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Eleftherios Papantonopoulos

Publications and source records attributed to Eleftherios Papantonopoulos.

At least 127 records · Page 7Linked to original sources
arXiv2010

A New Class of Exact Hairy Black Hole Solutions

We present a new class of black hole solutions with minimally coupled scalar field in the presence of a negative cosmological constant. We consider a one-parameter family of self-interaction potentials parametrized by a dimensionless parameter $g$. When $g=0$, we recover the conformally invariant solution of the Martinez-Troncoso-Zanelli (MTZ) black hole. A non-vanishing $g$ signals the departure from conformal invariance. All solutions are perturbatively stable for negative black hole mass and they may develop instabilities for positive mass. Thermodynamically, there is a critical temperature at vanishing black hole mass, where a higher-order phase transition occurs, as in the case of the MTZ black hole. Additionally, we obtain a branch of hairy solutions which undergo a first-order phase transition at a second critical temperature which depends on $g$ and it is higher than the MTZ critical temperature. As $g\to 0$, this second critical temperature diverges.

hep-th↗
arXiv2010

Chern-Simons black holes: scalar perturbations, mass and area spectrum and greybody factors

We study the Chern-Simons black holes in d-dimensions and we calculate analytically the quasi-normal modes of the scalar perturbations and we show that they depend on the highest power of curvature present in the Chern-Simons theory. We obtain the mass and area spectrum of these black holes and we show that they have a strong dependence on the topology of the transverse space and they are not evenly spaced. We also calculate analytically the reflection and transmission coefficients and the absorption cross section and we show that at low frequency limit there is a range of modes which contributes to the absorption cross section.

hep-th↗
arXiv2010

Holographic Superconductors with various condensates in Einstein-Gauss-Bonnet gravity

We study holographic superconductors in Einstein-Gauss-Bonnet gravity. We consider two particular backgrounds: a $d$-dimensional Gauss-Bonnet-AdS black hole and a Gauss-Bonnet-AdS soliton. We discuss in detail the effects that the mass of the scalar field, the Gauss-Bonnet coupling and the dimensionality of the AdS space have on the condensation formation and conductivity. We also study the ratio $ω_g/T_c $ for various masses of the scalar field and Gauss-Bonnet couplings.

hep-th↗
arXiv2009

Exact Gravity Dual of a Gapless Superconductor

A model of an exact gravity dual of a gapless superconductor is presented in which the condensate is provided by a charged scalar field coupled to a bulk black hole of hyperbolic horizon in asymptotically AdS spacetime. Below a critical temperature, the black hole acquires its hair through a phase transition while an electromagnetic perturbation of the background Maxwell field determines the conductivity of the boundary theory.

hep-th↗
arXiv2009

Shear Viscosity and Chern-Simons Diffusion Rate from Hyperbolic Horizons

We calculate the shear viscosity and anomalous baryon number violation rate in quantum field theories at finite temperature having gravity duals with hyperbolic horizons. We find the explicit dependence of these quantities on the temperature. We show that the ratio of shear viscosity to entropy density is below $1/(4π)$ at all temperatures and it can be made arbitrarily small in the low temperature limit.

hep-th↗
arXiv2009

Discontinuities in Scalar Perturbations of Topological Black Holes

We study the perturbative behaviour of topological black holes. We calculate both analytically and numerically the quasi-normal modes of scalar perturbations. In the case of small black holes we find discontinuities of the quasi-normal modes spectrum at the critical temperature and we argue that this is evidence of a second-order phase transition.

hep-th↗
arXiv2008

Black Holes on Thin 3-branes of Codimension-2 and their Extension into the Bulk

We discuss black hole solutions in six-dimensional gravity with a Gauss-Bonnet term in the bulk and an induced gravity term on a thin 3-brane of codimension-2. We show that these black holes can be localized on the brane, and they can further be extended into the bulk by a warp function. These solutions have regular horizons and no other curvature singularities appear apart from the string-like ones. The projection of the Gauss-Bonnet term on the brane imposes a constraint relation which requires the presence of matter in the extra dimensions.

hep-th↗
arXiv2008

Black Holes and Black String-like Solutions in Codimension-2 Braneworlds

We discuss black hole solutions with a Gauss-Bonnet term in the bulk and an induced gravity term on a thin brane of codimension-2. We show that these black holes can be localized on the brane, and they can be extended further into the bulk by a warp function. These solutions have regular horizons and no other curvature singularities appear apart from the string-like ones. The projection of the Gauss-Bonnet term on the brane imposes a constraint relation which dictates the form of matter on the brane and in the bulk.

