arXiv subjects

Stefano Viaggiu

Publications and source records attributed to Stefano Viaggiu.

At least 19 recordsLinked to original sources

arXiv2026

Viaggiu holographic dark energy in light of DESI DR2

We test the cosmological viability of the Viaggiu holographic dark energy (VHDE) model by using late-time observational data. In particular, we place constraints on the free parameters of the model using Type Ia supernovae from the PantheonPlus, Union3.0, and DES-Dovekie catalogues, the Cosmic Chronometers, and the Baryon Acoustic Oscillations from the DESI DR2. Our analysis suggests that the VHDE model fits the observational data better or similar to the $\Lambda$CDM for all dataset combinations considered. The value obtained for $H_0$ is similar to the $\Lambda$CDM, while the current matter density parameter is constrained around $\Omega_{m0}\simeq 0.24$, smaller to that obtained by the $\Lambda$CDM. Moreover, the parameter introduced by the VHDE is found to have a mean value within the range $\frac{\pi}{3} \delta^2 \sim 0.27-0.33$. Finally, we used Akaike's Information Criterion (AIC) and Bayesian evidence to test the VHDE model against the $\Lambda$CDM scenario. The AIC demonstrates that the two models are statistically indistinguishable, while Bayesian evidence reveals that the data have a mild preference for the $\Lambda$CDM model for most of the dataset combinations considered. Nevertheless, the VHDE model remains consistent with current late-time cosmological observations and offers a feasible mechanism for describing the late-time accelerating scenario.

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arXiv2025

Corrections to the Black hole entropy from a Bose Einstein condensate: a semi-classical phenomenological approach

In this paper we obtain logarithmic corrections to the black hole entropy. Motivated by our recent proposal concerning the nature of the degrees of freedom leading to the black hole entropy in terms of a Bose Einstein (BEC) condensate of gravitons, we study how to introduce logarithmic corrections. In fact we show that, after modifying the internal energy by means of simple by physically sound arguments dictated by ordinary quantum mechanics and possible non-commutative effects at Planckian scales, a logarithmic term does appear in the Bekenstein Hawking entropy law. We also obtain that the entropy $S_{BH}$ of a ball of Planckian areal radius is $2\pi K_B$, i.e. $S_{BH}(R=L_P)=2\pi K_B$. Our approach show that the possibility that the interior of a black hole is composed with a BEC of gravitons is a viable physically motivated possibility.

hep-th↗

arXiv2024

Physically viable rotating mass solutions surrounding Kerr black hole

There exists in literature an increasing interest in the study of mass distributions surrounding black holes as describing dark matter halo in spiral galaxies. Motivated by this interest, we study a very recent new class of rotating solutions that are suitable to build anisotropic matter sources surrounding rotating black holes. Contrary to the mainstream approach, instead of use the so called regular black holes as central objects, we perform a smooth matching between the aforementioned anisotropic matter and a central vacuum Kerr black hole. In this framework, we study in full generality energy conditions near the matching surface. As a result, we found that, after imposing the vanishing of the energy density $E$ at the matching surface, if weak and dominant energy conditions (WEC,DEC) are satisfied, then unavoidable strong energy conditions is violated, i.e. near the event horizon only matter with dark energy-like features is allowed. As an application, we present two solutions everywhere satisfying DEC. The first one is asymptotically flat and equipped with a non vanishing electric charge, while the second solution presented is equipped with a non-vanishing energy flow around the symmetry axis and it is not asymptotically flat

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arXiv2023

A new class of anisotropic rotating fluids and some throat-like sources for Kerr metric as examples

Motivated by the increasing interest in finding physically viable rotating sources, we present a new class of anisotropic rotating solutions. The energy-momentum tensor compatible with the metric is composed of anisotropic matter with a non-vanishing energy flow around the symmetry axis and vanishing viscosity. The new class of solutions can be used to find new possible sources for the Kerr metric, to obtain new regular black hole solutions and to study galaxies with a central rotating black hole and an halo of dark matter. As an example, we obtain a 5-parameter class of solutions representing a two-way traversable wormhole smoothly matched to the Kerr one and satisfying all energy conditions outside the wormhole for a wide range of parameters, in particular for compact objects. Finally, with a simple modification of the aforementioned solution, we obtain a source for Kerr metric with a throat geometry, non-representing a two-way traversable wormhole and satisfying all energy conditions.

