arXiv subjects

Francesco Pace

Publications and source records attributed to Francesco Pace.

At least 19 recordsLinked to original sources

arXiv2026

Linear perturbation theory and structure formation in a Horndeski theory of gravity without dark matter

We study the formation and evolution of the cosmic large-scale structure (LSS) in a scalar-tensor Horndeski theory of gravity without dark matter. We impose that this theory reduces to Refracted Gravity (RG) in the weak-field limit. RG is a non-relativistic theory where the gravitational permittivity $ε(ρ)$, with $ρ$ the baryonic matter density, provides the gravitational boost required to describe both the dynamics of galaxies and galaxy clusters without dark matter. We show that $ε(ρ)$ equals the function $G_4(φ)$ of the scalar field $φ$ of our Horndeski model. The background evolution of a flat, matter-dominated, homogeneous and isotropic universe shows that $φ$ is also responsible for the accelerated expansion of the universe and that the model is capable of reproducing the observed redshift evolution of the Hubble-Lemaître parameter. We finally develop the linear perturbation theory and study the growth factor equation. The model fails to correctly describe the LSS by predicting a late onset of the formation of cosmic structure at redshift $z\lesssim 1$.

astro-ph.CO↗

arXiv2026

Redshift dependence and dipolar velocity corrections in cosmographic reconstructions through Type Ia supernova samples

We perform a cosmographic analysis of Type Ia supernovae using the Pantheon+\&SH0ES, DES-SNY5 and Union3 compilations. We consider different redshift intervals, analyzed through a third-order Taylor expansion and a Padé $(1,2)$ approximation of the luminosity distance. First, we first infer the cosmographic parameters directly from the observed supernova redshifts. Afterwards, we extend the analysis by including a dipole correction associated with the local peculiar velocity field, constraining both its amplitude and direction. For Pantheon+\&SH0ES, the Cepheid calibration allows a direct determination of the Hubble constant, whereas for DES-SNY5 and Union3 we fix $H_0$ to set the absolute distance scale. By progressively increasing the maximum redshift of the sample, we study how the inferred cosmographic parameters depend on the adopted redshift interval. We find that the Hubble constant obtained from Pantheon+\&SH0ES remains consistent with previous determinations for both cosmographic parameterizations. Instead, the agreement of the deceleration $q_0$ and jerk $j_0$ parameters with the $Λ$CDM values depends on the adopted compilation and improves mainly for Pantheon+\&SH0ES as the redshift interval is enlarged. Moreover, the reconstructed dipole parameters remain stable across the redshift intervals considered, with velocity amplitudes of order $300\,\mathrm{km\,s^{-1}}$, for all three supernova samples. Finally, fixing the dipole parameters with the cosmic microwave background values leads to a slight shift from the fiducial $Λ$CDM values of the deceleration $q_0$ and jerk $j_0$ parameters.

astro-ph.CO↗

arXiv2026

Cosmic voids evolution in modified gravity via hydrodynamics

We present a hydrodynamical description of isolated spherical voids in modified gravity (MG), extending the standard General Relativity (GR) and dynamical dark energy treatment by encoding gravity modifications into effective couplings that enter the Euler and Poisson equations. This yields a compact non-linear evolution equation for the Eulerian density contrast, controlled by a time- and density-dependent effective gravitational strength, and provides a direct map between model functions and void observables. We apply the framework to the luminal Galileon class of models, where derivative self-interactions generate Vainshtein screening and might lead to a breakdown of the physical branch in sufficiently underdense regions. Exploiting this feature, we apply the void-informed viability requirement that translates into bounds on the theory parameter space and, equivalently, on the minimum attainable void depth as a function of redshift. For viable parameters of a concrete model, we quantify the impact of MG on isolated void evolution, the Lagrangian to Eulerian mapping, and the shell-crossing threshold. Relative to GR, we find a clear hierarchy of MG effects, with ${\cal O}(10\%)$ modifications in the gravitational couplings, percent-level shifts in the void density evolution, and sub-percent deviations in both the mapping and the shell-crossing thresholds. Moreover, within the adopted parametrization, we show analytically that voids always lie in an unscreened regime on the physical branch. Overall, the formalism provides a self-consistent route to predict void dynamics and consistency constraints in a broad class of MG models.

astro-ph.CO↗

arXiv2026

How deep can a cosmic void be? Voids-informed theoretical bounds in Galileon gravity

