Search results for "Relativity"
showing 10 items of 1213 documents
On the relativistic compressibility conditions
2018
The constraints imposed by the relativistic compressibility hypothesis on the square of the speed of sound in a medium are obtained. This result allows to obtain purely hydrodynamic conditions for the physical reality of a perfect energy tensor representing the energetic evolution of a perfect fluid in local thermal equilibrium. The results are applied to the paradigmatic case of the generic ideal gases. Then the physical reality of the ideal gas Stephani models is analyzed and the Rainich-like theory for ideal gas solutions is built.
3D Evolution of a Good-Bad-Ugly-F Model on Compactified Hyperboloidal Slices
2023
The Good-Bad-Ugly-F model is a system of semi-linear wave equations that mimics the asymptotic form of the Einstein field equations in generalized harmonic gauge with specific constraint damping and suitable gauge source functions. These constraint additions and gauge source functions eliminate logarithmic divergences appearing at the leading order in the asymptotic expansion of the metric components. In this work, as a step towards using compactified hyperboloidal slices in numerical relativity, we evolve this model numerically in spherical symmetry, axisymmetry and full 3d on such hyperboloidal slices. Promising numerical results are found in all cases. Our results show that nonlinear sys…
MOTS in Schwarzschild: multiple self-intersections and extreme mass ratio mergers
2020
We study the open and closed axisymmetric marginally outer trapped surfaces contained in leaves of constant Painlev\'e-Gullstrand time for Schwarzschild spacetimes. We identify a family of closed MOTS in the black hole interior characterized by an arbitrary number of self-intersections. This suggests that the self-intersecting behaviour reported in [Phys.Rev.D 100, 084044 (2019)] may be a far more generic phenomenon than expected. We also consider open surfaces, finding that their behaviour is highly constrained but includes surfaces with multiple self-intersections inside the horizon. We argue that the behaviour of open MOTS identifies and constrains the possible local behaviour of MOTS du…
Effect of a non null pressure on the evolution of perturbations in the matter dominated epoch
2003
We analyze the effect of pressure on the evolution of perturbations of an Einstein-de Sitter Universe in the matter dominated epoch assuming an ideal gas equation of state. For the sake of simplicity the temperature is considered uniform. The goal of the paper is to examine the validity of the linear approximation. With this purpose the evolution equations are developed including quadratic terms in the derivatives of the metric perturbations and using coordinate conditions that, in the linear case, reduce to the longitudinal gauge. We obtain the general solution, in the coordinate space, of the evolution equation for the scalar mode, and, in the case of spherical symmetry, we express this s…
A diffeomorphism invariant family of metric-affine actions for loop cosmologies
2023
In loop quantum cosmology (LQC) the big bang singularity is generically resolved by a big bounce. This feature holds even when modified quantization prescriptions of the Hamiltonian constraint are used such as in mLQC-I and mLQC-II. While the later describes an effective description qualitatively similar to that of standard LQC, the former describes an asymmetric evolution with an emergent Planckian de-Sitter pre-bounce phase even in the absence of a potential. We consider the potential relation of these canonically quantized non-singular models with effective actions based on a geometric description. We find a 3-parameter family of metric-affine $f(\mathcal{R})$ theories which accurately a…
Cosmic primordial density fluctuations and Bell inequalities
2023
The temperature measurements, $T$, of the perturbed cosmic microwave background, performed by the cosmic background explorer satellite (COBE), are considered. A dichotomist function, $f = \pm 1$, is defined such that $f =+1$ if $\delta T > 0$ and $f =-1$ if $\delta T < 0$, where $\delta T$ stands for the fluctuation of $T$. Then, it is assumed that behind the appearance of these fluctuations there is local realism. Under this assumption, some specific Clauser-Horne-Shimony-Holt (CHSH) inequalities are proved for these fluctuation temperatures measured by COBE in the different sky directions. The calculation of these inequalities from the actual temperature measurements shows that these ineq…
On Newtonian frames
2007
In Newtonian space-time there exist four, and only four, causal classes of frames. Natural frames allow to extend this result to coordinate systems, so that coordinate systems may be also locally classified in four causal classes. These causal classes admit simple geometric descriptions and physical interpretations. For example, one can generate representatives of the four causal classes by means of the {\em linear synchronization group}. Of particular interest is the {\em local Solar time synchronization}, which reveals the limits of the frequent use of the concept of `causally oriented oordinate', such as that of `time-like coordinate'. Classical {\em positioning systems}, based in sound …
Comment to the reply to "On the thermodynamics of inhomogeneous perfect fluid mixtures"
2004
In spite of a recent reply by Quevedo and Z\'arate (gr-qc/0403096), their assertion that their thermodynamic scheme for a perfect fluid binary mixture is incompatible with Szekeres and Stephani families of universes, except those of FRW ones, remains wrong.
Physics of Relativistic Perfect Fluids
2003
A criterion is presented and discussed to detect when a divergence-free perfect fluid energy tensor in the space-time describes an evolution in local thermal equilibrium. This criterion is applied to the class II Szafron-Szekeres perfect fluid space-times solutions, giving a very simple characterization of those that describe such thermal evolutions. For all of them, the significant thermodynamic variables are explicitly obtained. Also, the specific condition is given under which the divergence-free perfect fluid energy tensors may be interpreted as an ideal gas.
Metric-affine f(R,T) theories of gravity and their applications
2018
We study f(R,T) theories of gravity, where T is the trace of the energy-momentum tensor T_{\mu\nu}, with independent metric and affine connection (metric-affine theories). We find that the resulting field equations share a close resemblance with their metric-affine f(R) relatives once an effective energy-momentum tensor is introduced. As a result, the metric field equations are second-order and no new propagating degrees of freedom arise as compared to GR, which contrasts with the metric formulation of these theories, where a dynamical scalar degree of freedom is present. Analogously to its metric counterpart, the field equations impose the non-conservation of the energy-momentum tensor, wh…