arXiv subjects

Elisa Fazzari

Publications and source records attributed to Elisa Fazzari.

6 recordsLinked to original sources

arXiv2026

Exploring Hu-Sawicki-like modified gravity with Genetic Algorithms

We investigate whether viable Hu-Sawicki-like $f(R)$ models can produce deviations from $\Lambda\mathrm{CDM}$ that can be tested against current background cosmological data. We adopt a machine-learning approach based on Genetic Algorithms (GA) to reconstruct analytical perturbations around the Hu-Sawicki class of models. We develop a pipeline that interfaces the \texttt{GATO} GA code with the \texttt{CANDI} cosmology code. Each $f(R)$ function generated by the GA is first tested against theoretical viability conditions, including stability, the recovery of a standard matter-dominated epoch, the General Relativity limit, and chameleon screening mechanism. Viable candidates are then passed to \texttt{CANDI} to reconstruct the corresponding background cosmology and are tested against DESI DR2 BAO measurements and the Pantheon+ Type Ia supernova catalogue. %\newline The deviations we find are largest at late times, where the lower curvature makes modified-gravity effects more relevant, and are rapidly suppressed at higher redshift, in agreement with the imposed matching to the matter-dominated era. To further quantify deviations from the standard cosmological model, we compute the $Om(z)$ diagnostic. It shows only a very small departure from the constant $\Lambda\mathrm{CDM}$ behaviour. The effective dark energy equation of state associated with the reconstructed $f(R)$ function also evolves only weakly, showing a mild transition from an effective quintessence-like nature to an effective phantom-like regime. Overall, our results indicate that, within perturbations around the Hu-Sawicki class of models, current background data allow only limited deviations from $\Lambda\mathrm{CDM}$.

astro-ph.CO↗

arXiv2025

Creation of Viscous Dark Energy by the Hubble Flow: Comparison with SNe Ia Master Sample Binned Data

We study a family of cosmological models featuring dynamical dark energy (DE), based on the idea that the creation of its constituents arises from the gravitational field of the expanding universe, whose non-equilibrium physics is described by a non-zero bulk viscosity coefficient. We consider the complete scenario, in which both matter creation and bulk viscosity are present, together with its two limiting cases, in which only one of the two effects is retained. Once each model is constrained by requiring its present-day deceleration parameter $q_0$ to match specific values, the complete scenario introduces up to two additional free parameters with respect to the $\Lambda$CDM model, one of which is the equation of state parameter $w$ of the created dark energy. \textcolor{blue}{We consider two choices for $q_0$: the value predicted by the $\Lambda$CDM model, and one obtained from a background analysis of Fazzari et. al. (2025). To perform the analysis, we construct the effective running Hubble constant, i.e. a theoretical function corresponding to the ratio between the Hubble parameter of our models and the $\Lambda$CDM expansion rate. The theoretical predictions for the effective running Hubble constant of the three models are tested against the Master binned sample of Type Ia Supernovae (SNe Ia), through a Markov Chain Monte Carlo procedure with up to four free parameters. The most important result emerging from this analysis is that, when using the cosmographic $q_0$, all three models exhibit a quintessence-to-phantom transition in the effective equation of state parameter of the dark energy; on the contrary, when using the $q_0$ coming from the $\Lambda$CDM limit, the transition cannot happen, and the effective equation of state parameter is entirely of phantom nature across the considered redshift range.

