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Samuel Lepe

Publications and source records attributed to Samuel Lepe.

At least 19 recordsLinked to original sources
arXiv2026

The logotropic dark fluid as an energy diffusion in unimodular gravity

We show that the logotropic dark fluid, proposed as a single-fluid unification of dark matter and dark energy, is exactly dissipative unimodular gravity with pressureless matter and the logarithmic energy diffusion function $Q(a)=3A\ln a+Q_{c}$. The correspondence is exact at the level of the homogeneous background, rather than asymptotic, and introduces no parameters beyond those already present in either description. It provides a dynamical reinterpretation of the logotropic temperature $A$, whose origin was left open in the original proposal: up to the expansion rate, $A$ is the rate at which energy is diffused from matter into the geometric sector, $\dot Q=3AH$. The adiabatic sound speed of the effective fluid is $c_{s}^{2}=A/ρ$, where $ρ$ is the unimodular matter energy density, and reaches the speed of light at $a_{s}=a_{M}/2^{1/3}$; the matter energy density then vanishes and changes sign at $a_{M}$, which is precisely the scale factor at which the model is known to become phantom. Since $ρ$ coincides with the enthalpy density $ε+P$ of the logotropic fluid, the phantom branch is the branch on which the matter sector carries negative energy density and on which the apparent horizon entropy decreases. Finally, we show that a one-parameter saturating diffusion law removes these late-time pathologies while preserving the low-redshift phenomenology.

gr-qc↗
arXiv2026

Kaniadakis Holographic Dark Energy: UV$-$IR Duality, Horizon Thermodynamics and Cosmological Constraints

We investigate the cosmological and thermodynamic implications of holographic dark energy derived from the Kaniadakis deformation of the Bekenstein--Hawking entropy. In a spatially flat FLRW universe, the generalized entropy generates an effective dark-energy density with an infrared correction proportional to $H^{-2}$, naturally producing an ultraviolet--infrared structure in the cosmological dynamics. Using the Hayward--Kodama formulation of apparent-horizon thermodynamics, we derive a geometric equation of state and identify Van der Waals-type criticality, characterized by an inverted first-order phase transition and an inverted swallowtail Gibbs potential. We interpret this behavior as the thermodynamic signature of a geometric phase transition of the apparent horizon rather than standard fluid coexistence. We also consider an extended model with a $\dot H$ contribution inspired by the Granda--Oliveros cutoff and show that this unconventional critical behavior persists. At the perturbative level, the same ultraviolet--infrared competition controls the effective sound speed and dark-energy response through a dimensionless coupling $A(z)$, which acts as a geometric order parameter. In the minimal model, $A(z)$ may cross zero during matter domination, marking a transition between infrared- and ultraviolet-dominated regimes, whereas the extended scenario remains ultraviolet dominated. Finally, we confront both models with cosmic chronometers, PantheonPlus Type Ia supernovae, DESI DR2 baryon acoustic oscillations, and redshift-space distortion data. Bayesian comparison with $Λ$CDM, CPL, and standard holographic dark energy strongly disfavors the minimal model. The extended model yields the best maximum-likelihood fit among those considered, but remains disfavored relative to $Λ$CDM after accounting for its larger parameter volume.

gr-qc↗
arXiv2026

Does spatial curvature generate new thermodynamic criticality at the FLRW apparent horizon?

We generalize the apparent-horizon thermodynamic construction of [1] to a FLRW universe with non-zero spatial curvature, non-interacting cold dark matter and holographic-type dark energy. For nonzero curvature, the scale factor is an additional geometric variable in the horizon equation of state. The usual criticality conditions are thus not well defined until a closure prescription for the curvature sector is provided. We introduce a dimensionless curvature variable and restrict the thermodynamic variations to slices of constant value of this variable. For each such slice, a positive holographic coupling and nonlinear powers larger than one guaranty the existence of a unique positive critical point. The critical specific volume, temperature and pressure are shifted by the spatial curvature, while the critical ratio and the mean-field critical exponents remain unchanged. It thus rescales the critical quantities, without generating a new local universality class. We construct a Helmholtz potential out of the physical thermodynamic volume and the entropy conjugate to the rescaled horizon temperature. In the quadratic holographic model, the parametric coexistence curve which is equivalent to the Maxwell construction is derived from the equality of the Gibbs free energies of the competing branches. The associated latent heat disappears at the critical endpoint, resulting in a global first-order coexistence in the fixed-curvature ensemble. Whether a physical FLRW trajectory intersects the critical or coexistence locus is a separate dynamical question because the dimensionless curvature variable generally evolves during cosmological expansion.

