arXiv subjects

Hristu Culetu

Publications and source records attributed to Hristu Culetu.

At least 55 records · Page 3Linked to original sources

arXiv2013

On a regular modified Schwarzschild spacetime

A modified version of the Schwarzschild geometry is proposed. The source of curvature comes from an anisotropic fluid with $p_{r} = -ρ$ and fluctuating tangential pressures. The event horizon has zero surface gravity but the invariant acceleration of a static observer is finite there. The Tolman - Komar energy of the gravitational fluid changes sign on the horizon, equals the black hole mass at infinity and is vanishing at $r = 0$. There is a nonzero surface stress tensor on the horizon due to a jump of the extrinsic curvature. Near-horizon geometry resembles the Robinson - Bertotti metric and not the Rindler metric which corresponds to the standard Schwarzschild spacetime. The modified metric has no signature flip when the horizon is crossed and all physical parameters are finite throughout the spacetime.

gr-qc↗

arXiv2013

On a de Sitter-like spacetime with cylindrical symmetry

A curved static de Sitter-like metric is analyzed. The source of curvature is rooted from a constant stress tensor with positive energy density and negative pressures. All the curvature invariants are constant everywhere and the geometry is conformally flat. The horizon surface gravity equals the parameter $ω$ from the metric that is also interpreted as an angular velocity. The Tolman-Komar gravitational energy is investigated. One finds that the horizon entropy satisfies the relation $S = A_{H}/4$, as for the black hole horizon.

gr-qc↗

arXiv2013

Anisotropic fluid in a time dependent conformally flat spacetime

The special conformal transformation (composed by inversion - translation - inversion) is used to generate a time dependent conformally flat spacetime. In order to be an exact solution of Einstein's equations, we need as a source a stress tensor corresponding to an anisotropic fluid with negative regular energy density and positive pressures. For the static approximation, the generators of the infinitesimal transformation resemble those recently obtained by Majhi and Padmanabhan for the coordinate transformation leading to the "near horizon" Schwarzschild metric in Kruskal coordinates. The static approximation corresponds to an energy momentum tensor of $Λ$ - form, the "cosmological constant" $Λ$ being proportional to the acceleration squared.

gr-qc↗

arXiv2012

Comment on "Empty Black Holes, Firewalls, and the Origin of Bekenstein-Hawking Entropy"

Saravani, Afshordi and Mann \cite{SAM} considered a surface fluid with vanishing energy density on the stretched horizon of a black hole, taken as the new boundary of spacetime. We show that their entropy per unit area of the fluid does not correspond to the Bekenstein-Hawking entropy, both for a Schwarzschild black hole and for a Kerr-Newman black hole. Due to the lack of a horizon, we argue that the Unruh effect could not be taken into account and the concept of local thermodynamic equilibrium is not sustained because of the $θ$-dependence of the temperature.

hep-th↗

arXiv2012

Comment on "Non-vacuum conformally flat space-times: dark energy"

Ibohal, Ishwarchandra and Singh proposed a class of exact, non-vacuum and conformally flat solutions of Einstein's equations whose stress tensor $T_{ab}$ has negative pressure. We show that $T_{ab}$ corresponds to an anisotropic fluid and the equation of state parameter seems not to be $ω= -1/2$. We consider the authors' constant cannot be the mass of a test particle but is related to a Rindler acceleration of a spherical distribution of uniformly accelerating observers.

gr-qc↗

arXiv2012

Time dependent embedding of spherically symmetric Rindler-like spacetime

An anisotropic cosmic fluid with radial heat flux which sources a time dependent Rindler-like geometry is investigated. Even though its energy density $ρ$ is positive, the radial and transversal pressures are negative and the strong energy condition is not satisfied. The congruence of "static" observers is not geodesic and the heat flux is oriented outward. We computed the Misner-Sharp energy associated with the Rindler-type metric embedded in a spatially flat FLRW universe and found that the Weyl energy is vanishing thanks to the conformally flat form of the spacetime. The null geodesic expansions are computed and one finds that only one of the two apparent horizons is located inside the event horizon. The properties of the Rindler-like geometry embedded in the conformally-flat de Sitter spacetime are investigated.

gr-qc↗

arXiv2012

On the conformally flat Rindler-like geometry

The time dependent conformally-flat spherical Rindler spacetime is investigated. The geometry has an apparent horizon that coincides with the causal horizon. The scalar acceleration of a static observer is constant and equals to the acceleration $g$ from the static expression of the Rindler-like metric. The timelike radial geodesics are computed and proves to be hyperbolae when a specific choice of the constants of integration is operated.

gr-qc↗

arXiv2012

Rindler-type geometry inside a black hole

An anisotropic fluid with positive energy density and negative pressures is proposed in the black hole interior. The gravitational field is constant everywhere inside and is given by the horizon surface gravity. Even though the geometry is of Rindler type it is curved because of the spherical symmetry used and is singular at the origin. The Israel junction conditions are studied on the stretched horizon instead of using a matching process on the null surface $r = 2M$. The entropy of any inner sphere is mass independent, maximally packed and unaffected by the outer layers.

