this model, with a dark confining gauge sector and its associated axion, although embedding the full details within microphysics remains an urgent open question. Our scenario will be decisively probed with future CMB and LSS surveys.

astro-ph.CO↗

arXiv2021

Decay of Boson Stars with Application to Glueballs and Other Real Scalars

One of the most interesting candidates for dark matter are massive real scalar particles. A well-motivated example is from a pure Yang-Mills hidden sector, which locks up into glueballs in the early universe. The lightest glueball states are scalar particles and can act as a form of bosonic dark matter. If self-interactions are repulsive this can potentially lead to very massive boson stars, where the inward gravitational force is balanced by the repulsive self-interaction. This can also arise from elementary real scalars with a regular potential. In the literature it has been claimed that this allows for astrophysically significant boson stars with high compactness, which could undergo binary mergers and generate detectable gravitational waves. Here we show that previous analyses did not take into proper account $3 \to 2$ and $4 \to 2$ quantum mechanical annihilation processes in the core of the star, while other work misinterpreted the classical $3 \to 1$ process. In this work, we compute the annihilation rates, finding that massive stars will rapidly decay from the $3 \to 2$ or $4 \to 2$ processes (while the $3 \to 1$ process is typically small). Using the Einstein-Klein-Gordon equations, we also estimate the binding energy of these stars, showing that even the densest stars do not have quite enough binding energy to prevent annihilations. For such boson stars to live for the current age of the universe and to be consistent with bounds on dark matter scattering in galaxies, we find the following upper bound on their mass for $O(1)$ self-interaction couplings: $M_*<10^{-18}M_{sun}$ when $3 \to 2$ processes are allowed and $M_*<10^{-11}M_{sun}$ when only $4 \to 2$ processes are allowed. We also estimate destabilization from parametric resonance which can considerably constrain the phase space further. Furthermore, such stars are required to have very small compactness to be long lived.

hep-ph↗

arXiv2021

Recipes for Oscillon Longevity

Oscillons are localized states of scalar fields sustained by self interactions. They decay by emitting classical radiation, but their lifetimes are surprisingly large. We revisit the reasons behind their longevity, aiming at how the shape of the scalar potential $V(ϕ)$ determines the lifetime. The corpuscular picture, where the oscillon is identified with a bound state of a large number of field quanta, allows to understand lifetimes of order of $10^3$ cycles in generic potentials. At the non-perturbative level, two properties of the scalar potential can substantially boost the lifetime: the flattening of $V(ϕ)$ and the positivity of $V''(ϕ)$. These properties are realized in the axion monodromy family of potentials. Moreover, this class of models connects continuously with an exceptional potential that admits eternal oscillon solutions. We check these results with a new fast-forward numerical method that allows to evolve in time to stages that cannot be otherwise simulated on a computer. The method exploits the attractor properties of the oscillons and fully accounts for nonlinearities. We find lifetimes up to $10^{14}$ cycles, but larger values are possible. Our work shows that oscillons formed in the early Universe can be stable on cosmological time scales and thus contribute to the abundance of (ultra)light scalar dark matter.

hep-ph↗

arXiv2020

The DFSZ axion in the CMB

We perform for the first time a dedicated analysis of cosmological constraints on DFSZ QCD axion models. Such constructions are especially interesting in light of the recent Xenon-1T excess and of hints from stellar cooling. In DFSZ models, for $m_a\gtrsim 0.1$ eV, scatterings of pions and muons can produce a sizable cosmic background of thermal axions, that behave similarly to massive neutrinos. However, the pion coupling depends on the alignment between the vevs of two Higgs doublets, and can be significantly suppressed or enhanced with respect to the KSVZ scenario. Using the latest Planck and BAO data, we find $m_a\leq 0.2~\text{eV}$ at $95\%$ C.L., when the axion coupling to pions $c_{aπ}$ is maximal. Constraints on $m_a$, instead, can be significantly relaxed when $c_{aπ}$ is small. In particular, we point out that in the so-called DFSZ-II model, where the axion coupling to leptons does not vanish simultaneously with $c_{aπ}$, production via muons gives $m_a\leq 0.6~\text{eV}$ at $95\%$ C.L., whereas in the DFSZ-I model bounds on $m_a$ can be fully lifted. We then combine cosmological data with recent hints of a DFSZ axion coupled to electrons from the Xenon-1T experiment, finding in this case that the axion mass is constrained to be in the window $0.07 ~\text{eV} \lesssim m_a \lesssim 1.8\, (0.3)~\text{eV}$ for the DFSZ-I (DFSZ-II) model. A similar analysis with stellar cooling hints gives $3 ~\text{meV} \lesssim m_a \lesssim 0.2 ~\text{eV}$ for DFSZ-II, while no constraint arises in the DFSZ-I case. Forthcoming CMB Stage 4 experiments will be able to further test such scenarios; for instance the Xenon-1T window should be fully probed at $2σ$ for a DFSZ-I axion.

