Search results for "GRAVITATION"

The Adiabatic Invariance of the Action Variables

2001

We shall first use an example to explain the concept of adiabatic invariance. Let us consider a “super ball” of mass m, which bounces back and forth between two walls (distance l) with velocity \(\boldsymbol{v}_{0}\). Let gravitation be neglected, and the collisions with the walls be elastic. If F m denotes the average force onto each wall, then we have $$\displaystyle{ F_{m}T = -\int _{\mathrm{coll.\,time}}f\,dt\;. }$$ (9.1) f is the force acting on the ball during one collision, and T is the time between collisions.

Mean-field correlations in the core of rich galaxy clusters

1991

We develop a theory for the contribution to the clustering correlation function from gravitational interactions of neighboring pairs of galaxies in clusters. This is based on the «Hypernetted Chain Equation», a self-consistent integral equation relating the correlation function to the interaction potential.

R2phase diagram of quantum Einstein gravity and its spectral dimension

2012

Within the gravitational asymptotic safety program, the renormalization group (RG) flow of the ${R}^{2}$ truncation in three and four spacetime dimensions is analyzed in detail. In particular, we construct RG trajectories which emanate from the non-Gaussian UV fixed point and possess long classical regimes where the effective average action is well approximated by the classical Einstein-Hilbert action. As an application we study the spectral dimension of the effective quantum Einstein gravity spacetimes resulting from these trajectories, establishing that the picture of a multifractal spacetime is robust under the extension of the truncated theory space. We demonstrate that regimes of const…

Revisiting the quantum scalar field in spherically symmetric quantum gravity

2012

We extend previous results in spherically symmetric gravitational systems coupled with a massless scalar field within the loop quantum gravity framework. As starting point, we take the Schwarzschild spacetime. The results presented here rely on the uniform discretization method. We are able to minimize the associated discrete master constraint using a variational method. The trial state for the vacuum consists of a direct product of a Fock vacuum for the matter part and a Gaussian centered around the classical Schwarzschild solution. This paper follows the line of research presented by Gambini, Pullin and Rastgoo and a comparison between their result and the one given in this work is made.

Gravitational waves from neutron stars at different evolutionary stages

2003

We study how the internal structure of a neutron star and the physical processes that may occur during its evolution affect the quasi-normal mode spectrum, and consequently the gravitational radiation it emits. We discuss whether these modes can be excited and how much energy they should carry for the gravitational signal to be detectable by the first generation of interferometric antennas or by the new generation of high-frequency gravitational detectors, interferometric or resonant, that are under investigation.

Post-Newtonian constraints onf(R)cosmologies in metric and Palatini formalism

2005

We compute the complete post-Newtonian limit of both the metric and Palatini formulations of $f(R)$ gravities using a scalar-tensor representation. By comparing the predictions of these theories with laboratory and solar system experiments, we find a set of inequalities that any lagrangian $f(R)$ must satisfy. The constraints imposed by those inequalities allow us to find explicit bounds to the possible nonlinear terms of the lagrangian. We conclude that in both formalisms the lagrangian $f(R)$ must be almost linear in $R$ and that corrections that grow at low curvatures are incompatible with observations. This result shows that modifications of gravity at very low cosmic densities cannot b…

Focus on modern frontiers of matter wave optics and interferometry

2012

The level of experimental control and the detailed theoretical understanding of matter wave physics have led to a renaissance of experiments testing the very foundations of quantum mechanics and general relativity, as well as to applications in metrology. A variety of interferometric quantum sensors surpasses, or will surpass, the limits of their classical counterparts, for instance in the measurement of frequency and time or forces such as accelerations due to rotation and gravity with applications in basic science, navigation and the search for natural resources. The collection of original articles published in this focus issue of New Journal of Physics is intended as a snapshot of the cu…

Cosmological solutions in theD=5 Einstein-Maxwell theory coupled to matter

1991

We study the Einstein-Maxwell theory in five dimensions coupled to matter in two distinct ways. In the first we reduce the Lagrangian to an effective four-dimension one and then we couple it to matter; in the second, we introduce matter directly in the original five-dimensional theory. In both cases we use a non trivial configuration for the Maxwell potential. We find non singular solutions which present a repulsive gravitational phase. When this phase is absent, the initial singularity is unavoidable.

NEUTRINO OSCILLATION AND STELLAR COLLAPSE.

1984

General Relativistic Simulations of the Collapsar Scenario

2013

We are exploring the viability of the collapsar model for long-soft gamma-ray bursts. For this we perform state-of-the-art general relativistic hydrodynamic simulations in a dynamically evolving space-time with the CoCoNuT code. We start from massive low metallicity stellar models evolved up to core gravitational instability, and then follow the subsequent evolution until the system collapses forming a compact remnant. A preliminary study of the collapse outcome is performed by varying the typical parameters of the scenario, such as the initial stellar mass, metallicity, and rotational profile of the stellar progenitor. 1D models (without rotation) have been used to test our newly developed…