arXiv subjects

Nakia Carlevaro

Publications and source records attributed to Nakia Carlevaro.

At least 19 recordsLinked to original sources

arXiv2026

QCD CP-violation scenario for a revised cosmological dynamics: analysis of the binned Pantheon Sample of Super Novae Ia

We investigate a modified cosmological dynamics in which the Universe is composed of baryonic matter and a complex (classical) scalar field. The phase component of this field is identified with the axion field, which accounts for the dark matter contribution, while its modulus follows a $\lambda\phi^4$-like theory, associated with a dominant constant energy density and describing the dark energy component of the Universe. When the potential term of this complex scalar field is studied near its maximum, it naturally provides an interaction term between dark matter and dark energy. The cosmological model that emerges from this physical framework leads to a modified $\Lambda$CDM dynamics, in which the dark matter contribution is slightly and monotonically suppressed. We then construct the effective running Hubble constant associated with this revised cosmological scenario and we compare this diagnostic tool with the binned data of the Pantheon Sample of Type Ia Supernovae. As a result of the fitting procedure, we are able to provide a satisfactory interpretation of the data in terms of our theoretical conjecture that results statistically favored with respect to the $\Lambda$CDM model.

astro-ph.CO↗

arXiv2026

Modifying $\Lambda$CDM dynamics via out-of-equilibrium axions: reconciling SH0ES and DESI $H_0$ values

We investigate late-Universe dynamics in which the dark matter component is described by axion particles. The proposed framework departs from the standard $\Lambda$CDM paradigm due to a small fraction of axions driving the system away from thermal equilibrium. We analyze the evolution of the axion energy density using both a kinetic and a classical field approach, yielding an identical macroscopic evolution equation for the dark matter density. We emphasize that the BGK parameter is introduced phenomenologically at the kinetic level and this does not supply an independent microscopic derivation. The present work therefore explores the phenomenological consequences of late-time, out-of-equilibrium axion production rather than claiming a completed microphysical model. The resulting scenario modifies $\Lambda$CDM dynamics in the late Universe (specifically at $z \lesssim 1$), while asymptotically recovering the standard baseline at earlier cosmic epochs. We compare the theoretical predictions of our formulation against a comprehensive suite of late-Universe datasets. Our statistical analysis reveals that when the SH0ES local calibration is included, the collisional axion model becomes significantly favored over $\Lambda$CDM, yielding a best-fit Hubble constant of $H_0 \simeq 73~{\rm km\,s^{-1}\,Mpc^{-1}}$. Ultimately, this cosmological scenario successfully accommodates local distance-ladder measurements while maintaining excellent agreement with Baryon Acoustic Oscillation data from the DESI Collaboration.

astro-ph.CO↗

arXiv2026

Reproducing anomalous transport coefficients from electro-static tokamak edge turbulent dynamics

Turbulent transport near the X-point of a large tokamak is examined using local, gradient-driven simulations that determine the saturated plasma profiles. The distribution of a representative set of particle tracers evolving within these profiles is then analyzed. The study demonstrates that the resulting transport is diffusive, characterized by a coefficient that depends on the spectral properties of the turbulent energy and attains anomalous high values under broad conditions. These findings suggest that anomalous transport is an inherent outcome of the fundamental non-linear drift dynamics of plasmas. The scaling of transport with turbulent energy is also addressed, with implications for future progress toward a mean-field framework for turbulent transport.

physics.plasm-ph↗

arXiv2025

Decay of $f(R)$ quintessence into dark matter: mitigating the Hubble tension?

We propose a revised cosmological scenario that extends the $\Lambda$ Cold Dark Matter ($\Lambda$CDM) framework by incorporating metric $f(R)$ gravity in the Jordan frame. In this model, the dark energy component arises from a non-minimally coupled scalar field, decomposed into a smooth background (set to unity to recover General Relativity) and a rapidly varying, massive fluctuation that decays into the dark matter sector. In the near-GR limit, this setup provides a phenomenological extension of $\Lambda$CDM characterized by two additional parameters: the present-day value of the scalar fluctuation and a normalized decay rate. Using a Markov Chain Monte Carlo analysis of low-redshift cosmological data, comprising Type Ia Supernovae, Baryon Acoustic Oscillation (BAO), and Cosmic Chronometer measurements, we find that the proposed model achieves a better overall fit than $\Lambda$CDM, while the Bayesian evidence remains statistically inconclusive given the inclusion of two extra parameters. The model predicts a moderate increase in the inferred value of $H_0$ and an improved consistency with DESI BAO data when adopting the SH0ES prior. Furthermore, describing dark matter particle creation as a transition phase in the late Universe offers an intriguing physical interpretation, potentially capturing features already present in current data and providing a promising avenue to explore extensions of the standard cosmological model within modified gravity frameworks.

