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Hristu Culetu

Publications and source records attributed to Hristu Culetu.

69 records · Page 4Linked to original sources
arXiv2010

Boundary stress tensors for spherically symmetric conformal Rindler observers

The boundary energy - momentum tensors for a static observer in the conformally flat Rindler geometry are considered. We found the surface energy is positive far form the Planck world but the transversal pressures are negative. The kinematical parameters associated to a nongeodesic congruence of static observers are computed. The entropy $S$ corresponding to the degrees of freedom on the two surface of constant $ρ$ and $t$ equals the horizon entropy of a black hole with a time dependent mass and the Padmanabhan expression $E = 2 S T$ is obeyed. The two surface shear tensor is vanishing but the coefficient of the bulk viscosity $ζ$ is $1/16 π$ and therefore the negative pressure due to it acts as a surface tension.

hep-th↗
arXiv2010

On the near horizon approximation for the Schwarzschild black hole

An anisotropic fluid with a negative radial pressure $p = - ρ$ is supposed to exist near the horizon of a Schwarzschild black hole. The constant energy density $ρ$ depends only on the black hole mass. The radial acceleration of the static observers is constant but its modulus has exactly the Rindler form, as was recently suggested by Dahia and Felix da Silva. The structure of the stress tensor is similar with that obtained previously for the interior region of a black hole, excepting the time dependence.

hep-th↗
arXiv2010

Comments on "On the Origin of Gravity and the Laws of Newton", by Erik Verlinde

We argue that the relativistic Unruh temperature cannot be associated with the bits on the screen, in the form considered by Verlinde. The acceleration $a$ is a scalar quantity (the modulus of the acceleration four vecor) and not a vector. When the mass $m$ approaches the holographic screen, viewed as a stretched horizon, the shift $Δx$ from Verlinde's Eq. (3.15) becomes $c^{2}/a$ and the entropy variation equals $(1/2) k_{B} ΔN$, in accordance with Gao's calculations. 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.

hep-th↗
arXiv2009

The membrane paradigm for a dynamic wormhole

A classical membrane model is developed for a dynamic wormhole. Its throat expands hyperbolically with an acceleration proportional to the surface energy density $σ$. The model leads to a negative bulk viscosity coefficient $ζ= - 1/6 π$ on the throat, expressing the instability against perturbations (expansions or contractions).

gr-qc↗
arXiv2008

Comments on : Frame dragging anomalies for rotating bodies

It is shown that Collas and Klein (ArXiv : 0811.2471 [gr-qc]) wrongly concluded that "negative frame dragging" phenomenon takes place at all finite $r$ and $z$ coordinate values . We argue that a test particle with zero angular momentum counter-rotates with respect to the source in the "time machine" region only. In addition, Bonnor's spacetime has an event horizon at $r_{H}$ = 0.

gr-qc↗
arXiv2008

Boundary sources in the Doran - Lobo - Crawford spacetime

We take a null hypersurface (the causal horizon) generated by a congruence of null geodesics as the boundary of the Doran-Lobo-Crawford spacetime, to be the place where the Brown-York quasilocal energy is located. The components of the outer and inner stress tensors are computed and shown to depend on time and on the impact parameter $b$ of the test particle trajectory. The surface energy density $σ$ on the boundary is given by the same expression as that obtained previously for the energy stored on a Rindler horizon.

hep-th↗
arXiv2008

Anisotropic fluid with time dependent viscosity coefficients

A spacetime endowed with an anisotropic fluid is proposed for the interior of a Schwarzschild black hole. The geometry has an instantaneous Minkowski form and is a solution of Einstein's equations with a stress tensor on the r.h.s. obeying all the energy conditions. The interior fluid is compressible, with time dependent shear and bulk viscosity coefficients. The energy density $ρ$ and the ''radial'' pressure $p$ observe the equation of state $p + ρ= 0$ (as for dark energy), with no pressures on $θ-$ and $ϕ-$ directions. However, the angular components of the viscous part of the stress tensor are nonvanishing and equals the energy density of the fluid.

