arXiv subjects

G. Franciolini

Publications and source records attributed to G. Franciolini.

At least 19 recordsLinked to original sources

arXiv2026

Isocurvature Constraints on Dark Matter from Evaporated Primordial Black Holes

We revisit the scenario in which stable particles of a dark sector are produced through the complete evaporation of light primordial black holes (PBHs) formed in the early Universe. We investigate in detail the role of isocurvature perturbations that may arise in this framework. PBHs inherit Poisson fluctuations on unobservable small scales at formation; however, in the presence of primordial non-Gaussianity that couples long- and short-wavelength modes, these fluctuations can source isocurvature perturbations on cosmological scales. Such perturbations are unavoidably transferred to the dark sector particles emitted via Hawking evaporation. We highlight the potential impact of isocurvature constraints on dark sector particles produced through PBH evaporation. Along the way, we re-assess the constraints on this scenario arising from the overproduction of dark matter (DM), accounting for both PBH evaporation and gravitational production (freeze-in) during (after) inflation, as well as bounds from warm DM and the overproduction of scalar-induced gravitational waves.

astro-ph.CO↗

arXiv2026

Dark Matter from Eternity

We propose that the totality of dark matter in the universe might ascribe its origin to one of the key properties of cosmological inflation, that it may be eternal: regions that at the end of the primordial accelerated expansion of the universe never reheated, but keep eternally inflating, manifest themselves as primordial black holes in our observable universe. This mechanism can provide a primordial black hole abundance which is larger than the standard one due to the gravitational collapse of sizeable overdensities in the radiation phase. It also predicts a broad spectrum for the curvature perturbation and a flat stochastic gravitational wave background at a level of $\Omega_\text{GW} h^2 \simeq 10^{-10}$ up to the mHz.

astro-ph.CO↗

arXiv2026

Standard Model Higgs Peaks: a Note on the Vacuum Instability during Inflation

In the Standard Model, the Higgs potential develops an instability at high field values when the quartic self-coupling runs negative. Large quantum fluctuations during cosmic inflation could drive the Higgs field beyond the potential barrier, creating regions that would be catastrophic for our observable universe. We point out that the extreme-value statistics describing the peaks (maxima) of the Higgs values is the correct statistics to infer the condition to avoid vacuum instability. Even if this statistics delivers a bound on the Hubble rate during inflation which is only a factor $\sqrt{2}$ stronger than the one commonly adopted in the literature, it is qualitatively distinct and we believe worthwhile communicating it.

hep-ph↗

arXiv2025

Uncovering the population of compact binary mergers and their formation pathways with gravitational waves through the Einstein Telescope

Ground-based gravitational-wave (GW) observatories have transformed our view of compact-object mergers, yet their reach still limits a comprehensive reconstruction of the processes that generate these systems. Only next-generation observatories, with order-of-magnitude improvements in sensitivity and access to lower frequencies, will be capable of radically extending this detection horizon. GW observations will make it possible to detect the complete population of binary black hole (BBH) mergers out to redshifts of $z \simeq 100$. This capability will deliver an unprecedented map of merger events across cosmic time and enable precise reconstruction of their mass and spin distributions, while for several thousand events the signal-to-noise ratio will surpass 100, enabling precision physics of BHs and neutron stars (NSs). The access to lower frequencies will also open the intermediate-mass window, detecting systems of order $\sim 10^3 M_\odot$, potentially in coordination with multi-band observations from LISA. At higher redshifts, where Population III stars have so far remained beyond reach - even for the James Webb Space Telescope - GW observations by next-generation detectors will routinely provide observations of BH mergers thought to originate from these primordial stellar populations. Such measurements are expected to play a central role in clarifying the early assembly of supermassive black holes. A single detection of a binary BH system at $z \gtrsim 30$, or of a compact object with sub-solar mass and no tidal deformability, would constitute strong evidence for the existence of primordial black holes. Such a discovery would have profound consequences for our understanding of dark matter and the early Universe. Ultimately, the GW observations will become revolutionary for identifying the physical channels responsible for compact binary formation.

