Fabrizio Rompineve

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 ≃ 10 ⁸ GeV, that is the region targeted by axion helioscopes and partially constrained by astrophysical observations. The resulting PBHs have stupendously large masses, above 10 ¹¹ M _⊙ , 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 𝒪(10 ^-3 ) DM fraction in PBHs of masses (10 ⁵ -10 ⁶ ) M _⊙ . These can act as seeds for the formation of massive black holes at large redshifts, as suggested by recent JWST observations.

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 ∼ 2H _gw, where H _gw is the Hubble rate at the end of GW production, shortly after annihilation, (2) a doubly broken power law spectrum ∝ k^-n , with initial slope n ∼ 0.5 after the main peak and n ∼ 1.8 at high f, while the low frequency region f < f_p agrees with the causality behavior ∼ k ³. Additionally, extending previous results, we find that GW production continues to be efficient until a value of the Hubble scale H _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.

We investigate the presence of extra relativistic degrees of freedom in the early Universe, contributing to the effective number of neutrinos N _eff, as Δ N _eff ≡ N _eff-3.044 ≥ 0, in light of the recent measurements of Baryon Acoustic Oscillations (BAO) by the DESI collaboration. We analyze one-parameter extensions of the ΛCDM 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 Δ N _eff, with respect to previous BAO data from SDSS+6dFGS, when additionally employing Planck data (and supernovae data from Pantheon+), setting Δ N _eff ≤ 0.39 (95% C.L.) for free streaming DR, and a very mild preference for fluid DR, Δ N _eff = 0.221^+0.088 _-0.18 (≤ 0.46, 95% C.L.). Applying constraints from primordial element abundances leads to tighter constraints on Δ N _eff, but they are avoided if DR is produced after Big Bang Nucleosynthesis (BBN). For fluid DR we estimate the tension with the SH₀ES determination of H₀ 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 SH₀ES in these cases, resulting in a 4.4σ-5σ evidence for dark radiation with Δ N _eff ≃ 0.6 and large improvements in χ ² over ΛCDM, -18 ≲ Δχ ² ≲ -25. Upcoming data releases by DESI and other CMB and LSS surveys will decisively confirm or disfavour this conclusion.

Third generation ground-based gravitational wave (GW) detectors, such as Einstein Telescope and Cosmic Explorer, will operate in the ( 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 ≳ 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 LISA⁷ 7 Invited review for a CQG Focus Issue on the science case for next generation ground based GW detectors. .

This corrects the article DOI: 10.1103/PhysRevLett.132.081001.

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 10 5 Mpc ^{-1}\lesssim − 1 ≲ k \lesssim 10^8 ≲ 10 8 Mpc ^{-1} − 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.