We establish a void-based consistency test for Galileon scalar-tensor theories. We show that the previously reported unphysical breakdown of the predicted Newtonian force in certain Galileon models is controlled by a single condition linking non-linear void dynamics to the cosmic expansion history. This connection yields a redshift-dependent upper bound on the allowed depth of voids and promotes this requirement to a new viability condition, complementary to standard stability criteria. As an example, we apply this void-based criterion to a linear parameterization in the scale factor constrained by theoretical and observational bounds; we find that $\sim 60\%$ of the parameter space is excluded, with most problematic models failing by $z\lesssim 10$. These results position cosmic voids as sharp, complementary and theory-informed filters for viable modified gravity, enabling more informed priors and parameter-space choices in future cosmological inference.

astro-ph.CO↗

arXiv2026

A spherical hydrodynamical model of cosmic voids in ΛCDM and beyond

Cosmic voids have emerged as powerful probes for cosmology, providing complementary information on the large-scale structure of the universe. We present the first application of a hydrodynamical framework to model the evolution of cosmic voids. This approach offers a physically intuitive characterization of void dynamics and can naturally be applied to non-standard cosmologies. We derive the cosmology-dependent mapping that relates the linear (Lagrangian) and fully non-linear (Eulerian) evolution of the matter density contrast, a central component for accurate theoretical modeling of void statistics. Furthermore, we present a new method for determining the shell-crossing epoch across arbitrary cosmological backgrounds, thereby extending previous treatments restricted to the Einstein-de Sitter universe. Motivated by recent DESI results hinting at dynamical dark energy, we investigate void evolution in $ w_0w_a$CDM cosmologies by varying $ w_0$ and $w_a$. We also consider the impact of varying the matter density parameter, $ Ω_{\mathrm{m},0}$. We find that the evolution of isolated, spherically symmetric cosmic voids is most sensitive to $ Ω_{\mathrm{m},0} $ and $ w_0 $, which can alter the non-linear density contrast by up to 20-30%. Variations in $w_a$ have a smaller impact, but may still lead to measurable effects. We also show that the cosmology-dependent mapping between linear and non-linear density contrasts may provide a sensitive probe of dynamical dark energy in precision void analyses.

astro-ph.CO↗

arXiv2026

Linear perturbation theory and structure formation in a Brans-Dicke theory of gravity without dark matter

We investigate the formation of the large-scale cosmic structure in a scalar-tensor theory of gravity belonging to the class of the Brans--Dicke theories. The universe contains baryonic matter alone and neither dark matter nor dark energy. The two arbitrary functions of the scalar field characterizing the kinetic term and the self-interaction potential are set to $W(φ)=-1$ and $V(φ) = -Ξφ$, respectively, with $Ξ$ a positive constant. In the weak-field limit, the theory reduces to Refracted Gravity, a non-relativistic theory whose modified Poisson equation contains the scalar field $φ$ that provides the gravitational boost required to describe the dynamics of galaxies and galaxy clusters without dark matter. In a flat, matter-dominated, homogeneous and isotropic universe the same scalar field $φ$ drives the accelerated expansion of the universe and describes the observed redshift evolution of the Hubble-Lemaître parameter $H(z)$. However, in the equation of the growth factor of the linear perturbation theory, the form of $V(φ)$ makes the coefficient of the source of the gravitational field proportional to $H^{-1}(z)$; therefore the gravitational field is strongly suppressed at early times and structure formation is delayed to redshift $z< 1$, in disagreement with the observation of formed galaxies at much larger redshifts. In addition, the form of $W(φ)$ and a linear $V(φ)$ imply that $φ$ generates twice the gravitational boost on massive particles than on photons, with possible observable consequences on the gravitational lensing phenomenon. It remains to be investigated whether different choices of $W(φ)$ and $V(φ)$, that can still make the theory reduce to Refracted Gravity in the weak-field limit, might alleviate these problems.