astro-ph.HE↗

arXiv2025

The Irrelevance of Primordial Black Hole Clustering in the LVK mass range

We show that in realistic models where primordial black holes are formed due to the collapse of sizeable inflationary perturbations, their initial spatial clustering beyond Poisson distribution does not play any role in the binary mergers, including sub-solar primordial black holes, responsible for the gravitational waves detectable by LIGO-Virgo-KAGRA. This is a consequence of the existing FIRAS CMB distortion constraints on the relevant scales. This conclusion might not hold for lighter masses potentially accessible by future gravitational wave observations.

astro-ph.CO↗

arXiv2024

Can we identify primordial black holes? Tidal tests for subsolar-mass gravitational-wave observations

The detection of a subsolar object in a compact binary merger is regarded as one of the smoking gun signatures of a population of primordial black holes~(PBHs). We critically assess whether these systems could be distinguished from stellar binaries, for example composed of white dwarfs or neutron stars, which could also populate the subsolar mass range. At variance with PBHs, the gravitational-wave signal from stellar binaries is affected by tidal effects, which dramatically grow for moderately compact stars as those expected in the subsolar range. We forecast the capability of constraining tidal effects of putative subsolar neutron star binaries with current and future LIGO-Virgo-KAGRA (LVK) sensitivities as well as next-generation experiments. We show that, should LVK O4 run observe subsolar neutron-star mergers, it could measure the (large) tidal effects with high significance. In particular, for subsolar neutron-star binaries, O4 and O5 projected sensitivities would allow measuring the effect of tidal disruption on the waveform in a large portion of the parameter space, also constraining the tidal deformability at ${\cal O}(10\%)$ level, thus excluding a primordial origin of the binary. Vice versa, for subsolar PBH binaries, model-agnostic upper bounds on the tidal deformability can rule out neutron stars or more exotic competitors. Assuming events similar to the subthreshold candidate SSM200308 reported in LVK O3b data are PBH binaries, O4 projected sensitivity would allow ruling out the presence of neutron-star tidal effects at $\approx 3 \sigma$ C.L., thus strengthening the PBH hypothesis. Future experiments would lead to even stronger ($>5\sigma$) conclusions on potential discoveries of this kind.

astro-ph.HE↗

arXiv2023

Stochastic gravitational-wave background at 3G detectors as a smoking gun for microscopic dark matter relics

Microscopic horizonless relics could form in the early universe either directly through gravitational collapse or as stable remnants of the Hawking evaporation of primordial black holes. In both cases they completely or partially evade cosmological constraints arising from Hawking evaporation and in certain mass ranges can explain the entirety of the dark matter. We systematically explore the stochastic gravitational-wave background associated with the formation of microscopic dark-matter relics in various scenarios, adopting an agnostic approach and discussing the limitations introduced by existing constraints, possible ways to circumvent the latter, and expected astrophysical foregrounds. Interestingly, this signal is at most marginally detectable with current interferometers but could be detectable by third-generations instruments such as the Einstein Telescope, strengthening their potential as discovery machines.

astro-ph.CO↗

arXiv2023

Fermion soliton stars

A real scalar field coupled to a fermion via a Yukawa term can evade no-go theorems preventing solitonic solutions. For the first time, we study this model within General Relativity without approximations, finding static and spherically symmetric solutions that describe fermion soliton stars. The Yukawa coupling provides an effective mass for the fermion, which is key to the existence of self-gravitating relativistic solutions. We systematically study this novel family of solutions and present their mass-radius diagram and maximum compactness, which is close to (but smaller than) that of the corresponding Schwarzschild photon sphere. Finally, we discuss the ranges of the parameters of the fundamental theory in which the latter might have interesting astrophysical implications, including compact (sub)solar and supermassive fermion soliton stars for a standard gas of degenerate neutrons and electrons, respectively.

gr-qc↗

arXiv2023

Searching for Primordial Black Holes with the Einstein Telescope: impact of design and systematics