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 Hubble constant: evolutionary Dark Energy

We discuss an evolutionary dark energy model, based on the presence of non-equilibrium effects on the dark energy constituents, which are described via a bulk viscosity contribution. We implement the proposed dynamics by the analysis of the 40-bins Type Ia Supernovae (SNe) Pantheon sample data, in order to outline the existence of a running Hubble constant with the redshift. Via a fitting procedure, we determine the value of the additional parameter that our model possesses with respect a standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) scenario. As important result, the evolutionary dark energy proposal seems more appropriate to describe the binned SN analysis with respect to the $\Lambda$CDM Hubble parameter, i.e. a non running value for the Hubble constant over the bins.

gr-qc↗

arXiv2024

Analysis of non-diffusive avalanche transport of energetic particles

The dynamics of energetic particles (EPs) interacting with Alfv\'en eigenmodes (AEs) for the ITER 15MA baseline scenario was described using a reduced 1D model in [Carlevaro et al. PPCF 64, 035010 (2022)], and successfully tested against nonlinear wave-particle simulations. In this paper, we introduce a detailed phase-space and statistical analysis of this case to characterize the emerging EP transport regimes. Deviations from pure diffusive dynamics are quantitatively addressed, indicating the limitations of standard quasi-linear descriptions. The phase space diagnostics introduced allows to describe the emergence of a very complex dynamics of overlapping resonances and substructure formation, reinforcing the evidence of non-diffusive domino-like AEs.

physics.plasm-ph↗

arXiv2024

Turbulent transport regimes in the presence of an X-point magnetic configuration

We analyze the transport properties of the two-dimensional electrostatic turbulence characterizing the edge of a Tokamak device from the study of test particles motion (passive fluid tracers) following the EXB drift. We perform statistical tests on the tracer population in order to assess both the magnitude and the main features of transport. The role of other physical properties, such as viscosity and inverse energy cascade in the spectrum, is also considered. We outline that large scale eddies are responsible for greater transport coefficients, while the presence of an X-point magnetic field reduces the mean free path of the particles, however generating a larger outliers population with respect to a Gaussian profile.

physics.plasm-ph↗

arXiv2024

Modified gravity in the presence of matter creation: scenario for the late Universe

We consider a dynamic scenario for characterizing the late Universe evolution, aiming to mitigate the Hubble tension. Specifically, we consider a metric $f(R)$ gravity in the Jordan frame which is implemented to the dynamics of a flat isotropic Universe. This cosmological model incorporates a matter creation process, due to the time variation of the cosmological gravitational field. We model particle creation by representing the isotropic Universe (specifically, a given fiducial volume) as an open thermodynamic system. The resulting dynamical model involves four unknowns: the Hubble parameter, the non-minimally coupled scalar field, its potential, and the energy density of the matter component. We impose suitable conditions to derive a closed system for these functions of the redshift. In this model, the vacuum energy density of the present Universe is determined by the scalar field potential, in line with the modified gravity scenario. Hence, we construct a viable model, determining the form of the $f(R)$ theory a posteriori and appropriately constraining the phenomenological parameters of the matter creation process to eliminate tachyon modes. Finally, by analyzing the allowed parameter space, we demonstrate that the Planck evolution of the Hubble parameter can be reconciled with the late Universe dynamics, thus alleviating the Hubble tension.

gr-qc↗

arXiv2024

Kinetic Model for Dark Energy - Dark Matter Interaction: Scenario for the Hubble Tension

We analyze a model for Dark Energy - Dark Matter interaction, based on a decaying process of the former into the latter. The dynamical equations are constructed following a kinetic formulation, which separates the interacting fluctuations from an equilibrium distribution of both species. The emerging dynamical picture consists of coupled equations, which are specialized in the case of a Dark Energy equation of state parameter; we deal with a modified Lambda Cold Dark Matter ($\Lambda$CDM) model, which is investigated versus a possible interpretation of the Hubble tension. Using an optimized set of the model's free parameters, it can be shown that the obtained Hubble parameter can, in principle, address the tension. We then use the most recent datasets from late Universe sources and compressed information from the Cosmic Microwave Background data to constrain the free parameters and compare the addressed scenario to the standard $\Lambda$CDM model. The study outlines how our proposal is preferred by the data in all cases, based on fit quality, while also alleviating the tension.