hep-ph↗

arXiv2020

The $H_0$ tension: $ΔG_N$ vs. $ΔN_{\rm eff}$

We investigate whether the $4.4σ$ tension on $H_0$ between SH$_{0}$ES 2019 and Planck 2018 can be alleviated by a variation of Newton's constant $G_N$ between the early and the late Universe. This changes the Hubble rate before recombination, similarly to adding $ΔN_{\rm eff}$ extra relativistic degrees of freedom. We implement a varying $G_N$ in a scalar-tensor theory of gravity, with a non-minimal coupling $(M^2+βϕ^2)R$. If the scalar $ϕ$ starts in the radiation era at an initial value $ϕ_I \sim 0.5~M_p$ and with $β<0$, a dynamical transition occurs naturally around the epoch of matter-radiation equality and the field evolves towards zero at late times. As a consequence, the $H_0$ tension between SH$_{0}$ES (2019) and Planck 2018+BAO slightly decreases, as in $ΔN_{\rm eff}$ models, to the 3.8$σ$ level. We then perform a fit to a combined Planck, BAO and supernovae (SH$_0$ES and Pantheon) dataset. When including local constraints on Post-Newtonian (PN) parameters, we find $H_0=69.08_{-0.71}^{+0.6}~\text{km/s/Mpc}$ and a marginal improvement of $Δχ^2\simeq-3.2$ compared to $Λ$CDM, at the cost of 2 extra parameters. In order to take into account scenarios where local constraints could be evaded, we also perform a fit without PN constraints and find $H_0=69.65_{-0.78}^{+0.8}~\text{km/s/Mpc}$ and a more significant improvement $Δχ^2=-5.4$ with 2 extra parameters. For comparison, we find that the $ΔN_{\rm eff}$ model gives $H_0=70.08_{-0.95}^{+0.91}~\text{km/s/Mpc}$ and $Δχ^2=-3.4$ at the cost of one extra parameter, which disfavors the $Λ$CDM limit just above 2$σ$, since $ΔN_{\rm eff}=0.34_{-0.16}^{+0.15}$. Overall, our varying $G_N$ model performs similarly to the $ΔN_{\rm eff}$ model in respect to the $H_0$ tension, if a physical mechanism to remove PN constraints can be implemented.

astro-ph.CO↗

arXiv2020

Quantitative Analysis of the Stochastic Approach to Quantum Tunneling

Recently there has been increasing interest in alternate methods to compute quantum tunneling in field theory. Of particular interest is a stochastic approach which involves (i) sampling from the free theory Gaussian approximation to the Wigner distribution in order to obtain stochastic initial conditions for the field and momentum conjugate, then (ii) evolving under the classical field equations of motion, which leads to random bubble formation. Previous work showed parametric agreement between the logarithm of the tunneling rate in this stochastic approach and the usual instanton approximation. However, recent work [1] claimed excellent agreement between these methods. Here we show that this approach does not in fact match precisely; the stochastic method tends to overpredict the instanton tunneling rate. To quantify this, we parameterize the standard deviations in the initial stochastic fluctuations by $εσ$, where $σ$ is the actual standard deviation of the Gaussian distribution and $ε$ is a fudge factor; $ε= 1$ is the physical value. We numerically implement the stochastic approach to obtain the bubble formation rate for a range of potentials in 1+1-dimensions, finding that $ε$ always needs to be somewhat smaller than unity to suppress the otherwise much larger stochastic rates towards the instanton rates; for example, in the potential of [1] one needs $ε\approx 1/2$. We find that a mismatch in predictions also occurs when sampling from other Wigner distributions, and in single particle quantum mechanics even when the initial quantum system is prepared in an exact Gaussian state. If the goal is to obtain agreement between the two methods, our results show that the stochastic approach would be useful if a prescription to specify optimal fudge factors for fluctuations can be developed.

hep-th↗