Primordial Black Holes (PBHs) have recently attracted much attention as they may explain some of the LIGO/Virgo/KAGRA observations and significantly contribute to the dark matter in our universe. The next generation of Gravitational Wave (GW) detectors will have the unique opportunity to set stringent bounds on this putative population of objects. Focusing on the Einstein Telescope (ET), in this paper we analyse in detail the impact of systematics and different detector designs on our future capability of observing key quantities that would allow us to discover and/or constrain a population of PBH mergers. We also perform a population analysis, with a mass and redshift distribution compatible with the current observational bounds. Our results indicate that ET alone can reach an exquisite level of accuracy on the key observables considered, as well as detect up to tens of thousands of PBH binaries per year, but for some key signatures (in particular high--redshift sources) the cryogenic instrument optimised for low frequencies turns out to be crucial, both for the number of observations and the error on the parameters reconstruction. As far as the detector geometry is concerned, we find that a network consisting of two separated L--shaped interferometers of 15 (20)~km arm length, oriented at $45^{\circ}$ with respect to each other performs better than a single triangular shaped instrument of 10 (15)~km arm length, for all the metrics considered.

gr-qc↗

arXiv2023

From inflation to black hole mergers and back again: Gravitational-wave data-driven constraints on inflationary scenarios with a first-principle model of primordial black holes across the QCD epoch

Recent population studies have searched for a subpopulation of primordial black holes (PBHs) in the gravitational-wave (GW) events so far detected by LIGO/Virgo/KAGRA (LVK), in most cases adopting a phenomenological PBH mass distribution. When deriving such population from first principles in the standard scenario, however, the equation of state of the Universe at the time of PBH formation may strongly affect the PBH abundance and mass distribution, which ultimately depend on the power spectrum of cosmological perturbations. Here we improve on previous population studies on several aspects: (i) we adopt state-of-the-art PBH formation models describing the collapse of cosmological perturbations across the QCD epoch; (ii) we perform the first Bayesian multi-population inference on GW data including PBHs and directly using power spectrum parameters instead of phenomenological distributions; (iii) we critically confront the PBH scenario with LVK phenomenological models describing the GWTC-3 catalog both in the neutron-star and in the BH mass ranges, also considering PBHs as subpopulation of the total events. Our results confirm that LVK observations prevent the majority of the dark matter to be in the form of stellar mass PBHs. We find that the best fit PBH model can comprise a small fraction of the total events, in particular it can naturally explain events in the mass gaps. If the lower mass-gap event GW190814 is interpreted as a PBH binary, we predict that LVK should detect up to a few subsolar mergers and one to $\approx 30$ lower mass gap events during the upcoming O4 and O5 runs. Finally, mapping back the best-fit power spectrum into an ultra slow-roll inflationary scenario, we show that the latter predicts detectable PBH mergers in the LVK band, a stochastic GW background detectable by current and future instruments, and may include the entirety of dark matter in asteroid-mass PBHs.

astro-ph.CO↗

arXiv2023

High-redshift JWST Observations and Primordial Non-Gaussianity

Several bright and massive galaxy candidates at high redshifts have been recently observed by the James Webb Space Telescope. Such early massive galaxies seem difficult to reconcile with standard $Λ$ Cold Dark Matter model predictions. We discuss under which circumstances such observed massive galaxy candidates can be explained by introducing primordial non-Gaussianity in the initial conditions of the cosmological perturbations.

astro-ph.CO↗

arXiv2023

Lensing constraints on ultradense dark matter halos

Cosmological observations precisely measure primordial variations in the density of the Universe at megaparsec and larger scales, but much smaller scales remain poorly constrained. However, sufficiently large initial perturbations at small scales can lead to an abundance of ultradense dark matter minihalos that form during the radiation epoch and survive into the late-time Universe. Because of their early formation, these objects can be compact enough to produce detectable microlensing signatures. We investigate whether the EROS, OGLE, and HSC surveys can probe these halos by fully accounting for finite source size and extended lens effects. We find that current data may already constrain the amplitudes of primordial curvature perturbations in a new region of parameter space, but this conclusion is strongly sensitive to yet undetermined details about the internal structures of these ultradense halos. Under optimistic assumptions, current and future HSC data would constrain a power spectrum that features an enhancement at scales $k \sim 10^7/{\rm Mpc}$, and an amplitude as low as $\mathcal{P}_\zeta\simeq 10^{-4}$ may be accessible. This is a particularly interesting regime because it connects to primordial black hole formation in a portion of the LIGO/Virgo/Kagra mass range and the production of scalar-induced gravitational waves in the nanohertz frequency range reachable by pulsar timing arrays. These prospects motivate further study of the ultradense halo formation scenario to clarify their internal structures.