hep-th↗
arXiv2008

Phase Transitions in Charged Topological-AdS Black Holes

We study the perturbative behaviour of charged topological-AdS black holes. We calculate both analytically and numerically the quasi-normal modes of the electromagnetic and gravitational perturbations. Keeping the charge-to-mass ratio constant, we show that there is a second-order phase transition at a critical temperature at which the mass of the black hole vanishes. We pay special attention to the purely dissipative modes appearing in the spectrum as they behave singularly at the critical point.

hep-th↗
arXiv2007

Induced cosmology on a regularized brane in six-dimensional flux compactification

We consider a six-dimensional Einstein-Maxwell system compactified in an axisymmetric two-dimensional space with one capped regularized conical brane of codimension one. We study the cosmological evolution which is induced on the regularized brane as it moves in between known static bulk and cap solutions. Looking at the resulting Friedmann equation, we see that the brane cosmology at high energies is dominated by a five-dimensional rho^2 energy density term. At low energies, we obtain a Friedmann equation with a term linear to the energy density with, however, negative coefficient in the small four-brane radius limit (i.e. with negative effective Newton's constant). We discuss ways out of this problem.

hep-th↗
arXiv2007

Observational constrains on the DGP brane-world model with a Gauss-Bonnet term in the bulk

Using the data coming from the new 182 Gold type Ia supernova samples, the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey and the H(z) data, we have performed a statistical joint analysis of the DGP brane-world model with a high curvature Gauss-Bonnet term in the bulk. Consistent parameters estimations show that the Gauss-Bonnet-Induced Gravity model is a viable candidate to explain the observed acceleration of our universe.

gr-qc↗
arXiv2007

UV stable, Lorentz-violating dark energy with transient phantom era

Phantom fields with negative kinetic energy are often plagued by the vacuum quantum instability in the ultraviolet region. We present a Lorentz-violating dark energy model free from this problem and show that the crossing of the cosmological constant boundary w=-1 to the phantom equation of state is realized before reaching a de Sitter attractor. Another interesting feature is a peculiar time-dependence of the effective Newton's constant; the magnitude of this effect is naturally small but may be close to experimental limits. We also derive momentum scales of instabilities at which tachyons or ghosts appear in the infrared region around the present Hubble scale and clarify the conditions under which tachyonic instabilities do not spoil homogeneity of the present/future Universe.

hep-th↗
arXiv2007

Regularization of conical singularities in warped six-dimensional compactifications

We study the regularization of the codimension-2 singularities in six-dimensional Einstein-Maxwell axisymmetric models with warping. These singularities are replaced by codimension-1 branes of a ring form, situated around the axis of symmetry. We assume that there is a brane scalar field with Goldstone dynamics, which is known to generate a brane energy momentum tensor of a particular structure necessary for the above regularization to be successful. We study these compactifications in both a non-supersymmetric and a supersymmetric setting. We see that in the non-supersymmetric case, there is a restriction to the admissible warpings and furthermore to the quantum numbers of the bulk gauge field and the brane scalar field. On the contrary, in the supersymmetric case, the warping can be arbitrary.

hep-th↗
arXiv2006

Curvaton Dynamics in Brane-worlds

We study the curvaton dynamics in brane-world cosmologies. Assuming that the inflaton field survives without decay after the end of inflation, we apply the curvaton reheating mechanism to Randall-Sundrum and to its curvature corrections: Gauss-Bonnet, induced gravity and combined Gauss-Bonnet and induced gravity cosmological models. In the case of chaotic inflation and requiring suppression of possible short-wavelength generated gravitational waves, we constraint the parameters of a successful curvaton brane-world cosmological model. If density perturbations are also generated by the curvaton field then, the fundamental five-dimensional mass could be much lower than the Planck mass

gr-qc↗
arXiv2006

Geometrical Tachyon Dynamics in the Background of a Bulk Tachyon Field

We study the dynamics of a D3-brane moving in the background of a bulk tachyon field of a D3-brane solution of Type-0 string theory. We show that the dynamics on the probe D3-brane can be described by a geometrical tachyon field rolling down its potential which is modified by a function of the bulk tachyon and inflation occurs at weak string coupling, where the bulk tachyon condenses, near the top of the geometrical tachyon potential. We also find a late accelerating phase when the bulk tachyon asymptotes to zero which in the geometrical tachyon picture corresponds to the minimum of the geometrical potential.

hep-th↗