gr-qc↗
arXiv2026

Thermodynamic behavior of cosmological models with fractional entropy

We investigate the thermodynamic and phenomenological implications of a cosmological model governed by fractional entropy applied to the apparent horizon of a flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe. By utilizing the unified first law of thermodynamics alongside the Kodama-Hayward temperature, we derive a generalized set of Friedmann equations characterized by a fractional parameter $α\in (1,2]$. The thermodynamic analysis reveals that the specific heats $C_V$ and $C_p$ share the same sign and depend solely on the deceleration parameter, demonstrating that the fractional model is thermodynamically stable during the late-time accelerated expansion and does not exhibit phase transitions. To constrain the background dynamics, we confront the truncated fractional model with a joint sample of late-time observational data, including Cosmic Chronometers, Pantheon+SH0ES supernovae, and the latest DESI DR2 Baryon Acoustic Oscillations. Exploring the physically motivated range $1<α\le 2$, we find that the fit quality degrades monotonically as $α$ decreases from the General Relativity limit. Rather than limiting the model's physical value, this demonstrates its theoretical robustness: the fractional framework acts as a continuous deformation parameter that preserves macroscopic thermodynamic stability. The data favors $α$ close to 2 (yielding $H_0=69.50\pm 0.42$ km/s/Mpc and $Ω_{m0}=0.292\pm 0.008$), revealing that while the late-time background expansion strongly constrains deviations from the standard area law, the fractional model smoothly and stably accommodates these constraints without exhibiting thermodynamic pathologies.

gr-qc↗
arXiv2026

Curvature-induced phantom behavior and cosmological bounces without violating the Dominant Energy Condition

We show that a spatially closed universe can exhibit effective phantom dynamics ($q<-1$) and reach a finite-time cosmological event without violating the Dominant Energy Condition (DEC). Whereas the standard Big Rip is driven by a phantom fluid that breaks the energy conditions, here non-null spatial curvature acts as an active geometric contributor: for matter that strictly obeys the DEC ($ω\ge-1$), positive spatial curvature by itself drives the deceleration parameter to values below $-1$ and brings the expansion to a stop at a finite value of the scale factor, where the Hubble parameter vanishes while the energy density stays finite. Since the curvature also keeps the Hubble radius regular at this point, the event is naturally interpreted as a cosmological bounce rather than a disruptive singularity. We contrast this DEC-preserving mechanism with the genuine phantom case ($ω<-1$), for which the same closed geometry produces an early bounce followed by a late Big Rip, and we comment on the observational status of the curvature contribution.

gr-qc↗
arXiv2026

Torsional pseudo-inflation beyond Einstein-Cartan

Torsion entered early-universe cosmology twice: through the nonsingular Einstein--Cartan bounce, where the quantum spin of fermions halts the contraction, and through the proposal that the expansion following that bounce could take over the duties of cosmic inflation. We revisit the second idea in a constructive spirit. First we compute its budget: for a Weyssenhoff spin fluid with barotropic source, $w\in\left[0,1\right]$, the accelerated window that follows the bounce spans $N_e=\ln\left[4/\left(1+3w\right)\right]/3\left(1-w\right)\leq\tfrac{1}{3}\ln4\simeq0.46$ e-folds, and positive spatial curvature only shrinks it. The mechanism is sound; its torsional ``fuel'' dilutes as $a^{-6}$, and the engine stops two orders of magnitude short of inflationary needs. We then ask what torsion would need in order to sustain a quasi-de Sitter phase, and give a two-branch answer in Riemann--Cartan cosmology with nonminimal couplings, where torsion is fed by the couplings rather than by spin. The vectorial branch reproduces Palatini inflation, observationally alive. On the axial branch we exhibit an exact de Sitter solution sustained by a torsion condensate: constant axial torsion $f_{0}$, a linear Gauss--Bonnet coupling, and a potential fixed to $V_{0}=6f_{0}^{2}+\tfrac{5}{2}v^{2}$ by the Friedmann constraint. The solution is an attractor of the homogeneous dynamics, with eigenvalue exactly $-3H$. The phase ends when the slope $U^{\prime}$, a decreasing function of the roll speed at fixed flat potential, drifts down to the floor $5/16V_{0}$ and the condensate switches itself off; consistency of the effective theory pins the whole episode near the Planck scale, $H\gtrsim0.3$ in reduced Planck units. We studied this branch at background level; its perturbation spectrum remains open.

