Search results for "Relativity"
showing 10 items of 1213 documents
A scenario for critical scalar field collapse in $AdS_3$
2014
We present a family of exact solutions, depending on two parameters $\alpha$ and $b$ (related to the scalar field strength), to the three-dimensional Einstein-scalar field equations with negative cosmological constant $\Lambda$. For $b=0$ these solutions reduce to the static BTZ family of vacuum solutions, with mass $M = -\alpha$. For $b\neq0$, the solutions become dynamical and develop a strong spacelike central singularity. The $\alpha0$ agrees qualitatively with that observed in numerical simulations of subcritical collapse. We analyze the linear perturbations of the threshold solution, $\alpha=0$, in the $\Lambda=0$ approximation, and find that it has only one unstable growing mode, whi…
Testing Hawking particle creation by black holes through correlation measurements
2010
Hawking's prediction of thermal radiation by black holes has been shown by Unruh to be expected also in condensed matter systems. We show here that in a black hole-like configuration realized in a BEC this particle-creation does indeed take place and can be unambiguously identified via a characteristic pattern in the density-density correlations. This opens the concrete possibility of the experimental verification of this effect.
RG flows of Quantum Einstein Gravity in the linear-geometric approximation
2014
We construct a novel Wetterich-type functional renormalization group equation for gravity which encodes the gravitational degrees of freedom in terms of gauge-invariant fluctuation fields. Applying a linear-geometric approximation the structure of the new flow equation is considerably simpler than the standard Quantum Einstein Gravity construction since only transverse-traceless and trace part of the metric fluctuations propagate in loops. The geometric flow reproduces the phase-diagram of the Einstein-Hilbert truncation including the non-Gaussian fixed point essential for Asymptotic Safety. Extending the analysis to the polynomial $f(R)$-approximation establishes that this fixed point come…
Remarks on the reduced phase space of -dimensional gravity on a torus in the Ashtekar formulation
1998
We examine the reduced phase space of the Barbero-Varadarajan solutions of the Ashtekar formulation of (2 + 1)-dimensional general relativity on a torus. We show that it is a finite-dimensional space due to the existence of an infinite-dimensional residual gauge invariance which reduces the infinite-dimensional space of solutions to a finite-dimensional space of gauge-inequivalent solutions. This is in agreement with general arguments which imply that the number of physical degrees of freedom for (2 + 1)-dimensional Ashtekar gravity on a torus is finite.
Phase space coordinates and the Hamiltonian constraint of Regge calculus.
1994
We suggest that the phase space of Regge calculus is spanned by the areas and the deficit angles corresponding to the two-simplexes on the spacelike hypersurface of simplicial spacetime. Our proposal is based on a slight modification of the Ashtekar formulation of canonical gravity. In terms of these phase space coordinates we write an equation which we suggest to be a simplicial version of the Hamiltonian constraint of canonical gravity.
High Resolution and Broad Band Spectra of Low Mass X-ray Binaries: A Comparison between Black Holes and Neutron Stars
2005
A common question about compact objects in high energy astrophysics is whether it is possible to distinguish black hole from neutron star systems with some other property that is not the mass of the compact object. Up to now a few characteristics have been found which are typical of neutron stars (like quasi periodic oscillations at kHz frequencies or type-I X-ray bursts), but in many respects black hole and neutron star systems show very similar behaviors. We present here a spectral study of low mass X-ray binaries containing neutron stars and show that these systems have spectral characteristics that are very similar to what is found for black hole systems. This implies that it is unlikel…
Hydromagnetic instabilities and magnetic field amplification in core collapse supernovae
2011
Some of the most violent events in the universe, the gamma ray burst, could be related to the gravitational collapse of massive stellar cores. The recent association of long GRBs to some class of type Ic supernova seems to support this view. In such scenario fast rotation, strong magnetic fields and general relativistic effects are key ingredients. It is thus important to understand the mechanism that amplifies the magnetic field under that conditions. I present global simulations of the magneto-rotational collapse of stellar cores in general relativity and semi-global simulations of hydromagnetic instabilities under core collapse conditions. I discuss effect of the magneto-rotational insta…
Towards modelling the central engine of short GRBs
2011
Numerical relativity simulations of non-vacuum spacetimes have reached a status where a complete description of the inspiral, merger and post-merger stages of the late evolution of close binary neutron systems is possible. Determining the properties of the black-hole-torus system produced in such an event is a key aspect to understand the central engine of short-hard gamma-ray bursts (sGRBs). Of the many properties characterizing the torus, the total rest-mass is the most important one, since it is the torus' binding energy which can be tapped to extract the large amount of energy necessary to power the sGRB emission. In addition, the rest-mass density and angular momentum distribution in t…
The momentum and the angular momentum of the Universe revisited. Some preliminary results
2007
We consider the question of properly defining energy and momenta for non asymptotic Minkowskian spaces in general relativity. Only those of these spaces which have zero energy, zero linear 3-momentum, and zero intrinsic angular momentum would be candidates to creatable universes, that is, to universes which could have arisen from a vacuum quantum fluctuation. Given a universe, we completely characterize the family of coordinate systems in which it would make sense saying that this universe can be a creatable universe.
Dynamical spacetimes and gravitational radiation in a Fully Constrained Formulation
2010
This contribution summarizes the recent work carried out to analyze the behavior of the hyperbolic sector of the Fully Constrained Formulation (FCF) derived in Bonazzola et al. 2004. The numerical experiments presented here allows one to be confident in the performances of the upgraded version of CoCoNuT's code by replacing the Conformally Flat Condition (CFC) approximation of the Einstein equations by the FCF.