arXiv subjects

Gabriele Montefalcone

Publications and source records attributed to Gabriele Montefalcone.

At least 19 recordsLinked to original sources

arXiv2026

Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics

Neutrinos and other light relics leave characteristic imprints in the cosmic microwave background anisotropies, making their observation a sensitive probe of the particle content and thermal history of the early universe. The energy density in these relativistic species is parameterized by their effective number $N_\mathrm{eff}$. Measuring this parameter at the percent level, which is a long-standing science goal of CMB-S4 and other experiments, would test a wide range of well-motivated physics within and beyond the Standard Model of particle physics. In this paper, we present Fisher-matrix forecasts of the projected sensitivity to $N_\mathrm{eff}$ of several CMB-S4 survey configurations considered during its extensive design phase. The conceptual design reaches $\sigma(N_\mathrm{eff}) < 0.03$ over its seven-year observing period, while the revised configuration achieves the same precision over a longer timescale. We complement these results with a cosmic-variance-limited survey over the same multipole range to quantify the room for improvement accessible with additional instrumental, observational, and theoretical efforts. Finally, we discuss the broad implications of precise $N_\mathrm{eff}$ measurements for the radiation sector, big bang nucleosynthesis, light thermal relics, and other early-universe physics. The forecasts presented in this work are performed with the publicly released DRAFT (Dark Radiation Anisotropy Flowdown Team) tool. It provides an end-to-end pipeline from simulated foreground maps and component separation to delensing and projected sensitivities for any cosmological parameter, and it can be directly applied to other cosmic microwave background survey designs.

astro-ph.CO↗

arXiv2026

The Rise and Fall of Acoustic Oscillations at Cosmic Dawn

Cosmic dawn 21-cm observations will extend standard-ruler cosmology into the first billion years, unlocking epochs inaccessible by the cosmic microwave background and large-scale structure. Realizing this promise requires an accurate model of the acoustic structure imprinted onto early star formation. At such early times, two counterbalancing phenomena -- matter overdensities and streaming velocities between cold dark matter and baryons -- modulate the spatial statistics of star formation. While overdensities dictate where early galaxy-bearing haloes form, regions of high velocity suppress star formation. Their combined influence on the intergalactic medium yields 21-cm fluctuations with both baryon (BAO) and velocity-induced (VAO) acoustic oscillations. Here we present the first prescription to decompose the 21-cm power spectrum into its constituent acoustic features. We find a percent-level offset between the BAO and VAO shapes which, if ignored in standard-ruler analyses, would bias inferred values of $H(z)$ by $\sim 2\%$; we provide a correction. Moreover, as the relative prominence of BAOs and VAOs ebbs and flows non-monotonically across cosmic dawn, we demonstrate how their evolution is sensitive to the physics of early galaxy evolution and the first stars. Finally, we forecast how sensitive SKA will be to the BAO-VAO combined standard ruler. Our results establish joint BAO-VAO modeling as an essential ingredient of 21-cm acoustic inference, enabling robust constraints on both cosmic expansion and the first stars.

astro-ph.CO↗

arXiv2026

WIMP-like Dark Matter Without Thermalization At Freeze-Out

In the standard thermal relic scenario, dark matter remains in chemical equilibrium with the Standard Model radiation bath until freeze-out occurs at $T \sim m_X/20$, where $m_X$ is the dark matter mass. In this familiar class of models, the observed relic density is obtained for annihilation cross sections of order $\sigma v \sim 10^{-26}$ cm$^3$/s. We show that comparable cross sections can naturally be realized in hidden-sector models in which the dark matter and Standard Model sectors decouple at a very high temperature, $T \gg m_X$, and subsequently evolve with separate thermal histories. Despite this decoupling, the two sectors have similar temperatures during freeze-out, leading to the usual thermal relic abundance. As a consequence, the coupling between the Standard Model and hidden sectors can be extremely small, potentially placing direct detection and collider signals far below foreseeable sensitivities.

