arXiv subjects

Michele Redi

Publications and source records attributed to Michele Redi.

At least 19 recordsLinked to original sources

arXiv2026

Particle Production from Inhomogeneities: the off-shell side of gravitational waves

We continue the study of particle production from gravitational inhomogeneities in the early Universe. Focusing on sources active on sub-horizon scales, we derive general expressions relating particle production to the unequal-time two-point function of the stress-energy tensor sourcing scalar, vector and tensor metric perturbations. The resulting particle yield probes the time-like support of this correlator, and in the tensor case the same object controls gravitational wave emission when evaluated on the light-like support. This establishes a phenomenological link between dark matter production and gravitational wave signals, allowing the dark matter mass to be related to the amplitude of the stochastic gravitational wave background. Our results show that, on sub-horizon scales, particle production from inhomogeneous metric backgrounds practically reduces to gravitational scattering. This directly connects the formalism to gravitational freeze-in from the Standard Model thermal bath, while extending it to non-thermal and out of equilibrium sources. We apply the formalism to first order phase transitions and discuss the associated production from scalar and tensor perturbations. The mechanism can efficiently populate gravitationally coupled dark sectors, especially when the perturbations are generated shortly after inflation.

hep-ph↗

arXiv2026

The Too Visible QCD Axion

Murayama proposed a GeV-scale axion theory where the up-quark mass term is generated dynamically by the QCD chiral condensate, spontaneously breaking a Peccei-Quinn symmetry. It predicts a too large mass splitting between neutral and charged pions. Trying to solve this problem we explore extensions. Despite some partial improvements, we identify a structural obstruction: the new Peccei-Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, leading to an enhanced higher-order operator. As a consequence, pion scatterings too are distorted. We also examine the limit in which the axion becomes light, finding that it is excluded by fifth-force constraints.

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arXiv2026

Mimicking Phantom Dark Energy with Evolving Dark Matter Mass

We present a general method to reproduce a given cosmological background through energy exchange between dark energy (DE) and dark matter (DM). This can be simply realized with a standard quintessence scalar field that controls the DM mass. In particular a background with phantom crossing can be effectively realized without introducing ghosts or other pathologies. For example one can reproduce exactly the background that gives the best fit to the recent DESI+CMB+DESY5 data, within the Chevallier-Polarski-Linder (CPL) parametrization of DE. Although the background evolution is identical, the perturbations differ, leading to modified growth of structures. If the DM mass varies at late times, early-time observables are not modified and can reproduce the main predictions of the target model, but late-time observables are affected. We discuss in particular the effects on the matter power spectrum, CMB lensing and ISW effect. When reproducing the best fit CPL background model, this scenario generically predicts $\mathcal{O}(10\%)$ deviations in such observables. However, for suitable choices of parameters, effects on the matter power spectrum can be smaller, motivating a detailed study. In general, energy exchange between DE and DM generates a mismatch between the matter power spectrum and the gravitational potential amplitudes compared to the decoupled case, that can lead to deviations observable in future experiments.

astro-ph.CO↗

arXiv2025

Particle production from inhomogeneities: general metric perturbations

We present universal formulas for particle production from gravitational inhomogeneities. In the massless limit the result is strikingly simple and completely determined by the two-point function of the energy-momentum tensor that is fixed up to a constant - the central charge - for conformally coupled scalars, massless fermions and gauge fields. This result can be applied to any conformally coupled theory, weakly or strongly interacting, unifying previous derivations for fields of different spin and for scalar and tensor perturbations. We derive the results using the Schwinger method of 1PI effective action and through Bogoliubov transformations that allows to compute exclusive information on the distribution of particles. We then apply these results to stochastic backgrounds of scalar and tensor perturbations that can be generated by various phenomena such us inflationary perturbations and first order phase transitions. Differently from particle production usually considered in cosmology this mechanism allows for the production of massless fields. In particular the abundance induced by inhomogeneities can easily reproduce the dark matter abundance if scalar perturbations produced from inflation are enhanced at short scales.