astro-ph.CO↗

arXiv2025

Effects of dynamical friction on perturbations for evolving dark energy

We explore the impact of dynamical friction on scales where the linear growth factor and the spherical collapse model can be applied and show its influence on the evolution of perturbations. In particular, considering smooth and clustering dark energy models, we describe the role played by friction by selecting two main hierarchical models, i.e., the first where the friction term is proportional to the Hubble rate, whereas the second where friction is induced by the dark energy pressure. The second approach generalises the first and translates the idea that pressure is a general relativistic effect, motivating why friction might arise once barotropic dark energy fluids are considered. The corresponding effects of friction are investigated at the level of linear and nonlinear perturbations, using the formalism of the spherical collapse model. Whilst dynamical friction has very small effects and thus it cannot be excluded a priori, dissipative pressure friction leads to a substantial slow down in the evolution of perturbations. This can be inferred particularly from the halo mass function, for which we also employ corrections due to dark energy clustering. To this end, in order to discern detectable deviations from the standard cosmological model, we thus highlight where dissipation effects might play a significant role at large scales.

astro-ph.CO↗

arXiv2025

The late-time heating Green's function and improvements to distortion frequency hierarchy treatment

Early energy injection leaves an imprint on the observed blackbody spectrum of the CMB, allowing us to study the thermal history of the Universe. For small energy release, the distortion can be efficiently computed using the quasi-exact Green's function method. For pre-recombination injections, the Green's function has already been studied previously. Here we reconsider the pre- and post-recombination periods, showcasing both the spectral distortion intensity and the relative temperature difference, which encrypt precious information about physical processes such as free-free interactions and thermal decoupling. We present the associated distortion visibility function, investigating the impact of various physical effects. We then study improvements to the so-called frequency hierarchy (FH) treatment, a method that was developed for the modelling of anisotropic distortions, which like the average distortion signals encode valuable cosmological information. Specifically, the FH treatment has shortcomings even in the $μ$ era, that in principle should be easy to overcome. In this paper, we introduce a new approach to reduce the mismatch, concluding with a redefinition of the $μ$ spectral shape using CosmoTherm. This solution takes into account double Compton and Bremsstrahlung effects in the low tail, which can be included in the FH. This opens the path towards a refined modeling of spectral distortion anisotropies.

astro-ph.CO↗

arXiv2024

Kinetic coupled tachyon: A dynamical system analysis

We present and examine a kinetically coupled tachyon dark energy model, where a tachyon scalar field interacts with the matter sector. More specifically, we deduce this cosmological setting from a generalised interacting dark energy model that allows for the kinetic term of the scalar field to couple to the matter species a priori in the action. A thorough dynamical system analysis and its cosmological implications unveil the appearance of a scaling solution which is also an attractor of the system, thanks to a novel critical point, with a period of accelerated expansion thereafter. This new solution, not present in the uncoupled case, has the enticing consequence of alleviating the coincidence problem.

gr-qc↗

arXiv2024

Linear perturbations in Horndeski theories with spatial curvature

We analyse the implications of the presence of spatial curvature in modified gravity models. As it is well known, the current standard cosmological model, the $Λ$CDM, is assumed to be spatially flat based on the results of many experiments. However, this statement does not necessarily hold for a modified gravity (MG) model, and this leads to couplings of the spatial curvature with the parameters of the chosen cosmological model. In this paper, we illustrate the theoretical development of how spatial curvature affects the equations of motion at linear order for scalar and tensor perturbations modes using a model-independent approach based on the formalism of the Effective Field Theory (EFT) of dark energy (DE). The results show that spatial curvature gives rise to a coupling with the scalar field perturbations and the functions parameterizing the model.

gr-qc↗

arXiv2024

Linear and nonlinear clusterings of Horndeski-inspired dark energy models with fast transition

We analyze time-dependent dark energy equations of state through linear and nonlinear structure formation and their quintessence potentials, characterized by fast, recent transitions, inspired by parameter space studies of selected classes of the more general Horndeski models. The influence of dark energy on structures comes from modifications to the background expansion rate and from perturbations as well. In order to compute the structures growth, we employ a generalization of the \emph{spherical collapse} formalism that includes perturbations of fluids with pressure. We numerically solve the equations of motion for the perturbations and the field. Our analysis suggests that a true Heaviside step transition is a good approximation for most of the considered models, since most of the quantities weakly depend on the transition speed. We find that transitions occurring at redshifts $z_{\rm t}\gtrsim 2$ cannot be distinguished from the $Λ$CDM model if dark energy is freezing, i.e, the corresponding equation of state tends to $-1$. For fast, recent transitions, the redshift at which the properties of dark energy have the most significant effect is $z=0.6\pm 0.2$. We also find that in the freezing regime, the $σ_8$ values can be lowered by about $8\%$, suggesting that those models could relieve the $σ_8$-tension. Additionally, freezing models generally predict faster late-time merging rates but a lower number of massive galaxies at $z=0$. Finally, the nonlinear matter power spectrum for smooth dark energy shows a valley centered in $k\approx1\,h\,{\rm Mpc}^{-1}$ which in the clustering case is replaced by a sharp increase for $k\gtrsim 0.2\,h\,{\rm Mpc}^{-1}$ and a peak at $k\approx 2\,h\,{\rm Mpc}^{-1}$.