gr-qc↗

arXiv2024

Symmetries of the Large Scale Structures of the Universe as a Phenomenology of a Fractal Turbulence: The Role of the Plasma Component

We present a new perspective on the symmetries that govern the formation of large-scale structures across the Universe, particularly focusing on the transition from the seeds of galaxy clusters to the seeds of galaxies themselves. We address two main features of cosmological fluid dynamics pertaining to both the linear and non-linear regimes. The linear dynamics of cosmological perturbations within the Hubble horizon is characterized by the Jeans length, which separates stable configurations from unstable fluctuations due to the gravitational effect on sufficiently large (and therefore, massive enough) overdensities. On the other hand, the non-linear dynamics of the cosmological fluid is associated with a turbulent behavior once the Reynolds numbers reach a sufficiently high level. This turbulent regime leads to energy dissipation across smaller and smaller scales, resulting in a fractal distribution of eddies throughout physical space. The proposed scenario suggests that the spatial scale of eddy formation is associated with the Jeans length of various levels of fragmentation from an original large-scale structure. By focusing on the fragmentation of galaxy cluster seeds versus galaxy seeds, we arrived at a phenomenological law that links the ratio of the two structure densities to the number of galaxies in each cluster and to the Hausdorff number of the Universe matter distribution. Finally, we introduced a primordial magnetic field and studied its influence on the Jeans length dynamics. The resulting anisotropic behavior of the density contrast led us to infer that the main features of the turbulence could be reduced to a 2D Euler equation. Numerical simulations showed that the two lowest wavenumbers contained the major energy contribution of the spectrum.

astro-ph.CO↗

arXiv2023

Slow-rolling scalar dynamics and as solution for the Hubble tension

We construct a theoretical framework to interpret the Hubble tension by means of a slow-rolling dynamics of a self-interacting scalar field. In particular, we split the Friedmann equation in order to construct a system for the three unknowns, corresponding to the Hubble parameter $H$, the scalar field $\phi$ and its self-interaction potential $V$, as functions of the redshift. In the resulting picture, the vacuum energy density is provided by a constant term in the potential $V(\phi)$, while the corresponding small kinetic term is responsible for reproducing the apparent variation of the Hubble constant $H_0$ with the redshift. The emerging solution depends on two free parameters, one of which is fixed to account for the discrepancy between the values of $H_0$ as measured by the Super Nova Ia sample ($H_0=73.6\pm1.1$ km s$^{-1}$ Mpc$^{-1}$ (Brout et al., 2022)) and the Planck satellite data ($H_0=67.4\pm0.5$ km s$^{-1}$ Mpc$^{-1}$ (Aghanim et al., 2020)), respectively. The other parameter is instead determined by a fitting procedure of the apparent Hubble constant variation across the data corresponding to a 40 bin analysis of the Super Nova Pantheon sample, in each of which $H_0$ has been independently determined. The fundamental result of the present analysis is the emerging Hubble parameter as function of the redshift, which correctly takes the Super Nova Ia prediction at $z=0$ and naturally approaches the profile predicted by a flat $\Lambda$CDM model corresponding to the cosmological parameters detected by Planck. It is remarkable that this achievement is reached without reducing the Super Nova Ia data to a single point for determining $H(z=0)$, but accounting for the distribution over their redshift interval of observation, via the binned analysis.

gr-qc↗

arXiv2023

On the Effects of Tokamak Plasma Edge Symmetries on Turbulence Relaxation

The plasma edge of a tokamak configuration is characterized by turbulent dynamics leading to enhanced transport. We construct a simplified 3D Hasegawa--Wakatani model reducing to a single partial differential equation for the turbulent electric potential dynamics. Simulations demonstrate how the 3D turbulence relaxes on a 2D axisymmetric profile, corresponding to the so-called interchange turbulence. The spectral features of this regime are found to be strongly dependent on the initialization pattern. We outline that the emergence of axisymmetric turbulence is also achieved when the corresponding mode amplitude is not initialized. Then, we introduce the symmetries of the magnetic X-point of a tokamak configuration. We linearize the governing equation by treating the poloidal field as a small correction. We show that it is not always possible to solve the electric potential dynamics following a perturbative approach. This finding, which is due to resonance between the modes of the background and the poloidal perturbation, confirms that the X-point symmetries can alter the properties of turbulent transport in the edge region.