hep-th↗

arXiv2024

BAO vs. SN evidence for evolving dark energy

We critically review the evidence for time-varying dark energy from recent Baryon Acoustic Oscillations (BAO) and Supernova (SN) observations. First, we show that such evidence is present at the 3$\sigma$ level, even without the new BAO data from the dark energy Spectroscopic Instrument (DESI), by instead using BAO data from the dark energy Survey (DES), combined with the DES5Y supernovae and Planck CMB data. Next, we examine the role of the DES5Y supernova dataset, showing that the preference for time-varying dark energy is driven by the low redshift supernovae common to both the DES5Y and Pantheon+ compilations. We find that combining Pantheon+ and DES5Y supernovae by removing the common supernovae leads to two different results, depending on whether they are removed from the DES5Y or the Pantheon+ catalog, leading to stronger or weaker exclusion of $\Lambda$CDM, at the (3.8$\sigma$) and (2.5$\sigma$) level, respectively. These common supernovae have smaller error bars in DES5Y compared to Pantheon+, and, as recently pointed out, there is an offset in magnitude in DES5Y between supernovae at ($z > 0.1$), where almost all the measurements taken during the full five years of DES are, and the low-redshift ones ($z < 0.1$), where all the historical set of nearby supernovae lies. We show that marginalizing over such an offset in DES5Y would lead to significantly weaker evidence for evolving dark energy.

astro-ph.CO↗

arXiv2024

Stochastic Dark Matter from Curvature Perturbations

We investigate the production of dark matter from curvature perturbations produced during inflation or in standard cosmology, for example during first order phase transitions. Perturbations break Weyl flatness of the Friedmann-Lemaitre-Robertson-Walker metric, allowing conformally coupled fields -- in particular fermions studied here -- to be produced even in the massless limit. Particle production can be computed by studying the Bogoliubov transformation induced by the stochastic background. For perturbations generated during inflation, we present a closed formula for the resulting abundance of particles that depends solely on the power spectrum of curvature perturbations at the end of inflation. This production mechanism can be dominant especially if the amplitude of curvature perturbations is enhanced for modes that exit the horizon towards the end of inflation. In the simplest scenario, the critical dark matter abundance is reproduced for $M \gtrsim 10^{6}$ GeV.

hep-ph↗

arXiv2024

Reconstructing the Inflaton Potential: Primordial Black Holes and Gravitational Waves in Slow Roll and Ultra Slow Roll Single Field Inflation

We present new single field inflationary scenarios that produce the critical abundance of primordial black holes as dark matter reconstructing the inflaton potential from an input power spectrum. The method is exact in the slow roll approximation but remains effective even when the slow roll conditions are temporarily violated such as in ultra slow roll models. With this method we construct new ultra slow roll scenarios and also models that reproduce the DM abundance within the slow roll regime. As a second application we consider a scalar power spectrum that generates a secondary gravitational wave background compatible with the one recently observed in Pulsar Timing Arrays experiments. These scenarios could be tested by future observations of $\mu-$distortions of the CMB.

astro-ph.CO↗

arXiv2024

Consistent Theories for the DESI dark energy fit

We search for physically consistent realizations of evolving dark energy suggested by the cosmological fit of DESI, Planck and Supernovae data. First we note that any lagrangian description of the standard Chevallier-Polarski-Linder (CPL) parametrization for the dark energy equation of state $w$, allows for the addition of a cosmological constant. We perform the cosmological fit finding new regions of parameter space that however continue to favour dark energy with $w<-1$ at early times, that is challenging to realize in consistent theories. Next, in the spirit of effective field theories, we consider the effect of higher order terms in the Taylor expansion of the equation of state of dark energy around the present epoch. We find that non-linear corrections of the equation of state are weakly constrained, thus opening the way to scenarios that differ from CPL at early times, possibly with $w>-1$ at all times. We present indeed scenarios where evolving dark energy can be realized through quintessence models. We introduce in particular the ramp model where dark energy coincides with CPL at late times and approximates to a cosmological constant at early times. The latter model provides a much better fit than $\Lambda$CDM, and only slightly worse than $w_0w_a$CDM, but with the notable advantage of being described by a simple and theoretically consistent lagrangian of a canonical quintessence model.