hep-ph↗

arXiv2026

CMB Limits on the Absorption of Light Vector and Axial-Vector Dark Matter

Leptophilic sub-MeV spin-1 dark matter (DM) can be converted into a photon via inelastic scattering with a free electron or absorption by a neutral hydrogen atom in the primordial plasma. We study for the first time the impact of the energy injection resulting from such processes on cosmic microwave background (CMB) anisotropies. We obtain upper limits on the vector and axial-vector DM-electron couplings using Planck 2018 temperature, polarization, and lensing data for DM masses between 100 eV and 100 keV. We find that, due to the suppression of the hydrogen atomic form factor at high energies, inelastic scattering provides the dominant constraint for DM masses above the keV scale. At lower masses, hydrogen ionization through DM absorption is the leading channel, driven by the higher efficiency of post-recombination energy injection in modifying the free-electron fraction. Although the bounds we derive are considerably weaker than existing laboratory and astrophysical limits, they provide a robust and independent cosmological probe of leptophilic DM interactions.

astro-ph.CO↗

arXiv2026

Parameterizing Dark Energy at the density level: A two-parameter alternative to CPL

We introduce a minimal two-parameter formulation of the dark energy (DE) density evolution normalized to its present-day value, $f_{\rm DE}(z) \equiv \rho_{\rm DE}(z)/\rho_{\rm DE,0}$, in terms of $f_p\equiv f_{\rm DE}(z_p)$ and the DE equation of state $w_p\equiv w(z_p)$, at a pivot redshift $z_p$. This provides an alternative framework for assessing the evidence for evolving DE, complementary to the established Chevallier-Polarski-Linder (CPL) parameterization. By parameterizing the DE density directly, the $(w_p,\,f_p)$ formulation avoids the approximate degeneracies intrinsic to the $(w_0,\,w_a)$ basis -- in particular the weak sensitivity of the expansion history to $w_a$ -- while reproducing the background evolution of representative quintessence models with equivalent accuracy. Confronting it with the latest baryon acoustic oscillation (BAO) measurements from DESI, a prior on early-universe parameters from Planck cosmic microwave background (CMB) observations, and Type Ia supernovae (SNe) data, we find that the $w_p$ and $f_p$ parameters are both tightly constrained and sensitive to distinct subsets of the data. Specifically, $w_p$ is measured to percent-level precision by BAO and CMB alone, while $f_p$ is pinned down by the independent matter density constraint that only SNe provide. Including the Pantheon+ SNe sample, we obtain $w_p = -1.04 \pm 0.04$ and $f_p = 1.07 \pm 0.04$, with similar results when using the DESY5 SNe sample. The preference for evolving DE over $\Lambda$CDM remains below $3\sigma$ across all dataset combinations, comparable to that obtained with CPL. Notably, the proximity of both $w_p$ and $f_p$ to their cosmological constant values of $(-1,1)$ -- precisely at the epoch where the data are most sensitive -- deepens the coincidence previously identified in the CPL framework, reinforcing the case for caution in interpreting the current evidence for dynamical DE.

astro-ph.CO↗

arXiv2025

Defying eternal inflation in warm inflation with a negative running

It was pointed out previously~\cite{Kinney:2014jya} that a sufficiently negative running of the spectral index of curvature perturbations from (ordinary i.e. cold) inflation is able to prevent eternal inflation from ever occurring. Here, we reevaluate those original results, but in the context of warm inflation, in which a substantial radiation component (produced by the inflaton) exists throughout the inflationary period. We demonstrate that the same general requirements found in the context of ordinary (cold) inflation also hold true in warm inflation; indeed an even tinier amount of negative running is sufficient to prevent eternal inflation. This is particularly pertinent, as models featuring negative running are more generic in warm inflation scenarios. Finally, the condition for the existence of eternal inflation in cold inflation -- that the curvature perturbation amplitude exceed unity on superhorizon scales -- becomes more restrictive in the case of warm inflation. The curvature perturbations must be even larger, i.e. even farther out on the potential, away from the part of the potential where observables, e.g. in the Cosmic Microwave Background, are produced.