gr-qc↗
arXiv2026

No late-time role for adiabatic torsion: a no-go result for Hubble-cutoff holographic dark energy in Einstein--Cartan cosmology

In Friedmann cosmology with Einstein--Cartan torsion, the homogeneous torsion mode compatible with a separately conserved matter sector scales as $Φ\propto a^{-3}$ and enters the Friedmann constraint as a stiff component of negative energy density, $-3Φ^{2}\propto a^{-6}$. It has recently been claimed that this mode rescues the Hubble radius as an infrared cutoff for holographic dark energy, producing late-time acceleration and a phantom-divide crossing of possible relevance to DESI. We show that it cannot. With the Hubble cutoff the holographic density drops out of the deceleration parameter, and the only accelerating regime is the transient window $\bar{a}\leq a<4^{1/3}\bar{a}$ around the torsion bounce at $H(\bar{a})=0$; placing that window at observable redshifts would force the Hubble rate to vanish in our recent past, against the measured expansion history. Requiring a viable history yields nested upper bounds on $Ω_Φ\equiv(Φ_{0}/H_{0})^{2}$: fitting the official DESI DR2 BAO likelihood gives $Ω_Φ<8.7\times10^{-4}$ ($95\%$ CL), the existence of a spectroscopically confirmed galaxy at $z=14.32$ gives $8.4\times10^{-5}$, the CMB gives $3.1\times10^{-10}$, and Big Bang nucleosynthesis gives $5\times10^{-24}$. Today's imprint on the dark energy equation of state, $|1+ω_{0}|\leq2Ω_Φ/Ω_Λ$, falls short of the DESI preference by two orders of magnitude at best and twenty-two at worst, and on the phantom side. In the Granda--Oliveros cutoff, torsion deepens rather than prevents the big-rip singularity. An appendix derives the Friedmann pair from the Einstein--Cartan field equations and shows that the $a^{-3}$ scaling is the kinematics of a diluting spin fluid; escaping the no-go requires breaking exactly that.

gr-qc↗
arXiv2026

Thermodynamic constraints and future singularities in Unimodular Gravity driven by phantom and non-phantom fluids

This work investigates future cosmological singularities in a flat FLRW universe filled with a single barotropic fluid, ($p = (γ- 1)ρ$), within the framework of unimodular gravity. In this setting, the non-conservation of the energy-momentum tensor is encoded through an energy diffusion function $Q$. While a constant diffusion term leads to an effective cosmological constant and preserves adiabatic evolution, a time-dependent $Q(t)$ induces non-adiabatic dynamics. We consider a power-law Ansatz for $Q$ as a function of the redshift and impose the condition of positive entropy production. This requirement leads to non-trivial constraints on the model parameters, with direct implications for the admissible singularity structure. In particular, within the thermodynamically allowed sector, we show that Big Rip singularities are excluded for non-phantom fluids when the cosmological constant is positive. For phantom fluids, the model reproduces the expected Big Rip behavior, as well as Big Crunch solutions for negative cosmological constant. More importantly, we show that diffusion can induce an effective phantom regime even when the fundamental fluid is non-phantom. In particular, for a negative cosmological constant, we present an explicit realization of a Big Rip singularity in unimodular gravity driven by diffusion, while consistently preserving a non-phantom equation of state and positive entropy production. These results reveal a novel mechanism for the emergence of future singularities, with no direct analogue in standard General Relativity.

gr-qc↗
arXiv2026

Beyond Comoving Volume: Horizon Flux and Matter Creation in Modified Cosmology from the Unified First Law of Thermodynamics