astro-ph.CO↗

arXiv2024

Spherical collapse and halo abundance in shift-symmetric Galileon theory

We present the nonlinear growth of bound cosmological structures using the spherical collapse approach in the shift-symmetric Galileon theories. In particular, we focus on the class of models belonging to the Kinetic Gravity Braiding by adopting a general parametrization of the action encoding a large set of models by means of four free parameters: two defining the background evolution and two affecting the perturbations. For the latter we identify their specific signatures on the linearised critical density contrast, nonlinear effective gravitational coupling and the virial overdensity and how they drive their predictions away from $Λ$CDM. We then use the results of the spherical collapse model to predict the evolution of the halo mass function. We find that the shift-symmetric model predicts a larger number of objects compared to $Λ$CDM for masses $M \gtrsim 10^{14} h^{-1} \mathrm{M}_\odot$ and such number increases for larger deviations from the standard model. Therefore, the shift-symmetric model shows detectable signatures which can be used to distinguish it from the standard scenario.

astro-ph.CO↗

arXiv2022

Report of the Topical Group on Cosmic Probes of Fundamental Physics for for Snowmass 2021

Cosmic Probes of Fundamental Physics take two primary forms: Very high energy particles (cosmic rays, neutrinos, and gamma rays) and gravitational waves. Already today, these probes give access to fundamental physics not available by any other means, helping elucidate the underlying theory that completes the Standard Model. The last decade has witnessed a revolution of exciting discoveries such as the detection of high-energy neutrinos and gravitational waves. The scope for major developments in the next decades is dramatic, as we detail in this report.

hep-ph↗

arXiv2022

A 3D Phase Space Analysis of Scalar Field Potentials

In this study, we present the phase-space analysis of Quintessence models specified by the choice of two potentials, namely the Recliner potential and what we call the broken exponential-law potential, which is a new proposal. Using a dynamical system analysis we provide a systematic study of the cosmological evolution of the two models and their properties. We find new scaling solutions characterised by a constant ratio between the energy density of the scalar field and that of the matter component. These solutions are of high interest in light of the possibility to alleviate the coincidence problem. Additionally, the models also show attractor solutions. We finally construct concrete models built using a double potential according to which one potential realises the early-time scaling regime and the second one allows to exit this regime and to enter in the epoch of cosmic acceleration driven by a scalar-field dominated attractor point.

gr-qc↗

arXiv2022

Tidal virialization of dark matter haloes with clustering dark energy

We extend the analysis of Pace et al., JCAP, 2019, 060, by considering the virialization process in the extended spherical collapse model for clustering dark-energy models, i.e., accounting for dark-energy fluctuations. Differently from the standard approach, here virialization is naturally achieved by properly modelling deviations from sphericity due to shear and rotation induced by tidal interactions. We investigate the time evolution of the virial overdensity $Δ_\mathrm{vir}$ in seven clustering dynamical dark energy models and compare the results to the $Λ$CDM model and to the corresponding smooth dark-energy models. Taking into account all the appropriate corrections, we deduce the abundance of convergence peaks for Rubin Observatory-LSST and Euclid-like weak-lensing surveys, of Sunyaev-Zel'dovich peaks for a Simon Observatory-like CMB survey, and of X-ray peaks for an eROSITA-like survey. Despite the tiny differences in $Δ_\mathrm{vir}$ between clustering and smooth dark-energy models, owing to the large volumes covered by these surveys, five out of seven clustering dark-energy models can be statistically distinguished from $Λ$CDM. The contribution of dark-energy fluctuation cannot be neglected, especially for the Chevallier-Polarski-Limber and Albrecht-Skordis models, provided the instrumental configurations provide high signal-to-noise ratio. These results are almost independent of the tidal virialization model.

astro-ph.CO↗

arXiv2021

Comparison of different approaches to the quasi-static approximation in Horndeski models