astro-ph.CO↗

arXiv2025

Free-Streaming Neutrinos and Their Phase Shift in Current and Future CMB Power Spectra

The cosmic neutrino background and other light relics leave distinct imprints in the cosmic microwave background anisotropies through their gravitational influence. Since neutrinos decoupled from the primordial plasma about one second after the big bang, they have been free-streaming through the universe. This induced a characteristic phase shift in the acoustic peaks as a unique signature. In this work, we constrain the free-streaming nature of these relativistic species and other light relics beyond the Standard Model of particle physics by establishing two complementary template-based approaches to robustly infer the size of this phase shift from the temperature and polarization power spectra. One template shifts the multipoles in these spectra, while the other novel template more fundamentally isolates the phase shift at the level of the underlying photon-baryon perturbations. Applying these methods to Planck data, we detect the neutrino-induced phase shift at about $10σ$ significance, which rises to roughly $14σ$ with additional data from the Atacama Cosmology Telescope and the South Pole Telescope. We also infer that the data is consistent with the Standard Model prediction of three free-streaming neutrinos. In addition, we forecast the capabilities of future experiments which will enable significantly more precise phase-shift measurements, with the Simons Observatory and CMB-S4 reducing the $1σ$ uncertainties to roughly 4.3% and 2.5%, respectively. More generally, we establish a new analysis pipeline for the phase shift induced by neutrinos and other free-streaming dark radiation which additionally offers new avenues for exploring physics beyond the Standard Model in a signature-driven and model-agnostic way.

astro-ph.CO↗

arXiv2025

CMB Constraints on Loop-Induced Decays of Leptophilic Dark Matter

Leptophilic sub-MeV spin-zero dark matter (DM) decays into photons via one-loop processes, a scenario that has been in part overlooked in current literature. In this work, we provide updated and comprehensive upper limits on scalar, pseudo-scalar, and axion-like DM-electron couplings based on the latest NPIPE cosmic microwave background data from Planck. Our bounds on the couplings are not only competitive with astrophysical and terrestrial experiments, but outperform them in certain regions of parameter space. Notably, we present the most stringent limits to date on scalar DM with masses around a few keV and pseudo-scalar DM with masses between 100 eV and a few keV. Additionally, we explore, for the first time, the impact of implementing a cosmology-consistent treatment of energy deposition into the cosmic medium.

hep-ph↗

arXiv2025

Minimal Dark Matter Freeze-in with Low Reheating Temperatures and Implications for Direct Detection

We investigate the influence of the reheating temperature of the visible sector on the freeze-in dark matter (DM) benchmark model for direct detection experiments, where DM production is mediated by an ultralight dark photon. Here we consider a new regime for this benchmark: we take the initial temperature of the thermal Standard Model (SM) bath to be below the DM mass. The production rate from the SM bath is drastically reduced due to Boltzmann suppression, necessitating a significant increase in the portal coupling between DM and the SM to match the observed relic DM abundance. This enhancement in coupling strength increases the predicted DM-electron scattering cross section, making freeze-in DM more accessible to current direct detection experiments.