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arXiv2023

On the perturbative formalism and a possible quantum discrete spectrum for the Regge-Wheeler equation of a de Sitter spacetime

In this paper we study the perturbative regime in the static patch of de Sitter metric in the Regge-Wheeler formalism. After realizing that perturbative regime in a de Sitter spacetime depicted in terms of usual spherical coordinates cannot be extended up to the cosmological horizon, we study perturbative equations, in particular the axial ones, in terms of the tortoise coordinate $r_*$. We show that perturbative regime can be extended up to the cosmological horizon, provided that suitable boundary conditions are chosen. As an application, we explore the Regge-Wheeler equation at short distances by performing a taylor expansion. In order to study some possible quantum effects at short distances, we impose to the equation so obtained the same boundary conditions suitable for a quantum 3D harmonic oscillator. As a result, a discrete spectrum can be obtained. The aforementioned spectrum is analysed and a relation with possible effects denoting quantum behavior of gravitons is suggested.

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arXiv2021

The physical nature of the cosmological constant and the decoherence scale in a renormalization-group approach

In this paper we consider the nature of the cosmological constant as due by quantum fluctuations. Quantum fluctuations are generated at Planckian scales by noncommutative effects and watered down at larger scales up to a decoherence scale $L_D$ where classicality is reached. In particular, we formally depict the presence of the scale at $L_D$ by adopting a renormalization group approach. As a result, an analogy arises between the expression for the observed cosmological constant $\overline{\Lambda}$ generated by quantum fluctuations and the one expected by a renormalization group approach, provided that the renormalization scale $\mu$ is suitably chosen. In this framework, the decoherence scale $L_D$ is naturally identified with the value ${\mu}_D$, with $\hbar{\mu}_D$ representing the minimum allowed particle-momentum for our visible universe. Finally, by mimicking renormalization group approach, we present a technique to formally obtain a non-trivial infrared (IR) fixed point at $\mu=\mu_D$ in our model.

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arXiv2020

A proposal for Heisenberg uncertainty principle and STUR for curved backgrounds: an application to white dwarf, neutron stars and black holes

After a critical overview of the Generalized Uncertainty Principle (GUP) applied to compact objects, we propose a texture of Heisenberg uncertainty principle in curved spacetimes (CHUP). CHUP allows to write down physically motivated STUR (spacetime uncertainty relations) in a generic background for a non commutative spacetime in terms of tetrad variables. In order to study possible quantum effects for compact astrophysical objects as white dwarf, neutron stars and black holes, an expression for quantum fluctuations is outlined. As a result, contrary to GUP-based claims, we found no evidence for quantum effects concerning equilibrium equation and critical mass $M_c$ for white dwarf and neutron stars. Conversely, our expression for CHUP confirms that general relativistic effects strongly reduce the Oppenheimer-Volkoff Newtonian limit for very compact astrophysical objects as neutron stars. In particular, we found that for a degenerate relativistic Fermi gas, the maximum mass decreases for increasing compactness of the star with a minimum critical mass $M_c\simeq 0.59 M_{\odot}$ at the Buchdahl limit. Finally, we study possible non commutative effects near the event horizon of a black hole.

