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We investigate the structure and radial oscillations of anisotropic compact stars composed of dark energy, using the vanishing complexity factor formalism within general relativity. This novel approach establishes a direct link between the energy density and anisotropic factor, providing a robust framework for studying these exotic stellar objects. Employing an Extended Chaplygin Gas equation of state, we numerically compute interior solutions for both isotropic and anisotropic stars, revealing distinct differences in their properties. Additionally, we examine the oscillation modes and frequencies of these stars, highlighting the impact of anisotropy on their pulsational behavior. Our results reveal distinct differences in the stellar properties, such as the metric potentials, pressure, speed of sound, and relativistic adiabatic index, between the two cases. Furthermore, we calculate the large frequency separation for the fundamental and first excited modes, offering insights relevant for future asteroseismology studies. Our findings shed light on the complex interplay of gravity, matter, and anisotropy in compact stars, providing a new perspective on dark energy's role in their structure and dynamics., Comment: 14 pages, 1 table, 4 figures; To appear in PLB

We investigate two novel models of charged black holes in the framework of non-linear electrodynamics of Born-Infeld type. In particular, starting from two concrete Lagrangian densities, the corresponding metric potentials, the electric field, the Smarr formula and finally, the (scalar) quasinormal modes are computed for each model. Our findings show that although the models look very similar, their quasinormal spectra are characterized by certain differences., Comment: 18 pages, 7 figures, 3 tables, to be published in Annals of Physics

We study dynamical dark energy models within Einstein's theory by means of matter perturbations and the growth index $\gamma$. Within four-dimensional General Relativity, we assume that dark energy does not cluster, and we adopt a linear ansatz for the growth index to investigate its impact on the deceleration parameter, $q$, and on the dark energy equation-of-state parameter, $w$. Following this approach, we identify a relationship between $q_0$ (today's value of $q$) and $\gamma$, which to the best of our knowledge is new. For $w(z)$, we find that in most of the cases considered it crosses the -1 line (quintom) ending at a present day value $w_0 > -1$. Furthermore, we show that an analytic expression for $w(z)$ may be obtained in the form of order (4,4) (or higher) Pad{\'e} parameterizations., Comment: 13 pages, 3 figures, to be published in IJMPD

We investigate exotic stars composed of dark energy within the context of Einstein's General Relativity, by applying an extended Chaplygin gas equation-of-state. To account for anisotropies, we utilize a formalism based on the complexity factor to obtain numerical solutions. By applying well-established criteria, we demonstrate that the solutions are physically valid and well-behaved. In addition, a comparison with a more conventional approach is also conducted., Comment: Minor corrections

In this work, motivated by the fact that higher-dimensional theories predict the existence of black holes which differ from their four-dimensional counterpart, we analyse the geodesics and black hole shadow cast by a general non-extremal five-dimensional black hole. The system under consideration corresponds to the Chong-Cveti\v{c}-L\"u-Pope (Phys. Rev.Lett.{\bf 95}, 161301 (2005)), which has the Myers--Perry black hole as a limit., Comment: Some reference added

We study radial oscillations of non-rotating neutron stars (NSs) in four-dimensional General Relativity. The interior of the NS was modelled within a recently proposed multicomponent realistic equation of state (EoS) with the induced surface tension (IST). In particular, we considered the IST EoS with two sets of model parameters, that both reproduce all the known properties of normal nuclear matter, give a high quality description of the proton flow constraint, hadron multiplicities created in nuclear-nuclear collisions, consistent with astrophysical observations and the observational data from the NS-NS merger. We computed the 12 lowest radial oscillation modes, their frequencies and corresponding eigenfunctions, as well as the large frequency separation for six selected fiducial NSs (with different radii and masses of 1.2, 1.5 and 1.9 solar masses) of the two distinct model sets. The calculated frequencies show their continuous growth with an increase of the NS central baryon density. Moreover, we found correlations between the behaviour of first eigenfunction calculated for the fundamental mode, the adiabatic index and the speed of sound profile, which could be used to probe the internal structure of NSs with the asteroseismology data., Comment: 10 pages, 7 figures, accepted to Phys. Rev. D

