arXiv subjects

E. N. Saridakis

Publications and source records attributed to E. N. Saridakis.

At least 19 recordsLinked to original sources

arXiv2026

Thick branes and fermion localization in five-dimensional $f(T,T_G)$ gravity

We investigate thick-brane configurations in five-dimensional $f(T,T_G)$ modified teleparallel gravity. In five dimensions, the torsional Gauss-Bonnet invariant $T_G$ contributes dynamically, leading to genuinely new effects even at linear order. Within a warped geometry supported by a scalar field, we construct explicit solutions and show that the $T_G$ sector significantly modifies the brane structure. In particular, the coupling parameter controls the deformation of the warp factor and energy density, allowing for the emergence of brane splitting and nontrivial internal structure. We further analyze the localization of spin-$1/2$ fermions via a Yukawa coupling. The system admits a normalizable chiral zero mode, while the opposite chirality remains delocalized. The massive Kaluza-Klein spectrum is strongly affected by the torsional Gauss-Bonnet term, which modifies the effective potentials and leads to the appearance of resonant quasi-localized states.Our results show that $f(T,T_G)$ gravity provides a richer framework for braneworld models, where torsional higher-order corrections play a key role in shaping both geometry and field localization.

hep-th↗

arXiv2026

Holographic dark energy from a new two-parameter entropic functional

We formulate an extended holographic dark energy scenario based on a recently proposed two-parameter generalized entropic functional. Unlike constructions that phenomenologically impose modified entropy-area relations at the horizon level, the present framework is rooted in a microscopic entropy functional and the corresponding microstate counting. For bounded systems, the entropy acquires a generalized holographic scaling with two independent area contributions, recovering the Bekenstein-Hawking entropy in the appropriate limits. Implementing this entropy within the holographic principle, we derive a generalized dark energy density containing two distinct holographic sectors, naturally embedding standard holographic dark energy and $Λ$CDM as limiting cases. We analyze the cosmological evolution for both Hubble and future event horizon cutoffs and show that the model successfully reproduces the matter-to-dark-energy transition. The two entropic exponents enrich the dynamics, allowing for quintessence-like behavior or phantom regimes, while remaining compatible with the standard thermal history of the Universe.

gr-qc↗

arXiv2026

New modified cosmology from a new generalized entropy

We develop new modified cosmological scenarios by applying the first law of thermodynamics at the Universe horizon, utilizing a new entropic functional that generalizes the standard Boltzmann-Gibbs-Shannon entropy. In particular, starting from the general theory of entropy in terms of the probability distribution over the accessible microstates, and by imposing violation of the separability requirement and thus considering a generalized microstate scaling, we result to a generalized entropy expression, which applied in systems with boundaries yields a generalized holographic-like area-law scaling with two exponents. Hence, incorporating it within the gravity-thermodynamics framework, we result to a modified cosmological scenario with additional terms, which eventually give rise to an effective dark energy sector. We extract analytical expressions for the dark energy density and equation-of-state parameters, and we show that the Universe experiences the usual thermal history, with the sequence of matter and dark-energy eras. Additionally, depending on the values of the entropic exponents, the dark energy can be quintessence-like, phantom-like or experience the phantom-divide crossing during its evolution, ultimately stabilizing at the cosmological constant value in the asymptotic far future, a behavior richer than other entropic modified cosmologies.

gr-qc↗

arXiv2025

White Paper and Roadmap for Quantum Gravity Phenomenology in the Multi-Messenger Era

The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 ``Quantum gravity phenomenology in the multi-messenger approach (QG-MM)'', in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology.

gr-qc↗

arXiv2024

Quasinormal Modes in Noncommutative Schwarzschild black holes

We investigate the quasinormal modes of a massless scalar field in a Schwarzschild black hole, which is deformed due to noncommutative corrections. We introduce the deformed Schwarzschild black hole solution, which depends on the noncommutative parameter $Θ$. We then extract the master equation as a Schrödinger-like equation, giving the explicit expression of the effective potential which is modified due to the noncommutative corrections. After that, we solve the master equation numerically. The significance of these results is twofold. Firstly, our results can be related to the detection of gravitational waves by the near future gravitational wave detectors, such as LISA, which will have a significantly increased accuracy. In particular, these observed gravitational waves produced by binary strong gravitational systems have oscillating modes which can provide valuable information. Secondly, our results can serve as an additional tool to test the predictions of GR, as well as to examine the possible detection of this kind of gravitational corrections.