gr-qc↗

arXiv2012

Conformal time dependent Painleve-Gullstrand spacetime

The properties of an anisotropic fluid outside a star or a black hole embedded in an expanding universe are investigated. One finds that, in Painleve-Gullstrand coordinates, the heat flux of the cosmological fluid vanishes, in spite of the nonzero value of a non-diagonal component of the stress tensor. The pressures and energy density of the fluid are regular at $r = 2m$, divergent at $r = 0$ and change sign at certain values of their arguments.

gr-qc↗

arXiv2012

On the time dependent Schwarzschild - de Sitter spacetime

An imperfect cosmic fluid with energy flux is analyzed. Even though its energy density $ρ$ is positive, the pressure $p = -ρ$ due to the fact that the metric is asymptotically de Sitter. The kinematical quantities for a nongeodesic congruence are computed. The scalar expansion is time independent but divergent at the singularity $r = 2m$. Far from the central mass $m$ and for a cosmic time $\bar{t} << H^{-1}$, the heat flux $q$ does not depend on Newton's constant $G$.

gr-qc↗

arXiv2012

The Milne spacetime and the hadronic Rindler horizon

A direct relation between the time dependent Milne geometry and the Rindler spacetime is shown. Milne's metric corresponds to that one beyond Rindler's event horizon (in the region $t \succ |x|)$). It was found that the shear tensor from the dissipative term of the RHIC expanding fireball has the same structure as that corresponding to the anisotropic fluid from the black hole interior, even though the latter geometry is curved.

hep-th↗

arXiv2012

Bjorken expansion in the isotropic Kasner spacetime

An isotropic expansion for the QGP is proposed in curved Kasner spacetime for an experimental configuration with three dimensional set of beams. The fluid of relativistic particles has no shear viscosity but the nonzero bulk viscosity $ζ$ is time dependent and its value could explain the enormous entropy per baryon of our Universe. In addition, $ζ$ equals the bulk viscosity of the anisotropic compressible fluid conjectured for the interior of a black hole.

hep-th↗

arXiv2011

Hyperbolic motion generated by inversion

An inversion transformation applied to an inertial observer is used to generate a nonstatic conformally flat geometry in spherical coordinates. A static observer in the new geometry is uniformly accelerating with respect to the inertial one and vice versa, but its acceleration $g$ undergoes the transformation $g \rightarrow 1/b^{2}g$, where $b$ is a constant. A nongeodesic congruence of a static observer has a scalar expansion which grows linearly with time but the acceleration is proportional to $r$, as for the classical rotation.

gr-qc↗

arXiv2011

Rindler horizon entropy from nonstationarity

Finite entropy and energy are obtained for the horizon of a Rindler observer on the grounds of the nonstatic character of the geometry beyond the horizon. Edery - Constantineau prescription is used to find the dynamical phase space of this particular spacetime. The number of microstates rooted from the ignorance of a Rindler observer of the parameter $t$ from the nonstationary region is calculated. We suggest that the gravitational energy density constructed by means of the horizon energy and using the Holographic Principle is proportional to $g^{2}$, similar with a result recently obtained by Padmanabhan and with the energy density of the electromagnetic field.

gr-qc↗

arXiv2011

Black hole horizon around a relativistic star

A star surrounded by a black hole generated by it is analyzed. For an outer static observer the black hole mass (and radius) depends on his position in the gravitational field of the star. In spite of the black hole presence, the geometry remains that of Schwarzschild. For an inner observer the black hole mass is constant and depends on the star's mass $m$ and radius $R$. The entropy for $r > R$ is one quarter of the area and is constant for $r < R$.

gr-qc↗

arXiv2011

On the conversion of rest energy in horizon energy

It is shown that the Verlinde formula for the entropy variation of a holographic screen is a consequence of the conversion of the particle energy in horizon energy. The special role played by the particular displacement $Δx = c^{2}/a$ is emphasized, $a$ being the particle acceleration. Using the Heisenberg Principle we show that the energy on the causal horizon (viewed as a holographic screen) of an inertial observer is proportional to its radius, as for a black hole.

gr-qc↗

arXiv2011

Anisotropic fluid inside a relativistic star

An anisotropic fluid with variable energy density and negative pressure is proposed, both outside and inside stars. The gravitational field is constant everywhere in free space (if we neglect the local contributions) and its value is of the order of $g = 10^{-8} cm/s^{2}$, in accordance with MOND model. With $ρ,~ p \propto 1/r$, the acceleration is also constant inside stars but the value is different from one star to another and depends on their mass $M$ and radius $R$. In spite of the fact that the spacetime is of Rindler type and curved even far from a local mass, the active gravitational energy on the horizon is $-1/4g$, as for the flat Rindler space, excepting the negative sign.

gr-qc↗

arXiv2010

Light Dragging Phenomenon and Expanding Wormholes

The null geodesic congruence for the Lorentzian version of Hawking's wormhole is studied, in spherical Rindler coordinates. One finds that the wormhole throat expands exponentially and the "flare - out" condition is satisfied. A time reversal is equivalent with an inversion applied to the radial coordinate. The stress energy is mostly located near wormhole's throat and the anisotropic fluid is comoving with the spherical distribution of accelerating observers. Far from the throat (the light cone in Cartesian coordinates) the negative energy density acquires an expression similar with the Casimir energy density between two perfectly reflecting plates. We conjecture that the light propagation follows the throat of a preexisting expanding wormhole (a "light dragging" phenomenon).

hep-th↗