arXiv subjects
Publications and source records attributed to Grigoris Panotopoulos.
We obtain an analytical expression for the electromagnetic quasinormal spectrum of the higher-dimensional nearly-extremal Schwarzschild-de Sitter black hole. The WKB method is used to verify the results, and a comparison with known results from previous works is briefly made as well.
We perturb the non-rotating BTZ black hole with a non-minimally coupled massless scalar field, and we compute the quasinormal spectrum exactly. We solve the radial equation in terms of hypergeometric functions, and we obtain an analytical expression for the quasinormal frequencies. In addition, we compare our analytical results with the 6th order semi-analytical WKB method, and we find an excellent agreement. The impact of the nonminimal coupling as well as of the cosmological constant on the quasinormal spectrum is briefly discussed.
We study is some detail the Cosmology of Oscillating Dark Energy described by concrete equations-of-state investigated recently in the literature. In particular, at the background level we compute the statefinder parameters, while at the level of linear cosmological perturbations we compute the growth index $γ$ as well as the combination parameter $A=f σ_8$. The comparison with $Λ$CDM is made as well.
We study the stability under scalar perturbations, and we compute the quasinormal modes of the Einstein-Born-Infeld dilaton spacetime in 1+3 dimensions. Solving the full radial equation in terms of hypergeometric functions, we provide an exact analytical expression for the spectrum. We find that the frequencies are purely imaginary, and we confirm our results by computing them numerically. Although the scalar field that perturbs the black hole is electrically neutral, an instability similar to that seen in charged scalar perturbations of the Reissner-Nordström black hole is observed.
We study for the first time how a new class of stars could impact an ensemble of pulsars with known masses and spin-periods. These new compact objects are strange stars admixed with condensed dark matter. In this exploratory theoretical work, our goal is to determine how the basic parameters of pulsars are modified for such a new class of compact objects. In particular we consider three different scenarios that correspond to a dark matter mass fraction of $~5\%$, $~11\%$ and $~25\%$. Within each scenario with fixed parameters we predict theoretically other properties of the pulsars, such as the radius, the compactness, the moment of inertia, as well as the angular momentum. Our numerical results are summarized in tables and also shown graphically for better visualization, where a comparison between the different scenarios can be made.
In the present work we study the propagation of a probe minimally coupled scalar field in Einstein-power-Maxwell charged black hole background in (1+2) dimensions. We find analytical expressions for the reflection coefficient as well as for the absorption cross-section in the low energy regime, and we show graphically their behaviour as functions of the frequency for several values of the free parameters of the theory.
The properties of dark matter admixed strange quark stars are investigated in the Starobinsky model of modified gravity. For quark matter we assume the MIT bag model, while self-interacting dark matter inside the star is modelled as a Bose-Einstein condensate with a polytropic equation of state. We numerically integrate the structure equations in the Einstein frame adopting the two-fluid formalism treating the curvature correction term non-perturbatively. Our findings show that strange quark stars (in agreement with current observational constraints) with the highest masses are equally affected by dark matter and modified gravity.
In the present work we study non-rotating dark stars in $f(R)$ modified theory of gravity. In particular, we have considered bosonic self-interacting dark matter modelled inside the star as a Bose-Einstein condensate, while as far as the modified theory of gravity is concerned we have assumed Starobinsky's model $R+aR^2$. We solve the generalized structure equations numerically, and we obtain the mass-to-ratio relation for several different values of the parameter $a$, and for two different dark matter equation-of-states. Our results show that the dark matter stars become more compact in the R-squared gravity compared to General Relativity, while at the same time the highest star mass is slightly increased in the modified gravitational theory. The numerical value of the highest star mass for each case has been reported.
We study for the first time the stability against scalar perturbations, and we compute the spectrum of quasinormal modes of three-dimensional charged black holes in Einstein-power-Maxwell non-linear electrodynamics assuming running couplings. Adopting the 6th order WKB approximation we investigate how the running of the couplings change the spectrum of the classical theory. Our results show that all modes corresponding to non-vanishing angular momentum are unstable both in the classical theory and with the running of the couplings, while the fundamental mode can be stable or unstable depending on the running parameter and the electric charge.
In the present work we study a concrete model of Scalar--Tensor theory of gravity characterized by two free parameters, and compare its predictions against observational data and constraints coming from supernovae, solar system tests as well as the stability of cosmic structures. First an exact analytical solution at the background level is obtained. Then using that solution the expression for the turnaround radius is computed. Finally we show graphically how current data and limits put bounds on the parameters of the model at hand.