A quasi-static approximation (QSA) for modified gravity can be applied in a number of ways. We consider three different analytical formulations based on applying this approximation to: (1) the field equations; (2) the equations for the two metric potentials; (3) the use of the attractor solution derived within the Equation of State (EoS) approach. We assess the veracity of these implementations on the effective gravitational constant ($μ$) and the slip parameter ($η$), within the framework of Horndeski models. In particular, for a set of models we compare cosmological observables, i.e., the matter power spectrum and the CMB temperature and lensing angular power spectra, computed using the QSA, with exact numerical solutions. To do that, we use a newly developed branch of the CLASS code: QSA_class. All three approaches agree exactly on very small scales. Typically, we find that, except for $f(R)$ models where all the three approaches lead to the same result, the quasi-static approximations differ from the numerical calculations on large scales ($k \lesssim 3 - 4 \times 10^{-3}\,h\,{\rm Mpc}^{-1}$). Cosmological observables are reproduced to within 1% up to scales ${\rm K} = k/H_0$ of the order of a few and $\ell>5$ for the approaches based on the field equations and on the EoS, and we also do not find any appreciable difference if we use the scale-dependent expressions for $μ$ and $η$ with respect to the value on small scales, showing that the formalism and the conclusions are reliable and robust, fixing the range of applicability of the formalism. We discuss why the expressions derived from the equations for the potentials have limited applicability. Our results are in agreement with previous analytical estimates and show that the QSA is a reliable tool and can be used for comparison with current and future observations to constrain models beyond $Λ$CDM.

astro-ph.CO↗

arXiv2021

Testing generalized logotropic models with cosmic growth

We check the dynamical and observational features of four typologies of logotropic dark energy models, leading to a \emph{thermodynamic cosmic speed up} fueled by a single fluid that unifies dark energy and dark matter. We first present two principal Anton-Schmidt fluids where the Grüneisen parameter $γ_{\rm G}$ is free to vary and then fixed to the special value $γ_{\rm G}=\tfrac{5}{6}$. We also investigate the pure logotropic model, corresponding to $γ_{\rm G}=-\frac{1}{6}$. Finally, we propose a new logotropic paradigm that works as a generalized logotropic fluid, in which we split the role of dark matter and baryons. We demonstrate that the logotropic paradigms may present drawbacks in perturbations, showing a negative adiabatic sound speed which make perturbations unstable. The Anton-Schmidt model with $γ_{\rm G}=\frac{5}{6}$ is ruled out while the generalized logotropic fluid seems to be the most suitable one, albeit weakly disfavored than the $Λ$CDM model. We combine low- and higher-redshift domains through experimental fits based on Monte Carlo Markov Chain procedures, taking into account supernovae Ia catalogue, Hubble measurements and $σ_8$ data points. We consider two model selection criteria to infer the statistical significance of the four models. We conclude there is statistical advantage to handle the Anton-Schmidt fluid with the Grüneisen parameter free to vary and/or fixed to $γ_{\rm G}=-\frac{1}{6}$. The generalized logotropic fluid indicates suitable results, statistically favored than the other models, until the sound speed is positive, becoming unstable in perturbations elsewhere. We emphasize that the $Λ$CDM paradigm works statistically better than any kinds of logotropic and generalized logotropic models, while the Chevallier-Polarski-Linder parametrization is statistically comparable with logotropic scenarios.

astro-ph.CO↗

arXiv2021

Cosmological gravity on all scales II: Model independent modified gravity $N$-body simulations

Model-independent constraints on modified gravity models hitherto exist mainly on linear scales. A recently developed formalism presented a consistent parameterisation that is valid on all scales. Using this approach, we perform model-independent modified gravity $N$-body simulations on all cosmological scales with a time-dependent $μ$. We present convergence tests of our simulations, and we examine how well existing fitting functions reproduce the non-linear matter power spectrum of the simulations. We find that although there is a significant variation in the accuracy of all of the fitting functions over the parameter space of our simulations, the ReACT framework delivers the most consistent performance for the matter power spectrum. We comment on how this might be improved to the level required for future surveys such as Euclid and the Vera Rubin Telescope (LSST). We also show how to compute weak-lensing observables consistently from the simulated matter power spectra in our approach, and show that ReACT also performs best when fitting the weak-lensing observables. This paves the way for a full model-independent test of modified gravity using all of the data from such upcoming surveys.

astro-ph.CO↗