astro-ph.CO↗

arXiv2022

Ruling out Initially Clustered Primordial Black Holes as Dark Matter

Combining constraints from microlensing and Lyman-$α$ forest, we provide a simple argument to show that large spatial clustering of stellar-mass primordial black holes at the time of formation, such as the one induced by the presence of large non-Gaussianities, is ruled out. Therefore, it is not possible to evade existing constraints preventing stellar-mass primordial black holes to be a dominant constituent of the dark matter by boosting their initial clustering.

astro-ph.CO↗

arXiv2022

Primordial Black Holes in Matter-Dominated Eras: the Role of Accretion

We consider the role of secondary infall and accretion onto an initially overdense perturbation in matter-dominated eras, like the one which is likely to follow the end of inflation. We show that primordial black holes may form through post-collapse accretion, namely the accretion onto an initial overdensity whose collapse has not given rise to a primordial black hole. Accretion may be also responsible for the growth of the primordial black hole masses by orders of magnitude till the end of the matter-dominated era.

astro-ph.CO↗

arXiv2022

Searching for mass-spin correlations in the population of gravitational-wave events: the GWTC-3 case study

One fundamental goal of the newly born gravitational wave astronomy is discovering the origin of the observed binary black hole mergers. Towards this end, identifying features in the growing wealth of data may help in distinguishing different formation pathways. While large uncertainties still affect the binary formation models, spin-mass relations remain characteristic features of specific classes of channels. By focusing on the effective inspiral spin $χ_\text{eff}$, the best reconstructed spin-related merger parameter, we show that current GWTC-3 data support the hypothesis that a fraction of events may display mass-spin correlations similar to one expected by dynamical formation channels of either astrophysical or primordial nature. We quantify the Bayesian evidence in favour of those models, which are substantially preferred when compared to the Gaussian phenomenological model adopted to describe the distribution of $χ_\text{eff}$ in the recent LIGO/Virgo/KAGRA population analyses.

astro-ph.HE↗

arXiv2021

Bubble Correlation in First-Order Phase Transitions

Making use of both the stochastic approach to the tunneling phenomenon and the threshold statistics, we offer a simple argument to show that critical bubbles may be correlated in first-order phase transitions and biased compared to the underlying scalar field spatial distribution. This happens though only if the typical energy scale of the phase transition is sufficiently high. We briefly discuss possible implications of this result, e.g. the formation of primordial black holes through bubble collisions.

hep-ph↗

arXiv2021

Primordial Black Holes: from Theory to Gravitational Wave Observations

Primordial Black Holes (PBH) can form in the early universe and might comprise a significant fraction of the dark matter. Interestingly, they are accompanied by the generation of Gravitational Wave (GW) signals and they could contribute to the merger events currently observed by the LIGO/Virgo Collaboration (LVC). In this thesis, we study the PBH scenario, addressing various properties at the formation epoch and the computation of abundance beyond the Gaussian paradigm, while also developing the theoretical description of PBH evolution through accretion and mergers, with particular focus on modelling their GW signatures. In a second part, we compare the primordial scenario with current GW data, seizing the possible contribution of PBH binaries to LVC signals and forecasting the potential of future GW detectors, such as Einstein Telescope and LISA, to detect mergers of primordial binaries and the stochastic GW background induced at second order by the PBH formation mechanism.

astro-ph.CO↗

arXiv2021

Constraining the Initial Primordial Black Hole Clustering with CMB-distortion

The merger rate of primordial black holes depends on their initial clustering. In the absence of primordial non-Gaussianity correlating short and large-scales, primordial black holes are distributed à la Poisson at the time of their formation. However, primordial non-Gaussianity of the local-type may correlate primordial black holes on large-scales. We show that future experiments looking for CMB $μ$-distortion would test the hypothesis of initial primordial black hole clustering induced by local non-Gaussianity, while existing limits already show that significant non-Gaussianity is necessary to induce primordial black hole clustering.

astro-ph.CO↗