hep-ph↗

arXiv2025

Thermal Gravitons from Warm Inflation

In warm inflation (WI), the persistent thermal bath that is sustained by dissipative interactions with the inflaton field produces a stochastic background of gravitational waves (GWs). In this paper we study the production and evolution of these GWs. Specifically, we investigate the emission of thermal gravitons (gravitons emitted by a thermal bath) from particle scattering in the bath and the evolution of the corresponding GWs. We find that the bulk of thermal graviton production in WI occurs during the transition to radiation domination after inflation. Further, the energy density of thermal gravitons is enhanced by roughly one to two orders of magnitude compared to that in a radiation-dominated scenario with the same reheating temperature. We also calculate the spectrum of the resulting stochastic GW background and find that it has a distinctive shape, consisting of a peak at high frequencies ~100 GHz and an almost flat spectrum extending to low frequencies. The peak arises from emission of sub-horizon modes that follow the temperature of the bath. The flat part of the spectrum corresponds to the modes that exit the horizon during WI and re-enter during radiation domination. We show that the detection prospects for the high-frequency peak of the GW spectrum, while improved slightly compared to the radiation-dominated case, still remain challenging. The thermal spectrum's low-frequency plateau is typically subdominant to the amplitude of the standard vacuum tensor modes from inflation, although WI models can exist where the thermal graviton plateau surpasses the vacuum contribution without exceeding current observational limits on the tensor-to-scalar ratio. Furthermore, we calculate the thermal graviton contribution from WI to dark radiation and show that WI models are generally expected to satisfy current observational bounds, including those from the cosmic microwave background.

hep-ph↗

arXiv2025

Directly Probing Neutrino Interactions through CMB Phase Shift Measurements

Perturbations in the cosmic neutrino background produce a characteristic phase shift in the acoustic oscillations imprinted in the anisotropies of the cosmic microwave background (CMB), providing a unique observational probe of neutrino physics. In this work, we explore how this phase shift signature is altered in the presence of neutrino interactions with temperature-dependent scattering rates, motivated by physical constructions for neutrino self-interactions and neutrino-dark matter couplings. A key finding is that the phase shift in these realistic models -- characterized by gradual rather than instantaneous decoupling -- maintains the same functional form as the free-streaming template, with only the asymptotic amplitude decreasing for stronger interactions that delay decoupling. This simple parametrization enables us to directly constrain neutrino interactions through phase shift measurements in the temperature and polarization power spectra from CMB observations. Analyzing the latest data from \textit{Planck}, the Atacama Cosmology Telescope, and the South Pole Telescope, we derive strong constraints on the neutrino decoupling redshift. Our global analysis indicates that neutrinos have been freely streaming since deep within the radiation-dominated epoch. We also explore flavor-dependent scenarios in which only one neutrino species interacts. Overall, our work establishes a signature-driven framework that exploits the clean phase shift signal in the acoustic oscillations of the CMB as a precise and robust probe of non-standard neutrino interactions in the early universe.

astro-ph.CO↗

arXiv2025

How Theory-Informed Priors Affect DESI Evidence for Evolving Dark Energy

Recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have been interpreted to suggest that dark energy may be evolving. In this work, we examine how prior choices affect such conclusions. Specifically, we study the biases introduced by the customary use of uniform priors on the Chevallier-Polarski-Linder (CPL) parameters, $w_0$ and $w_a$, when assessing evidence for evolving dark energy. To do so, we construct theory-informed priors on $(w_0, w_a)$ using a normalizing flow (NF), trained on two representative quintessence models, which learns the distribution of these parameters conditional on the underlying $Λ$CDM parameters. In the combined $\textit{Planck}$ CMB + DESI BAO analysis we find that the apparent tension with a cosmological constant in the CPL framework can be reduced from $\sim 3.1σ$ to $\sim 1.3σ$ once theory-informed priors are applied, rendering the result effectively consistent with $Λ$CDM. For completeness, we also analyze combinations that include Type Ia supernova data, showing similar shifts toward the $Λ$CDM limit. Taken together, the observed sensitivity to prior choices in these analyses arises because uniform priors - often mischaracterized as "uninformative" - can actually bias inferences toward unphysical parameter regions. Consequently, our results underscore the importance of adopting physically motivated priors to ensure robust cosmological inferences, especially when evaluating new hypotheses with only marginal statistical support. Lastly, our NF-based framework achieves these results by post-processing existing MCMC chains, requiring $\approx 1$ hour of additional CPU compute time on top of the base analysis - a dramatic speedup over direct model sampling that highlights the scalability of this approach for testing diverse theoretical models.