hep-th↗
arXiv2007

On a stationary spinning string spacetime

The properties of a stationary massless string endowed with intrinsic spin are discussed. The spacetime is Minkowskian geometrically but the topology is nontrivial due to the horizon located on the surface $r=0$, similar with Rindler's case. For $r$ less than the Planck length $b$, $g_{ϕϕ}$ has the same sign as $g_{tt}$ and closed timelike curves are possible. We assume an elementary particles' spin originates in the frame dragging effect produced by the rotation of the source. The Sagnac time delay is calculated and proves to be constant.

hep-th↗
arXiv2007

Does rotation generate a massive string ?

The properties of a stationary massive string endowed with intrinsic angular momentum are investigated. The spacetime is generated by an "improper" time translation combined with uniform rotation. The mass per unit length of the string is proportional to the angular velocity $ω$. The spacetime is Minkowskian geometrically but the topology is nontrivial thanks to the event horizon located on the surface $r = 0$ (similar with Rindler's spacetime) and to the deficit angle generated by rotation. The Sagnac time delay is calculated. It proves to be nonvanishing even when $ω= 0$ due to the intrinsic spin of the string.

hep-th↗
arXiv2007

On The Black Hole Interior Spacetime

A new version of the geometry inside a black hole is proposed, on the grounds of an idea given by Doran et al. The spacetime is still time dependent and is a solution of Einstein's equations with a stress tensor corresponding to an anisotropic fluid. The energy density of the fluid is proportional to $1/t^{2}$ as in many dark energy models and the Brown-York quasilocal energy of the black hole interior equals its mass $m$.

hep-th↗
arXiv2006

Is the Rindler horizon energy nonvanishing ?

A nonvanishing value for the Rindler horizon energy is proposed, by an analogy with the "near horizon" Schwarzschild metric. We show that the Rindler horizon energy is given by the same formula $E = α/2$ obtained by Padmanabhan for the Schwarzschild spacetime, where $α$ is the gravitational radius.

hep-th↗
arXiv2006

On the Hawking wormhole horizon entropy

The entropy S of the horizon $theta = pi/2$ of the Hawking wormhole written in spherical Rindler coordinates is computed in this letter. Using Padmanabhan's prescription,we found that the surface gravity of the horizon equals the proper acceleration of the Rindler observer. S is a monotonic function of the radial coordinate $xi$ and vanishes when $xi$ equals the Planck length. In addition, its expression is similar with the Kaul - Majumdar one for the black hole entropy, including logarithmic corrections in quantum gravity scenarios.

hep-th↗
arXiv2004

The zero point energy and gravitation

A possible connection between the energy W of the vacuum fluctuations of quantum fields and gravity in "empty space" is conjectured in this paper using a natural cutoff of high momenta with the help of the gravitational radius of the vacuum region considered. We found that below some "critical" length $L = 1 mm$ the pressure $sigma$ is one third of the energy density $epsilon$, as for dark matter, but above $1 mm$ the equation of state is $sigma = -(epsilon)$ (dark energy). In the case of a massive field, W does not depend on the mass of the field for $L<<1 mm$ but for $L>>1 mm$ it does not depend on the Planck constant. In addition, when the Newton constant tends to zero, W becomes infinite. The energy density is also a function of the volume V of the vacuum region taken into account.

hep-th↗
arXiv2003

On a Time Dependent Cosmological Constant

We conjecture a time dependent Lambda(t), in terms of the Gaussian curvature of the causal horizon, which is nonvanishing even in Minkowski space due to the lack of informations beyond the light cone. Using the Heisenberg Principle, the corresponding energy of the quantum fluctuations, localized on the past or future null horizon, is proportional to sqrt(Lambda(t)). We compute Lambda(t) for the (conformally flat) Lorentzian Hawking wormhole geometry, written in static spherical Rindler coordinates.

hep-th↗