We explore the derivation of the Friedmann equations from a thermodynamic perspective, applying the unified first law of thermodynamics to the apparent horizon of a flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe. We extend this framework to incorporate gravitationally induced particle creation, treating the region enclosed by the apparent horizon as an open thermodynamic system. A crucial aspect of our analysis is the recognition that the apparent horizon volume is not comoving; this requires consistent accounting of particle exchange across the moving boundary. We demonstrate that the evolution of the particle number, and explicitly the matter entropy, can be decomposed into two distinct physical contributions: genuine bulk particle production and a net flux induced by the dynamics of the horizon itself. Finally, we derive the Generalized Second Law (GSL) in this setting, showing transparently how the total entropy budget is balanced by horizon thermodynamics, bulk creation, and boundary fluxes.

gr-qc↗
arXiv2026

Spatial curvature in Unimodular Gravity

We investigate the cosmological implications of unimodular gravity (UG) featuring energy diffusion and spatial curvature. While standard diffusion models often suffer from thermodynamic inconsistencies, we propose a phenomenologically viable power-law Ansatz for the diffusion function, $Q(z) = Q_0(1+z)^β$, which strictly satisfies the second law of thermodynamics by demanding positive entropy production ($βQ_0 > 0$). Using a joint statistical analysis with the Pantheon+ Type Ia Supernova compilation and Baryon Acoustic Oscillation (BAO) measurements, we tightly constrain the parameter space. We find a diffusion exponent of $β= 0.503_{-0.126}^{+0.118}$ and a slight preference for a closed spatial geometry with $Ω_{k0} = -0.109_{-0.071}^{+0.076}$ at present time. Remarkably, the consideration of spatial curvature and diffusion naturally alleviates the Hubble tension, yielding $H_0 = 73.350_{-0.226}^{+0.221}$ km/s/Mpc while maintaining a consistent cosmic age of $t_0 \simeq 13.61$ Gyr. Furthermore, the constrained diffusion scales as a stable, quintessence-like effective dark energy ($ω_{\text{eff}} \simeq -0.832$). Thus, unimodular diffusion provides a thermodynamically consistent phenomenological alternative that can alleviate the Hubble tension while preserving both the cosmic age and the sound-horizon scale, with a preference for a closed spatial geometry.

gr-qc↗
arXiv2026

The holographic origin of future singularities and the role of spatial curvature in cosmic expansion

We investigate the fundamental cosmological implications of holographic dark energy using the Granda-Oliveros (GO) infrared cutoff, spatial curvature, and generalized entropies. We demonstrate that the GO cutoff establishes a geometric origin for phantom acceleration, inevitably leading to a big rip singularity without requiring exotic matter. Incorporating spatial curvature reveals that topology acts as a quantitative catalyst; positive curvature accelerates the singularity in closed universes, but cannot alter its fundamental behavior. Furthermore, we show that Kaniadakis generalized entropy modifications are structurally insufficient to prevent this finite-time divergence. To successfully soften the big rip and yield an asymptotic little rip, it is necessary (as first alternative) to integrate irreversible thermodynamical mechanisms, such as non-equilibrium particle creation. These macroscopic processes are sufficient to neutralize the geometric divergence of the GO cutoff, as we discuss in the work.

gr-qc↗
arXiv2026

First-order phase transitions and cosmic evolution: thermodynamic approach to generalized holographic dark energy

Focusing on the description of cosmic evolution at late times, this study examines a generalized holographic dark energy (HDE) framework constructed via a polynomial expansion in the Hubble parameter, which includes contributions proportional to $H^{2}$, $H^{4}$, and $H^{6}$, introduced through a variable parameter within the standard holographic formula. The analysis is carried out in the context of a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) Universe, consisting of non-interacting matter together with the HDE fluid. We obtain the full set of Friedmann equations to investigate cosmic evolution and then analyze the system to determine whether thermodynamic $P - v$ type phase transitions can occur.

gr-qc↗
arXiv2025

Revealing some cosmological aspects of Kaniadakis entropy

Adopting the modifications induced by the truncated version of the Kaniadakis entropy on the Friedmann equations, we explore some relevant aspects of this cosmological scenario at the background level. We analyze the constraint imposed on the parameter $K$ obtained from the accelerated cosmic expansion condition, and we also study the role of such a parameter as a cosmological constant.