In the present work we compute the spectrum of quasinormal frequencies of four-dimensional charged black holes in Einstein-power-Maxwell theory. In particular we study scalar perturbations adopting the 6th order WKB approximation. We analyze in detail the behaviour of the spectrum depending on the charge of the black hole, the quantum number of angular momentum and the overtone number. In addition, a comparison is made between the results obtained here and the results valid for charged black holes in other theories as well as for the Reissner-Nordstr${ö}$m black hole. Finally, we have provided an analytical expression for the quasinormal spectrum in the eikonal limit.
In the present work we study a warm inflationary model defined by a quasi-exponential inflaton potential and an inflaton decay rate proportional to the Hubble rate. The model is characterized by three free parameters. We compute the power spectrum, the scalar spectral index as well as the tensor-to-scalar ratio within the framework of the model, and we compare with the latest Planck data. On the $r-n_s$ plane we show both the theoretical curves and the contour plots allowed by observations, and we constrain the parameters of the model accordingly. The non-linear parameter $f_{NL}$, corresponding to primordial non-Gaussianities, is also discussed and we found that the value predicted by our model is within the bounds imposed by current observational data.
We apply Starobinsky's formalism of stochastic inflation to the case of a minimally coupled scalar field with linear self-interaction potential. We solve the corresponding Fokker-Planck equation exactly, and obtain analytical expressions for the stochastic expectation values.
In the present work we study the propagation of a probe nonminimally coupled scalar field in BTZ black hole background. We find analytical expressions for the reflection coefficient, the absorption cross-section, and the decay rate in the strong coupling case, where the nonminimal coupling is larger than its conformal value $ξ_{c}= 1/6$. Our main results are summarized in several figures, in which we show how the behavior of the aforementioned quantities depends on the parameters of the theory. Our findings show that i) the reflection coefficient tends to zero only for a nonminimal coupling $ξ\geq 0.25$, and ii) in the zero angular-momentum case the greybody factor in the low energy regime tends to a finite constant that generically does not coincide with the area of the horizon of the black hole. There is, however, a special case in which this holds.
We compute the 20 lowest frequency radial oscillation modes of strange stars admixed with condensed dark matter. We assume a self-interacting bosonic dark matter, and we model dark matter inside the star as a Bose-Einstein condensate. In this case the equation of state is a polytropic one with index $1+1/n=2$ and a constant $K$ that is computed in terms of the mass of the dark matter particle and the scattering length. Assuming a mass and a scattering length compatible with current observational bounds for self-interacting dark matter, we have integrated numerically first the Tolman-Oppenheimer-Volkoff equations for the hydrostatic equilibrium, and then the equations for the perturbations $ξ=Δr/r$ and $η=ΔP/P$. For a compact object with certain mass and radius we have considered here three cases, namely no dark matter at all and two different dark matter scenarios. Our results show that i) the separation between consecutive modes increases with the amount of dark matter, and ii) the effect is more pronounced for higher order modes. These effects are relevant even for a strange star made of 5\% dark matter.
In this work we apply relativistic mean-field theory in neutron stars assuming that fermionic dark matter is trapped inside the star and interacts directly with neutrons by exchanging Standard Model Higgs bosons. For realistic values of the parameters of the model we compute numerically the equation of state, and we compare it to the standard one. Furthermore, the mass-to-radius relation for both equations of state (pure neutron matter as well as admixed DM-neutron star) is shown, and the highest star mass for both cases is reported.
In the present work we study the branon dark matter particles impact on compact objects, and we provide the first constraints of the parameter space using white dwarf stars. The branon dark matter model is characterized by two free parameters, namely the branon mass particle M and the brane tension factor $f$. The latter determines the strength of the interaction of branon dark matter particles with baryons. By considering a typical white dwarf star we were able to obtain constraints on branon dark matter and compare with current limits obtained by direct detection searches and dark matter abundance. In particular our results show that i) for heavy branons with a mass $M > 10 GeV$ white dwarfs fail to provide us with bounds better than current limits from DM direct detection searches, and ii) for light branons in the mass range $2 keV < M < 1 GeV$, which cannot be probed neither with current dark matter experiments nor with the next generation of detectors, the dark matter abundance constrain determines $f$ as a function of $M$ in the range $0.1 GeV < M < 1 GeV$ for the branon mass and $1 GeV < f < 5 GeV$ for the brane tension factor. Furthermore, our findings indicate that the limits from white dwarfs are not stronger than the dark matter abundance constrain.
In the present work we study the gravitational effects of condensed dark matter on strange stars. We consider self-interacting dark matter particles with properties consistent with current observational constraints, and dark matter inside the star is modelled as a Bose-Einstein condensate. We integrate numerically the Tolman-Oppenheimer-Volkoff equations in the two-fluid formalism assuming that strange stars are made of up to 4 per cent of dark matter. It is shown that for a mass of the dark matter particles in the range $50 MeV-160 MeV$ strange stars are characterized by a maximum mass and radius similar to the ones found for neutron stars.