physics.plasm-ph↗

arXiv2023

Metric $f(R)$ gravity with dynamical dark energy as a scenario for the Hubble tension

We introduce a theoretical framework to interpret the Hubble tension, based on the combination of a metric $f(R)$ gravity with a dynamical dark energy contribution. The modified gravity provides the non-minimally coupled scalar field responsible for the proper scaling of the Hubble constant, in order to accommodate for the local SNIa pantheon+ data and Planck measurements. The dynamical dark energy source, which exhibits a phantom divide line separating the low red-shift quintessence regime ($-1<w<-1/3$) from the phantom contribution ($w<-1$) in the early Universe, guarantees the absence of tachyonic instabilities at low red-shift. The resulting $H_0(z)$ profile rapidly approaches the Planck value, with a plateau behaviour for $z\gtrsim 5$. In this scenario, the Hubble tension emerges as a low red-shift effect, which can be in principle tested by comparing SNIa predictions with far sources, like QUASARS and Gamma Ray Bursts.

gr-qc↗

arXiv2023

On the 3D turbulence regime in a Tokamak plasma edge

We derive a reduced model for the electrostatic turbulence in a Tokamak edge, when dealing with a resistive plasma and neglecting the spatial gradient of the background density which triggers the linear drift wave response. The obtained dynamics, de facto equivalent to a Hasegawa-Wakatani model, is characterized by a constitutive relation between the electric potential Laplacian and the density, which allows to deal with a single 3D equation governing the electric potential fluctuations. We study the evolution of the model, by separating the $n=0$ ($n$ indicating toroidal-like number) mode from all the other ones. Then, we linearize the dynamics of the $n\neq 0$ modes around the steady 2D spectrum, which describes the spectral features of the electrostatic 2D turbulence. We theoretically and numerically demonstrate the existence of decaying branch of the 3D turbulence, having such a 2D steady spectrum as a natural attractor. This result suggests that the basic constituent of the self-sustained non-linear drift response in the plasma edge has to be individualized in the non-linear 2D electrostatic turbulence.

physics.plasm-ph↗

arXiv2022

Energetic particle transport: diffusion vs convection and phase-space barriers

Energetic particle redistribution in the presence of multiple Alfv\'en eigenmodes is analyzed in [PPCF 58, 014019 (2016)] for the ITER 15MA baseline scenario: non-linear hybrid simulations (within their well known limits) point out that transport can be dominated by avalanches under certain conditions. These phenomena are properly reproduced by the 1D reduced description of [PPCF 64, 035010 (2022)]. Here, using this simplified 1D model, we define the transport character (convective/diffusive) of self-consistent energetic particle redistribution. Transport barriers in phase space are studied using the Lagrangian Coherent Structures technique.

physics.plasm-ph↗

arXiv2022

On the Turbulent Behavior of a Magnetically Confined Plasma Near the X-Point

We construct a model for the turbulence near the X-point of a Tokamak device and, under suitable assumptions, we arrive to a closed equation for the electric field potential fluctuations. The analytical and numerical analysis is focused on a reduced two-dimensional formulation of the dynamics, which allows a direct mapping to the incompressible Navier-Stokes equation. The main merit of this study is to outline how the turbulence near the X-point, in correspondence to typical operation conditions of medium and large size Tokamaks, is dominated by the enstrophy cascade from large to smaller spatial scales.

physics.plasm-ph↗

arXiv2022

One dimensional reduced model for ITER relevant energetic particle transport

We set up a mapping procedure able to translate the evolution of the radial profile of fast ions, interacting with Toroidal Alfvén Eigenmodes, into the dynamics of an equivalent one dimensional bump-on-tail system. We apply this mapping technique to reproduce ITER relevant simulations, which clearly outlined deviations from the diffusive quasi-linear (QL) model. Our analysis demonstrates the capability of the one-dimensional beam-plasma dynamics to predict the relevant features of the non-linear hybrid LIGKA/HAGIS simulations. In particular, we clearly identify how the deviation from the QL evolutive profiles is due to the presence of avalanche processes. A detailed analysis regarding the reduced dimensionality is also addressed, by means of phase-space slicing based on constants of motion. In the conclusions, we outline the main criticalities and outcomes of the procedure, which must be satisfactorily addressed to make quantitative prediction on the observed outgoing fluxes in a Tokamak device.

physics.plasm-ph↗