astro-ph.CO↗

arXiv2023

Early vs late string networks from a minimal QCD Axion

We propose a new regime of minimal QCD axion dark matter that lies between the pre- and post-inflationary scenarios, such that the Peccei-Quinn (PQ) symmetry is restored only on sufficiently large spatial scales. This leads to a novel cosmological evolution, in which strings and domain walls re-enter the horizon and annihilate later than in the ordinary post-inflationary regime, possibly even after the QCD crossover. Such dynamics can occur if the PQ symmetry is restored by inflationary fluctuations, i.e. the Hubble parameter during inflation $H_I$ is larger than the PQ breaking scale $f_a$, but it is not thermally restored afterwards. Solving the Fokker-Planck equation, we estimate the number of inflationary e-folds required for the PQ symmetry to be, on average, restored. Moreover, we show that, in the large parts of parameter space where the radial mode is displaced from the minimum by de Sitter fluctuations, a string network forms due to the radial mode oscillating over the top of its potential after inflation. In both cases we identify order one ranges in $H_I/f_a$ and in the quartic coupling $\lambda$ of the PQ potential that lead to the late-string dynamics. In this regime the cosmological dark matter abundance can be reproduced for axion decay constants as low as the astrophysical constraint $O(10^8)$ GeV, corresponding to axion masses up to $10^{-2}~{\rm eV}$, and with miniclusters with masses as large as $O(10)M_\odot$.

hep-ph↗

arXiv2023

Neutrinos, Dark Matter and Higgs Vacua in Parity Solutions of the strong CP problem

The strong CP problem can be solved if the laws of nature are invariant under a space-time parity exchanging the Standard Model with its mirror copy. We review and extend different realizations of this idea with the aim of discussing Dark Matter, neutrino physics, leptogenesis and collider physics within the same context. In the minimal realization of Ref. [1] the mirror world contains a massless dark photon, which leads to a rather interesting cosmology. Mirror electrons reproduce the dark matter abundance for masses between 500-1000 GeV with traces of strongly interacting dark matter. This scenario also predicts deviations from cold dark matter, sizable $\Delta N_{\rm eff}$ and colored states in the TeV range that will be tested in a variety of upcoming experiments. We also explore scenarios where the mirror photon is massive and the mirror particles are charged under ordinary electro-magnetism with very different phenomenology. We also show that, for the measured values of the SM parameters, the Higgs effective potential can give rise to a second minimum at large field value as required to break spontaneously the parity symmetry.

hep-ph↗

arXiv2022

The meso-inflationary QCD axion

We study the possibility that the axion Peccei-Quinn symmetry is spontaneously broken after the beginning of inflation. This scenario interpolates between pre-inflationary and post-inflationary axion cosmology with significant phenomenological differences from both. Since the axion is not present at the early stages of inflation large inflationary fluctuations are produced only at scales not constrained by CMB, avoiding the strongest isocurvature constraints. The energy density in isocurvature perturbations at short scales however can be comparable with the adiabatic contribution from misalignment. These large overdensities can lead to the formation of axion mini-clusters and also to constraints from Lyman-$\alpha$ forest and future CMB spectral distortion measurements. If Peccei-Quinn symmetry is broken during the first O(25) e-foldings of inflation no axions are produced from the string network but contributions from the annihilation of domain walls can further boost the abundance. This scenario is minimally realized if the Hubble scale during inflation drops below the Peccei-Quinn symmetry breaking scale but other realizations are possible.

hep-ph↗

arXiv2022

Jump Starting the Dark Sector with a Phase Transition

We study the possibility to populate the dark sector through a phase transition. We will consider secluded dark sectors made of gauge theories, Randall-Sundrum scenarios and conformally coupled elementary particles. These sectors have in common the fact that the action is approximately Weyl invariant, implying that particle production due to time dependent background is strongly suppressed. In particular no significant production takes place during inflation allowing to avoid strong isocurvature constraints from CMB. As we will show, if the scale of inflation is large compared to the dynamical mass scale, these sectors automatically undergo a phase transition that in the simplest cases is controlled by the Hubble parameter. If the phase transition takes place during reheating or radiation the abundance obtained can be larger than particle production and production from the SM plasma. For phase transitions completing during radiation domination, the DM mass is predicted in the range $10^8$ GeV while larger values are required for phase transitions occurring during reheating.

hep-ph↗

arXiv2022

Dark Photon Dark Matter without Stueckelberg Mass

We study the scenario of dark photon Dark Matter where the mass is generated through the Higgs mechanism rather than the constant Stueckelberg mass. In this construction the dark sector contains necessarily extra degrees of freedom and interactions that lead to non-trivial dynamics including thermalization, phase transitions, cosmic string production. As a consequence the predictions of Stueckelberg theories are vastly modified, strongly depending on the couplings to curvature and on the scale of inflation $H_I$ compared to the scale $f$ of spontaneous symmetry breaking. We find in particular that only in extreme regions of parameter space the phenomenology of Stueckelberg dark photon is reproduced. These scenarios are strongly constrained by isocurvature perturbations unless the dark sector is approximately Weyl invariant.