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arXiv2020

Einstein, Planck and Vera Rubin: relevant encounters between the Cosmological and the Quantum Worlds

In Cosmology and in Fundamental Physics there is a crucial question like: where the elusive substance that we call Dark Matter is hidden in the Universe and what is it made of?, that, even after 40 years from the Vera Rubin seminal discovery does not have a proper answer. Actually, the more we have investigated, the more this issue has become strongly entangled with aspects that go beyond the established Quantum Physics, the Standard Model of Elementary particles and the General Relativity and related to processes like the Inflation, the accelerated expansion of the Universe and High Energy Phenomena around compact objects. Even Quantum Gravity and very exotic DM particle candidates may play a role in framing the Dark Matter mystery that seems to be accomplice of new unknown Physics. Observations and experiments have clearly indicated that the above phenomenon cannot be considered as already theoretically framed, as hoped for decades. The Special Topic to which this review belongs wants to penetrate this newly realized mystery from different angles, including that of a contamination of different fields of Physics apparently unrelated. We show with the works of this ST that this contamination is able to guide us into the required new Physics. This review wants to provide a good number of these "paths or contamination" beyond/among the three worlds above; in most of the cases, the results presented here open a direct link with the multi-scale dark matter phenomenon, enlightening some of its important aspects. Also in the remaining cases, possible interesting contacts emerges.

gr-qc↗

arXiv2019

The cosmological constant from Planckian fluctuations and the averaging procedure

In this paper I continue the investigation in \cite{1,1b} concerning my proposal on the nature of the cosmological constant. In particular, I study both mathematically and physically the quantum Planckian context and I provide, in order to depict quantum fluctuations and in absence of a complete quantum gravity theory, a semiclassical solution where an effective inhomogeneous metric at Planckian scales or above is averaged. In such a framework, a generalization of the well known Buchert formalism \cite{2} is obtained with the foliation in terms of the mean value $s(\hat{t})$ of the time operator $\hat{t}$ in a maximally localizing state $\{s\}$ of a quantum spacetime \cite{3,4,5,6} and in a cosmological context \cite{7}. As a result, after introducing a decoherence length scale $L_D$ where quantum fluctuations are averaged on, a classical de Sitter universe emerges with a small cosmological constant depending on $L_D$ and frozen in a true vacuum state (lowest energy), provided that the kinematical backreaction is negligible at that scale $L_D$. Finally, I analyse the case with a non-vanishing initial spatial curvature $\mathcal{R}$ showing that, for a reasonable large class of models, spatial curvature and kinematical backreation $\mathcal{Q}$ are suppressed by the dynamical evolution of the spacetime.

gr-qc↗

arXiv2019

The physical origin of the cosmological constant: continuum limit and the analogy with the Casimir effect

In this paper we continue the investigations in \cite{1} concerning the origin of the cosmological constant. First of all, we generalise the results in \cite{1,2} by considering a continuum approximation for a radiation field in a cosmological background. In this way, we clearly show that the bare cosmological constant is obtained with wanishing temperature $T$ and that the specific heat $C$ is zero at the decoherence scale $L_D$. Moreover, we address the issue to fix the parameters present in our model. In particular, we push forward the analogy between our expression for $Λ_L$ at a given proper scale $L$ and the one extrapolated by the Casimir effect. As a consequence, we can fix the decoherence scale $L_D$ at which we have the crossover to classicality to be of the order of $\sim 10^{-5}$ meters. This implies that the actual observed value of the cosmological constant is fixed (frozen) at this new physical scale.