In this work, starting from a spherically symmetric polytropic black hole, a rotating solution is obtained by following the Newman--Janis algorithm without complexification. Besides studying the horizon, the static conditions and causality issues of the rotating solution, we obtain and discuss the shape of its shadow. Some other physical features as the Hawking temperature and emission rate of the rotating polytropic black hole solution are also discussed., Comment: 9 pages, 2 figures, some references added

We give a scalar field description of two dark energy parameterizations, and we analyze in detail its cosmology both at the level of background evolution and at the level of linear perturbations. In particular, we compute the statefinder parameters and the growth index as functions of the red-shift for both dark energy parameterizations, and the comparison with the $\Lambda CDM$ model as well as with a few well-known geometrical dark energy models is shown. In addition, the combination parameter $A=f \sigma_8$ of both models is compared against current data., Comment: Revtex, 11 pages, 10 figures, to be published in PRD

We compute the absorption cross section of space-time scalars on a static D2 rane, in global coordinates, wrapped on the S^2 of an AdS_2 X S^2 X CY_3 geometry. We discuss its relevance for the construction of the dual quantum mechanics of Calabi-Yau black holes., Comment: 18 pages, 2 figures

In the present work we study a dark energy model in which a non-linear scalar field (tachyon) with a Born-Infeld type of action is responsible for the observed cosmic acceleration. The potential of the tachyon is well-motivated since it comes from open string theory and the model is subjected to various cosmological constraints with data coming from supernovae as well as from microwave background radiation. Our analysis shows that in the particular model under study the tachyon can be an excellent candidate for dark energy in the universe, as the model agrees with a series of observational data and for a wide range of the parameters of the model., Comment: references added, submitted to JCAP

In the present work we study non-thermal leptogenesis and baryon asymmetry in the universe in different neutrino mass models discussed recently. For each model we obtain a formula relating the reheating temperature after inflation to the inflaton mass. It is shown that all but four cases are excluded and that in the cases which survive the inflaton mass and the reheating temperature after inflation are bounded from below and from above., Comment: 1+11 pages, 3 tables, 0 figures

In the present work we discuss inflation, dark matter and cosmological evolution in the context of the brane-world scenario. Being string theory inspired, the brane-world models provide corrections to the General Relativity, which is considered to be the low-energy limit of string theory. We find that novel cosmologies are obtained, which potentially can provide answers to some of the longstanding problems of modern cosmology, such as the origin and nature of dark energy. At the same time the successes of the standard four-dimensional cosmology are preserved, and in some cases the treatment in the framework of brane cosmology is even more satisfactory., Comment: PhD thesis has been withdrawn by the author

We discuss sneutrino inflation in the brane-world scenario. We work in the Randall-Sundrum type II brane-world, generalized with the introduction of the Gauss-Bonnet term, a correction to the effective action in string theories. We find that a viable inflationary model is obtained with a reheating temperature appropriate to lead to the right baryon asymmetry and render the gravitino safe for cosmology. In specific realizations we satisfy all the observational constaints without the unnaturally small Yukawa couplings required in other related approaches., Comment: one reference added, submitted to Nucl.Phys.B

The role of brane-bulk energy exchange and of an induced gravity term on a single braneworld of negative tension and vanishing effective cosmological constant is studied. It is shown that for the physically interesting cases of dust and radiation a unique global attractor which can realize our present universe (accelerating and 0

We discuss dark matter in the brane world scenario. We work in the Randall-Sundrum type II brane world and assume that the lightest supersymmetric particle is the axino. We find that the axinos can play the role of cold dark matter in the universe, provided that the five-dimensional Planck mass is bounded both from below and from above. This is possible for higher reheating temperatures compared to the conventional four-dimensional cosmology due to a novel expansion law for the universe., Comment: 1+11 pages, version submitted to JCAP

We consider hybrid inflation in the braneworld scenario. In particular, we consider inflation in global supersymmetry with the D-terms in the scalar potential for the inflaton field to be the dominant ones (D-term inflation). We find that D-term dominated inflation can naturally accomodate all requirements of the successful hybrid inflationary model also in the framework of D-brane cosmology with global supersymmetry. The reheating temperature after inflation can be high enough ($\sim 10^{10} GeV$) for successful thermal leptogenesis., Comment: 11 pages, 2 figures, one reference added