astro-ph.CO↗

arXiv2025

Tracing the Neutrino-Induced Phase Shift in the 21-cm Spectrum

We study the phase shift that free-streaming neutrinos imprint on the 21-cm power spectrum during cosmic dawn, computing for the first time its effect on both density- and velocity-induced acoustic oscillations. Neutrinos are known to generate a characteristic phase shift in the acoustic oscillations of the photon-baryon plasma before recombination, a signature already detected in the cosmic microwave background (CMB) as well as the spectrum of baryon acoustic oscillations (BAOs) extracted from galaxy surveys. We show that in the 21-cm signal this phase shift is distinct from that observed in the CMB and BAO spectra, exhibiting a characteristic mode and redshift dependence arising from the additional contribution of the so-called velocity acoustic oscillations (VAOs), sourced by the baryon-dark matter relative velocities. Our results establish the phase of acoustic oscillations in the 21-cm spectrum as a promising new avenue for probing free-streaming light relics at cosmic dawn, complementary to existing CMB and BAO measurements.

astro-ph.CO↗

arXiv2025

Explaining the "too massive" high-redshift galaxies in JWST data: numerical study of three effects and a simple relation

The James Webb Space Telescope has discovered high luminosity galaxies that appear to be "too many" and "too massive" compared to predictions of the Standard LCDM cosmology, suggesting that star formation in the early universe is more rapid than previously anticipated. In this paper we examine in detail the following three effects which can instead provide alternative explanations for these observations: (1) a "top heavy" initial mass function (IMF) for the stars, (2) a variety of star formation histories (constant, exponentially decreasing, and peaked star formation rates), and (3) a variety of initial metallicities. Due to any of these three effects, galaxies of a given luminosity in JWST may be interpreted as having a larger stellar mass than they actually do. Our results are obtained using the Pegase stellar population code, and are presented as the ratio of the modified star formation efficiency relative to the fiducial one (which uses a Salpeter IMF and constant star formation rate). As an example, if the high-mass end of the IMF goes as $M^{-1.35}$, the star formation efficiency and inferred stellar galactic mass could be lower by a factor of $\sim 10$ than in the fiducial case. Our examination (keeping the star formation rate constant) of a top-heavy IMF with slope $α$ leads to a simple relation that is a good approximation to the numerical results, $ε(α) \approx ε_{\rm fid}e^{2.66(α-2.35)}$. Since there are more low mass galaxies than high mass galaxies, these effects may result in a large number of seemingly overly massive galaxies compared to the expectations. Thus, the effects studied in this paper may explain both puzzling observations regarding high luminosity galaxies in JWST: the apparently overly massive galaxies as well as the profusion of apparently high mass galaxies.

astro-ph.CO↗

arXiv2025

Can a Breakdown of Hawking Evaporation Open a New Mass Window for Primordial Black Holes as Dark Matter?

Semi-classical Hawking evaporation is expected to break down at some point in a black hole's evolution as the effects of quantum gravity become important. In particular, it has been argued that the so-called memory-burden effect could cause black holes to become stabilized by the information that they carry, thereby suppressing the rate at which they undergo Hawking evaporation. It has furthermore been suggested that this opens a new mass window, between $10^{4}\,{\rm g} \lesssim M \lesssim 10^{10}\,{\rm g}$, over which primordial black holes could constitute the dark matter of our Universe. We show for the first time that this is true only if the transition from the semi-classical phase of a black hole to its memory-burdened phase is practically instantaneous. If this transition is instead more continuous, Hawking evaporation will persist at relevant levels throughout the eras of Big Bang Nucleosynthesis and recombination, leading to stringent constraints which rule out the possibility that black holes lighter than $\sim 4 \times 10^{16}\,{\rm g}$ could make up all or most of the dark matter. More broadly, our analysis demonstrates that even if departures from the semi-classical Hawking evaporation occur as proposed, they must be both drastic and abrupt to open viable new mass windows for primordial black hole dark matter.