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arXiv2018

An algorithm to generate anisotropic rotating fluids with vanishing viscosity

Starting with generic stationary axially symmetric spacetimes depending on two spacelike isotropic orthogonal coordinates $x^{1}, x^{2}$, we build anisotropic fluids with and without heat flow but with wanishing viscosity. In the first part of the paper, after applying the transformation $x^1\rightarrow J(x^1)$, $x^2\rightarrow F(x^2)$(with $J(x^1), F(x^2)$ regular functions) to general metrics coefficients $g_{ab}(x^1,x^2)\rightarrow g_{ab}(J(x^1), F(x^2))$ with $G_{x^1 x^2}=0$, being $G_{ab}$ the Einstein's tensor, we obtain that ${\tilde{G}}_{x^1 x^2}=0\rightarrow G_{x^1 x^2}(J(x^1),F(x^2))=0$. Therefore, the transformed spacetime is endowed with an energy-momentum tensor $T_{ab}$ with expression $g_{ab}Q_{i}+heat\;term$ (where $g_{ab}$ is the metric and $\{Q_{i}\}, {i=1..4}$ are functions depending on the physical parameters of the fluid), i.e. without viscosity and generally with a non-vanishing heat flow. We show that after introducing suitable coordinates, we can obtain interior solutions that can be matched to the Kerr one on spheroids or Cassinian ovals, providing the necessary mathematical machinery. In the second part of the paper we study the equation involving the heat flow and thus we generate anisotropic solutions with vanishing heat flow. In this frame, a class of asymptotically flat solutions with vanishing heat flow and viscosity can be obtained. Finally, some explicit solutions are presented with possible applications to a string with anisotropic source and a dark energy-like equation of state.

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arXiv2018

On a physical description and origin of the cosmological constant

In this paper we use and extend the results present in \cite{1,2,3,4} and in particular in \cite{4} to obtain a statistical description of the cosmological constant in a cosmological de Sitter universe in terms of massless excitations with Planckian effects. First of all, we show that at a classical level, the cosmological constant $Λ>0$ can be obtained only for $T\rightarrow 0$. Similarly to the black hole case, when quantum effects are taken into account, a representation for $Λ$ is possible in terms of massless excitations, provided that quantum corrections to the Misner-Sharp mass are considered. Moreover, thanks to quantum fluctuations, an effective cosmological constant arises depending on the physical scale under consideration, thus representing a possible solution to the cosmological constant problem without introducing a quintessence field. The smalness of the actual value for $Λ$ can be due to the existence of a quantum decoherence scale above the Planck length such that the spacetime evolves as a pure de Sitter universe with a small averaged cosmological constant frozen in the lowest energy state.

gr-qc↗

arXiv2018

Statistical description of massless excitations within a sphere with a linear equation of state and the dark energy case

In this paper we continue the investigations present in \cite{1}-\cite{3}. In particular, we extend the theorem proved in \cite{3} to any massless excitation in a given spherical box. As a first interesting result, we show that it is possible, contrary to the black hole case studied in detail in \cite{1,2,3}, to build macroscopic configurations with a dark energy equation of state. To this purpose, by requiring a stable configuration, a macroscopic dark fluid is obtained with an internal energy $U$ scaling as the volume $V$, but with a fundamental correction looking like $\sim 1/R$ motivated by quantum fluctuations. Thanks to the proposition in section 3 (and in \cite{3} for gravitons), one can depict the dark energy in terms of massless excitations with a discrete spectrum. This fact open the possibility to test a possible physical mechanism converting usual radiation into dark energy in a macroscopic configuration, also in a cosmological context. In fact, for example, in a Friedmann flat universe with a cosmological constant particles are marginally trapped at the Hubble horizon for any given comoving observer.

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arXiv2018

Building a linear equation of state for trapped gravitons from finite size effects and the Schwarzschild black hole case

In this paper we continue the investigations present in \cite{1} and \cite{2} concerning the spectrum of trapped gravitons in a spherical box, and in particular inside a Schwarzschild black hole (BH). We explore the possibility that, due to finite size effects, the frequency of the radiation made of trapped gravitons can be modified in such a way that a linear equation of state $PV=γU$ for the pressure $P$ and the internal energy $U$ arises. Firstly, we study the case with $U\sim R$, where only fluids with $γ>-\frac{1}{3}$ are possible. If corrections $\sim 1/R$ are added to $U$, for $γ\in[0,\frac{1}{3}]$ we found no limitation on the allowed value for the areal radius of the trapped sphere $R$. Moreover, for $γ>\frac{1}{3}$ we have a minimum allowed value for $R$ of the order of the Planck length $L_P$. Conversely, a fluid with $P<0$ can be obtained but with a maximum allowed value for $R$. With the added term looking like $\sim 1/R$ to the BH internal energy $U$, the well known logarithmic corrections to the BH entropy naturally emerge for any linear equation of state. The results of this paper suggest that finite size effects could modify the structure of graviton's radiation inside, showing a possible mechanism to transform radiation into dark energy.