gr-qc↗
arXiv2025

Matter creation, adiabaticity and phantom behavior

We present a novel cosmological framework that unifies matter creation dynamics with thermodynamic principles. Starting with a single-component fluid characterized by a constant equation of state parameter, $ω$, we introduce a generalized second law of thermodynamics by considering the entropy associated with the cosmic horizon. Imposing an adiabatic expansion condition uniquely determines the particle creation rate, $Γ$, a feature unprecedented in previous matter creation models. This mechanism yields a cosmology featuring phantom-like expansion while relying solely on a single constituent, which can be either a quintessence-like fluid or a non-exotic, non-relativistic dark matter component. Remarkably, this framework avoids the need for exotic physics while providing a consistent explanation for the accelerated expansion of the universe. Our results open new pathways for understanding the interplay between horizon thermodynamics, particle creation, and cosmic evolution, offering fresh insights into the nature of dark energy and its potential thermodynamic origins.

astro-ph.CO↗
arXiv2024

Exploring thermodynamics inconsistencies in unimodular gravity: a comparative study of two energy diffusion functions

In this work we study the thermodynamics formulation for unimodular gravity under the election of two different models for the energy diffusion function. Such function encodes the current for the non-conservation of the energy-momentum tensor and is usually termed as $Q(t)$. In analogy to the cosmological scenario where the cosmic expansion is influenced by $Q(t)$, the thermodynamics implications in this scheme are also determined by the choice of the function $Q(t)$, as we discuss in the work. Specifically, we consider the barotropic and the continuous spontaneous localization models as energy diffusion functions, commonly used in the literature as viable candidates to face the well-known $H_{0}$ tension. The consistency conditions demanded for the entropy of the system in terms of the cosmological parameters of the model: positive production ($dS/dt>0$) and convexity condition ($d^{2}S/dt^{2} <0$), are investigated. We show that these conditions strongly constraint the viability of both models. Additionally, we comment about our results and compare with those obtained in recent works where the restriction of the parameters for these two diffusion models was implemented with the use of cosmological data.

gr-qc↗
arXiv2024

Testing a nonlinear solution of the Israel-Stewart theory

In this work, we test the capability of an exact solution found in the framework of a nonlinear extension of the Israel-Stewart theory to fit the supernovae Ia, gravitational lensing, and black hole shadow data. This exact solution is a generalization of one previously found for a dissipative unified dark matter model in the context of the near-equilibrium description of dissipative processes, where we do not have the full regime of the nonlinear picture. This generalized solution is restricted to the case where a positive entropy production is guaranteed and is tested under the condition that ensures its causality, local existence, and uniqueness. From the observational constraints, we found that this generalized solution is a good candidate in the description of the observational late-time data used in this work, with best-fit values $H_{0}=73.2_{-0.9}^{+0.8}\,\frac{km/s}{Mpc}$, $q_{0}=-0.41_{-0.03}^{+0.03}$, $\hatξ_{0}=0.88_{-0.17}^{+0.09}$, $ε=0.34_{-0.04}^{+0.03}$, and $k=0.27_{-0.20}^{+0.37}$. Therefore, we show that the nonlinear regime of the Israel-Stewart theory consistently describes the recent accelerated expansion of the universe without the inclusion of some kind of dark energy component and also provides a more realistic description of the fluids that make up the late Universe.

gr-qc↗
arXiv2024

A new approach to $P-V$ phase transitions: Einstein gravity and holographic type dark energy

In the framework of Einstein's gravity, we study the thermodynamic equation state, $P=P(V,T)$, associated with a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. In this scenario, we consider the components of the dark sector as non-interacting fluids that dominate the universe's energy content at late times. Under these circumstances, the functional structure of the cosmological coincidence parameter plays a relevant role in admitting first-order $P-V$ phase transitions; specifically, the dark energy density and the coincidence parameter must be given in terms of the radius of the apparent horizon.

gr-qc↗
arXiv2024

Generalized second law of thermodynamics for the matter creation scenario and emergence of phantom regime

This work is focused on the exploration of the thermodynamics foundations of the matter creation scenario when a generalized form of the second law of thermodynamics for this scheme is implemented. In this scenario we consider an expanding cosmology in which the created matter is trapped by the apparent horizon. The scheme leads to phantom evolution but at first glance it lacks of physical consistency. However, the inclusion of chemical potential into the description solves the thermodynamics issues of the model and determines the behavior of the cosmic fluid, in other words, the cosmic fluid now can behave as phantom dark energy or as quintessence one.

gr-qc↗