astro-ph.CO↗

arXiv2024

Dark Matter production during Warm Inflation via Freeze-In

We present a novel perspective on the role of inflation in the production of Dark Matter (DM). Specifically, we explore the DM production during Warm Inflation via ultraviolet Freeze-In (WIFI). We demonstrate that in a Warm Inflation (WI) setting the persistent thermal bath, sustained by the dissipative interactions with the inflaton field, can source a sizable DM abundance via the non-renormalizable interactions that connect the DM with the bath. Compared to the (conventional) radiation-dominated (RD) UV freeze-in scenario for the same reheat temperature (after inflation), the resulting DM yield in WIFI is always enhanced showing a strongly positive dependence on the mass dimension of the non-renormalizable operator. Of particular interest, for a sufficiently large mass dimension of the operator, the entirety of the DM abundance of the Universe can be created during the inflationary phase. For the specific models we study, we find that the enhancement in DM yield, relative to RD UV freeze-in, is at least an order of magnitude for an operator of mass dimension 5, and as large as 18 order of magnitudes for an operator of mass dimension 10. Our findings also suggest a broader applicability for producing other cosmological relics, which may have a substantial impact on the evolution of the early Universe.

hep-ph↗

arXiv2023

WarmSPy: a numerical study of cosmological perturbations in warm inflation

We present WarmSPy, a numerical code in Python designed to solve for the perturbations' equations in warm inflation models and compute the corresponding scalar power spectrum at CMB horizon crossing. In models of warm inflation, a radiation bath of temperature $T$ during inflation induces a dissipation (friction) rate of strength $Q \propto T^c/ϕ^m$ in the equation of motion for the inflaton field $ϕ$. While for a temperature-independent dissipation rate ($c=0$) an analytic expression for the scalar power spectrum exists, in the case of a non-zero value for $c$ the set of equations can only be solved numerically. For $c>0$ ($c<0$), the coupling between the perturbations in the inflaton field and radiation induces a growing (decaying) mode in the scalar perturbations, generally parameterized by a multiplicative function $G(Q)$ which we refer to as the scalar dissipation function. Using WarmSPy, we provide an analytic fit for $G(Q)$ for the cases of $c=\{3,1,-1\}$, corresponding to three cases that have been realized in physical models. Compared to previous literature results, our fits are more robust and valid over a broader range of dissipation strengths $Q\in[10^{-7},10^{4}]$. Additionally, for the first time, we numerically assess the stability of the scalar dissipation function against various model parameters, inflationary histories as well as the effects of metric perturbations. As a whole, the results do not depend appreciably on most of the parameters in the analysis, except for the dissipation index $c$, providing evidence for the universal behaviour of the scalar dissipation function $G(Q)$.

astro-ph.CO↗

arXiv2023

Observational Constraints on Warm Natural Inflation

Warm natural inflation is studied for the case of the original cosine potential. The radiation bath during inflation induces a dissipation (friction) rate in the equation of motion for the inflaton field, which can potentially reduce the field excursion needed for an observationally viable period of inflation. We examine if the dissipation thus provides a mechanism to avoid the large decay constant $f \gtrsim M_{\mathrm{pl}}$ of cold cosine natural inflation. Whereas temperature independent dissipation has previously been shown to alleviate the need for a trans-Planckian decay constant $f$, we illustrate here the difficulties of accommodating a significantly sub-Planckian decay constant ($f<10^{-1}M_{\mathrm{pl}}$) in the case of the following temperature dependent dissipation rates, $Γ\propto T^c$, with $c=\{1,3\}$. Such dissipation rates represent physically well-motivated constructions in the literature. For each model, we map its location in the $r$-$n_s$ plane and compare with Cosmic Microwave Background data. For $c=1 \, (c=3)$, we find that agreement with CMB data requires that dissipation be in the weak (moderate) regime and that the minimum allowed value of the decay constant in the potential is $f_{\rm min} = 0.3 \, (0.8)\,M_{\mathrm{pl}}$ respectively.

gr-qc↗