gr-qc↗

arXiv2018

Updated constraints on $f(T)$ models using direct and indirect measurements of the Hubble parameter

We extract observational constraints on $f(T)$ gravity, using the recently proposed statistical method which is not affected by the value of $H_0$ and thus it bypasses the problem of the disagreement in its exact numerical value between Planck and direct measurements. We use direct measurements of the Hubble parameter with the corresponding covariance matrix, and for completeness we perform a joint analysis using the latest data from Supernovae type Ia based on JLA sample, quasi-stellar objects, and Cosmic Microwave Background shift parameter from Planck. We analyze a large family of $f(T)$ models, and we compare the fitting results with $Λ$CDM cosmology using the AIC statistical test. Utilizing only the Hubble parameter data we find that in the case of the power-law $f(T)$ model a small but non-zero deviation from $Λ$CDM cosmology is slightly favored at 1-$σ$, nevertheless the corresponding AIC value shows a statistical equivalence with it. Finally, the join analysis reveals that all $f(T)$ models are very efficient and in very good agreement with observations.

astro-ph.CO↗

arXiv2017

Constraining f(T) teleparallel gravity by Big Bang Nucleosynthesis

We use BBN observational data on primordial abundance of ${}^4He$ to constrain f(T) gravity. The three most studied viable $f(T)$ models, namely the power law, the exponential and the square-root exponential are considered, and the BBN bounds are adopted in order to extract constraints on their free parameters. For the power-law model, we find that the constraints are in agreement with those acquired using late-time cosmological data. For the exponential and the square-root exponential models, we show that for realiable regions of parameters space they always satisfy the BBN bounds. We conclude that viable f(T) models can successfully satisfy the BBN constraints.

astro-ph.CO↗

arXiv2016

Emergent universe in Horava-Lifshitz-like F(R) gravity

We investigate the Einstein static universe (ESU) and the emergent universe scenario in the framework of Hořava-Lifshitz-like $F(R)$ gravity. We first perform a dynamical analysis in the phase space, and amongst others we show that a spatially open universe filled with matter satisfying the strong energy condition can exhibit a stable static phase. Additionally, we examine the behavior of the scenario under scalar perturbations and extract the conditions under which it is free of perturbative instabilities, showing that the obtained background ESU solutions are free of such instabilities. However, in order for the Einstein static universe to give rise to the emergent universe scenario we need to have an exotic matter sector that can lead the universe to depart from the stable static state and enter into its usual expanding thermal history.

gr-qc↗

arXiv2014

New Schwarzschild-like solutions in f(T) gravity through Noether symmetries

Spherically symmetric solutions for f(T) gravity models are derived by the so called Noether Symmetry Approach. First, we present a full set of Noether symmetries for some minisuperspace models. Then, we compute analytical solutions and find that spherically symmetric solutions in f(T) gravity can be recast in terms of Schwarzschild-like solutions modified by a distortion function depending on a characteristic radius. The obtained solutions are more general than those obtained by the usual solution methods.

gr-qc↗

arXiv2013

Viable f(T) models are practically indistinguishable from LCDM

We investigate the cosmological predictions of several $f(T)$ models, with up to two parameters, at both the background and the perturbation levels. Using current cosmological observations (geometric supernovae type Ia, cosmic microwave background and baryonic acoustic oscillation and dynamical growth data) we impose constraints on the distortion parameter, which quantifies the deviation of these models from the concordance $Λ$ cosmology at the background level. In addition we constrain the growth index $γ$ predicted in the context of these models using the latest perturbation growth data in the context of three parametrizations for $γ$. The evolution of the best fit effective Newton constant, which incorporates the $f(T)$-gravity effects, is also obtained along with the corresponding $1σ$ error regions. We show that all the viable parameter sectors of the $f(T)$ gravity models considered practically reduce these models to $Λ$CDM. Thus, the degrees of freedom that open up to $Λ$CDM in the context of $f(T)$ gravity models are not utilized by the cosmological data leading to an overall disfavor of these models.

astro-ph.CO↗

arXiv2009

Statistical Approach to the Cosmological-Constant Problem on Brane Worlds

We investigate the physically accepted solutions of general Braneworld scenarios, scanning uniformly the associated parameter space. Without making any further assumptions we find that solutions which give "small" Hubble parameters on the physical brane, and therefore "small" effective cosmological constants on the 4D Universe, are far more probable than those with "large" ones. Eventually, their distribution tends to the $δ$-function in the limit of continuous covering of the parameter space.