hep-th↗

arXiv2017

A quantum Friedmann flat spacetime: Uncertainty Relations, Thermodynamics and some cosmological consequences

We present Friedmann flat spacetime uncertainty relations (STUR) together with some cosmological implications. An interesting link between the Principle of "gravitational stability against localization of events" (PGSL) and the holographic Bekenstein entropy bound (HEB) is also investigated. The same theorems leading to our STUR are used to calculate, thanks to the holographic principle, the entropy of the universe at its apparent horizon. The generalized entropy formula can be used to discuss interesting links with a quantum spacetime.

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arXiv2017

A possible new cosmological redshift effect due to $Λ$ on traveling gravitational waves in Friedmann universes

In this paper we continue the investigation concerning the propagation of gravitational waves in a cosmological background using Laplace transform \cite{A}. We analyze the possible physical consequences of the result present in \cite{A} where it is argued that a non-vanishing positive abscissa of convergence caused by the de Sitter expansion factor $a(t)=e^{Ht}$ implies a shift in the frequencies domain of a traveling gravitational waves as measured by a comoving observer. In particular, we show that in a generic asymptotically de Sitter cosmological universe this redshift effect does also arise. Conversely, in a universe expanding with, for example, a power law expansion, this phenomenon does not happen. This physically possible new redshift effect, although negligible for the actual very low value of $Λ$, can have interesting physical consequences concerning for example its relation with Bose-Einstein condensation or more speculatively with the nature of the cosmological constant in terms of gravitons, as recently suggested in \cite{B} near a Bose-Einstein condensation phase.

hep-th↗

arXiv2017

Entropy, temperature and internal energy of trapped gravitons and corrections to the Black Hole entropy

In this paper we study the proposal present in \cite{1} concerning the statistical description of trapped gravitons and applied to derive the semi-classical black hole (BH) entropy $S_{BH}$. We study the possible configurations depending on physically reasonable expressions for the internal energy $U$. In particular, we show that expressions for $U\sim R^k, k\geq 1$, with $R$ the radius of the confining spherical box, can have a semi-classical description, while behaviors with $k<1$ derive from thermodynamic or quantum fluctuations. There, by taking a suitable physically motivated expression for $U(R)$, we obtain the well known logarithmic corrections to the BH entropy, with the usual behaviors present in the literature of BH entropy. Moreover, a phase transition emerges with a positive specific heat $C$ at Planckian lengths instead of the usual negative oneat non-Planckian scales, in agreement with results present in the literature. Finally, we show that evaporation stops at a radius $R$ of the order of the Planck length.

hep-th↗

arXiv2017

Statistical mechanics of gravitons in a box and the black hole entropy

This paper is devoted to the study of the statistical mechanics of trapped gravitons obtained by 'trapping' a spherical gravitational wave in a box. As a consequence, a discrete spectrum dependent on the Legendre index $\ell$ similar to the harmonic oscillator one is obtained and a statistical study is performed. The mean energy $ $ results as a sum of two discrete Planck distributions with different dependent frequencies. As an important application, we derive the semiclassical Bekenstein-Hawking entropy formula for a static Schwarzschild black hole by only requiring that the black hole internal energy $U$ is provided by its ADM rest energy, without invoking particular quantum gravity theories. This seriously suggests that the interior of a black hole can be composed of trapped gravitons at a thermodynamical temperature proportional by a factor $\simeq 2$ to the horizon temperature $T_h$.

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