We profit by a recent paper of Visser claiming that Rastall gravity is equivalent to Einstein gravity to compare the two gravitational theories in a general way. Our conclusions are different from ...Visser’s ones. We indeed argue that these two theories are not equivalent. In fact, Rastall theory of gravity is an “open” theory when compared to Einstein general theory of relativity. Thus, it is ready to accept the challenges of observational cosmology and quantum gravity.
Employing the Verlinde's hypothesis, and considering two well-known generalized entropy formalisms, two modifications to the Newtonian gravity are derived. In addition, it has been shown that the ...generalized entropy measures may also provide theoretical basis for the Modified Newtonian Dynamics (MOND) theory and generate its modified forms. Since these entropy measures are also successful in describing the current accelerated universe, our results indicate that the origin of dark sectors of cosmos may be unified to meeting the generalized entropy measures instead of the Boltzmann–Gibbs entropy by the gravitational systems due to the long-range nature of gravity.
The Bousso's D-bound entropy for the various possible black hole solutions on a 4-dimensional brane is checked. It is found that the D-bound entropy here is apparently different from that of obtained ...for the 4-dimensional black hole solutions. This difference is interpreted as the extra loss of information, associated to the extra dimension, when an extra-dimensional black hole is moved outward the observer's cosmological horizon. Also, it is discussed that N-bound entropy is hold for the possible solutions here. Finally, by adopting the recent Bohr-like approach to black hole quantum physics for the excited black holes, the obtained results are written also in terms of the black hole excited states.
A quantum model for the Schwarzschild black hole was recently proposed by applying Rosen’s quantization approach to the historical Oppenheimer and Snyder gravitational collapse and by setting the ...constraints on the formation of the Schwarzschild black hole. An interesting picture emerges: the traditional Schwarzschild singularity is replaced by a quantum oscillator describing a nonsingular “two-particle ” system where the two components, the “nucleus ” and the “electron,”, strongly interact with each other through a quantum gravitational interaction. In agreement with the de Broglie hypothesis, the “electron ” is interpreted in terms of quantum oscillations of the black-hole horizon. In other words, the Schwarzschild black hole should be considered as a gravitational analogue of the hydrogen atom. In this paper, after a short review of our previous results, we analyze some of the consequences of this approach. We also show that, by performing a correct rescaling of the energy levels, the semiclassical Bohr-like approach to quantum black holes, previously developed by one of the authors (CC), is consistent with the results obtained here for large values of the black-hole principal quantum number. After this, Hawking radiation is analyzed by discussing its connection with the black-hole quantum structure. Finally, we conclude the paper by discussing the black-hole information problem and its possible resolution.
The production of a stochastic background of relic gravitational waves is well known in various works in the literature, where, using the so called adiabatically-amplified zero-point fluctuations ...process it has been shown how the standard inflationary scenario for the early universe can in principle provide a distinctive spectrum of relic gravitational waves. In this paper, it is shown that a weak modification of General Relativity produces a third massive polarization of gravitational waves and the primordial production of this polarization is analyzed adapting the adiabatically-amplified zero-point fluctuations process at this case.
The presence of the mass could also have important applications in cosmology as the fact that gravitational waves can have mass could give a contribution to the dark matter of the universe.
At the end of the paper an upper bound for these relic gravitational waves, which arises from the WMAP constrains, is also released.
A
bstract
The non-strictly continuous character of the Hawking radiation spectrum generates a natural correspondence between Hawking radiation and black hole (BH) quasi-normal modes (QNM). In this ...work, we generalize recent results on this important issue to the framework of Kerr BHs (KBH). We show that also for the KBH, QNMs can be naturally interpreted in terms of quantum levels. Thus, the emission or absorption of a particle is in turn interpreted in terms of a transition between two different levels. At the end of the paper, we also generalize some concepts concerning the “effective state” of a KBH.
PDF