hep-th↗

arXiv2009

Quintom dark energy models with nearly flat potentials

We examine quintom dark energy models, produced by the combined consideration of a canonical and a phantom field, with nearly flat potentials and dark energy equation-of-state parameter $w_{DE}$ close to -1. We find that all such models converge to a single expression for $w_{DE}(z)$, depending only on the initial field values and their derivatives. We show that this quintom paradigm allows for a description of the transition through -1 in the near cosmological past. In addition, we provide the necessary conditions for the determination of the direction of the -1-crossing.

astro-ph↗

arXiv2009

Braneworld models with a non-minimally coupled phantom bulk field: a simple way to obtain the -1-crossing at late times

We investigate general braneworld models, with a non-minimally coupled phantom bulk field and arbitrary brane and bulk matter contents. We show that the effective dark energy of the brane-universe acquires a dynamical nature, as a result of the non-minimal coupling which provides a mechanism for an indirect "bulk-brane interaction" through gravity. For late-time cosmological evolution and without resorting to special ansatzes or to specific areas of the parameter space, we show that the -1-crossing of its equation-of-state parameter is general and can be easily achieved. As an example we provide a simple, but sufficiently general, approximate analytical solution, that presents the crossing behavior.

hep-th↗

arXiv2008

Non-minimally coupled canonical, phantom and quintom models of holographic dark energy

We investigate canonical, phantom and quintom models, with the various fields being non-minimally coupled to gravity, in the framework of holographic dark energy. We classify them and we discuss their cosmological implications. In particular, we examine the present value of the dark energy equation-of-state parameter and the crossing through the phantom divide, and we extract the conditions for a future cosmological singularity. The combined scenarios are in agreement with observations and reveal interesting cosmological behaviors.

hep-th↗

arXiv2008

Classical evolution of fractal measures generated by a scalar field on the lattice

We investigate the classical evolution of a $ϕ^4$ scalar field theory, using in the initial state random field configurations possessing a fractal measure expressed by a non-integer mass dimension. These configurations resemble the equilibrium state of a critical scalar condensate. The measures of the initial fractal behavior vary in time following the mean field motion. We show that the remnants of the original fractal geometry survive and leave an imprint in the system time averaged observables, even for large times compared to the approximate oscillation period of the mean field, determined by the model parameters. This behavior becomes more transparent in the evolution of a deterministic Cantor-like scalar field configuration. We extend our study to the case of two interacting scalar fields, and we find qualitatively similar results. Therefore, our analysis indicates that the geometrical properties of a critical system initially at equilibrium could sustain for several periods of the field oscillations in the phase of non-equilibrium evolution.

physics.comp-ph↗

arXiv2008

Correspondence between Holographic and Gauss-Bonnet dark energy models

In the present work we investigate the cosmological implications of holographic dark energy density in the Gauss-Bonnet framework. By formulating independently the two cosmological scenarios, and by enforcing their simultaneous validity, we show that there is a correspondence between the holographic dark energy scenario in flat universe and the phantom dark energy model in the framework of Gauss-Bonnet theory with a potential. This correspondence leads consistently to an accelerating universe. However, in general one has not full freedom of constructing independently the two cosmological scenarios. Specific constraints must be imposed on the coupling with gravity and on the potential.

hep-th↗

arXiv2008

Cyclic Universes from General Collisionless Braneworld Models

We investigate the full 5D dynamics of general braneworld models. Without making any further assumptions we show that cyclic behavior can arise naturally in a fraction of physically accepted solutions. The model does not require brane collisions, which in the stationary case remain fixed, and cyclicity takes place on the branes. We indicate that the cosmological constants play the central role for the realization of cyclic solutions and we show that its extremely small value on the observable universe makes the period of the cycles and the maximum scale factor astronomically large.

hep-th↗

arXiv2008

Coupled oscillators as models of quintom dark energy

We investigate quintom cosmology in FRW universes using isomorphic models consisting of three coupled oscillators, one of which carries negative kinetic energy. In particular, we examine the cosmological paradigms of minimally-coupled massless quintom, of two conformally-coupled massive scalars and of conformally-coupled massive quintom, and we obtain their qualitative characteristics as well as their quantitative asymptotic behavior. For open or flat geometries, we find that, independently of the specific initial conditions, the universe is always led to an eternal expansion.

hep-th↗