astro-ph.CO↗

arXiv2025

Cosmographic Footprints of Dynamical Dark Energy

We introduce a novel cosmographic framework to trace the late-time kinematics of the Universe without assuming any underlying dynamics. The method relies on generalized Pad\'e-$(2,1)$ expansions around arbitrary pivot redshifts, which, compared to state-of-the-art calculations, reduce truncation errors by up to two orders of magnitude at high redshift and yield more precise constraints by defining cosmographic parameters exactly where the data lie. This avoids extrapolations, mitigates degeneracies, and enables a clean disentangling of their effects. Using the latest low-redshift datasets, we center the generalized expansion in multiple bins across $z\in[0,1]$ and obtain precise constraints on the redshift evolution of cosmographic parameters. We find that all key parameters deviate from their $\Lambda$CDM predictions in a redshift-dependent way that can be naturally explained within dynamical Dark Energy scenarios. The deceleration parameter $q(z)$ follows a redshift evolution consistent with the Chevallier-Polarski-Linder (CPL) parameterization, while the generalized $Om(z)$ diagnostic shows deviations of up to $\sim4\sigma$ from the constant $\Lambda$CDM expectation, closely matching the CPL predictions. Taken together, these results point to footprints of dynamical Dark Energy in the kinematics of the Universe at $z\lesssim 1$.

astro-ph.CO↗

arXiv2025

Investigating $f(R)$-Inflation: background evolution and constraints

In this work, we investigate the possibility of generating an inflationary mechanism within the framework of a metric-$f(R)$ modified gravity theory, formulated in the Jordan frame. We explore whether the scalar field, non-minimally coupled to gravity and emerging in the Jordan frame, can play the role of the primordial inflaton. Particular attention is devoted to constructing a dynamical scenario in the Jordan frame that exhibits a slow-rolling phase for the scalar field and admits a quasi-de Sitter solution for cosmic evolution. To ensure consistency with the standard cosmological model, we impose a matching condition with the $\Lambda$CDM model at the end of the inflationary phase. Furthermore, to address the problem of the absence of matter after inflation, we consider a radiation-type particle creation process that maintains an approximately constant energy density. We test our theoretical model against background observational data, specifically Pantheon$^+$ calibrated with SH0ES and DESI calibrated with BBN. We asses the model's viability by combining theoretical consistency tests with its predictions for primordial power spectrum observables, and we discuss the implications for alleviating the Hubble constant tension.

astro-ph.CO↗

arXiv2025

Two Dynamical Scenarios for Binned Master Sample Interpretation

We analyze two different scenarios for the late Universe dynamics, resulting into Hubble parameters deviating from the $\Lambda$CDM, mainly for the presence of an additional free parameter, which is the dark energy parameter. The first model consists of a pure evolutionary dark energy paradigm, as result of its creation by the gravitational field of the expanding Universe. The second model also considers an interaction of the evolutionary dark energy with the matter component, postulated via the conservation of the sum of their ideal energy-momentum tensors. These two models are then compared \textit{via} the diagnostic tool of the effective running Hubble constant, with the binned data of the so-called ``Master sample'' for the Type Ia Supernovae. The comparison procedures, based on a standard MCMC analysis, led to a clear preference of data for the dark energy - matter interaction model, which is associated to a phantom matter equation of state parameter (very close to $-1$) when, being left free by data (it has a flat posterior), it is fixed in order to reproduce the decreasing power-law behavior of the effective running Hubble constant, already discussed in literature.

astro-ph.CO↗

arXiv2024

Running Einstein Constant and a Possible Vacuum State of the Universe

We propose a revised formulation of General Relativity for cosmological settings, in which the Einstein constant varies with the energy density of the Universe. We demonstrate that this modification has only phenomenological impact of providing an effective dark energy density expression. Assuming a state close to vacuum, here defined by the vanishing product of the Einstein coupling constant and the Universe's energy density, we perform a Taylor expansion of the theory and hence extend it to the whole domain. In this framework, the (renormalized) vacuum energy problem is studied, and an additional constant pressure term, which induces a Chaplygin-like contribution to the dark energy sector, arises in the late-time dynamics. The correction to the late-time Hubble parameter is investigated by comparing theoretical predictions with the late Universe observational data. Our findings indicate that the current value of the stated vacuum energy is consistent with zero within 1$\sigma$. Implications of the modified $\Lambda$CDM model with respect to the Hubble tension are also discussed.

gr-qc↗