hep-ph↗

arXiv2022

Dark matter self-interactions in the matter power spectrum

We study the imprints of secluded dark sectors with a mass gap and self-interactions on the matter power spectrum. When Dark Matter (DM) is sufficiently light, in the ballpark of a few KeV, and self-interacting we find qualitative difference with respect to $\Lambda$CDM and also to free streaming DM. In order to emphasize the role of interactions for the evolution of the primordial perturbations we discuss various regimes: ranging from the ideal case of a tightly coupled perfect fluid to the free case of Warm Dark Matter, including the realistic case of small but non-vanishing self-interactions. We compute the matter power spectrum in all these regimes with the aid of Boltzmann solvers. Light dark sectors with self-interactions are efficiently constrained by Lyman-$\alpha$ data and we find that the presence of self-interactions relaxes the bound on the DM mass. As a concrete realization we study models with dark QCD-like sectors, where DM is made of light dark-pions.

hep-ph↗

arXiv2021

Dark QCD Matters

We investigate the nightmare scenario of dark sectors that are made of non-abelian gauge theories with fermions, gravitationally coupled to the Standard Model (SM). While testing these scenarios is experimentally challenging, they are strongly motivated by the accidental stability of dark baryons and pions, that explain the cosmological stability of dark matter (DM). We study the production of these sectors which are minimally populated through gravitational freeze-in, leading to a dark sector temperature much lower than the SM, or through inflaton decay, or renormalizable interactions producing warmer DM. Despite having only gravitational couplings with the SM these scenarios turn out to be rather predictive depending roughly on three parameters: the dark sector temperature, the confinement scale and the dark pion mass. In particular, when the initial temperature is comparable to the SM one these scenarios are very constrained by structure formation, $ΔN_{\rm eff}$ and limits on DM self-interactions. Dark sectors with same temperature or warmer than SM are typically excluded.

hep-ph↗

arXiv2021

General Freeze-in and Freeze-out

We use the framework of relativistic and non-relativistic conformal field theories (CFT) to derive general results relevant for the production of weakly coupled and strongly coupled dark sectors through thermal interactions. Our result reproduce trivially known formulas for $2\to n $ processes and extend to general $m\to n$ processes as well as interacting dark sectors. As concrete examples we consider freeze-in of a relativistic CFT coupled to the SM with contact interactions and derive Sommerfeld enhancement of non-relativistic cross-sections from the theory of fermions at unitarity.

hep-ph↗

arXiv2021

Gravitational Production of a Conformal Dark Sector

Dark sectors with purely gravitational couplings to the Standard Model are unavoidably populated from the SM plasma by graviton exchange, and naturally provide DM candidates. We examine the production in the relativistic regime where the dark sector is approximately scale invariant, providing general analytical formulas that depend solely on the central charge of the dark sector. We then assess the relevance of interactions that can lead to a variety of phenomena including thermalisation, nonperturbative mass gaps, out-of-equilibrium phase transitions and cannibalism in the dark sector. As an illustrative example we consider the dark glueball scenario in this light and show it to be a viable DM candidate due to the suppression of gravitational production. We go on to extend these results to strongly coupled CFTs and their holographic duals at large-N with the dark dilaton as the DM candidate.

hep-ph↗

arXiv2021

Dark Nucleosynthesis: Cross-sections and Astrophysical Signals

We investigate dark matter bound-state formation and its implication for indirect-detection experiments. We focus on the case where dark matter is a baryon of a strongly-coupled dark sector and provide generic formulae for the formation of shallow nuclear bound states on emission of photons, and W and Z gauge bosons. These processes can occur via electric and magnetic transitions, and give rise to indirect signals that are testable in monochromatic and diffuse photon measurements by Fermi and HESS. We also study the validity of factorizing the bound-state formation cross section into a short-distance nuclear part multiplied by Sommerfeld-enhancement factors. We find that the short-distance nuclear potential often violates factorization, modifying in particular the location of the peaks associated with zero-energy bound states. Finally we revisit bound-state formation of a (weakly-coupled) Minimal DM quintuplet including isospin-breaking effects, and find it gives rise to indirect-detection signals that are compatible with current bounds.

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