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We revisit the construction of diffeomorphic but not isometric metric solutions to the Schwarzschild metric. These solutions require the introduction of non-trivial areal-radial functions and are characterized by the key property that the radial horizon's location is displaced continuously towards the singularity ( r = 0). In the limiting case scenario the location of the singularity and horizon merges and any in-falling observer hits a null singularity at the very moment they cross the horizon. This fact may have important consequences for the resolution of the fire wall problem and the complementarity controversy in black holes. This construction allows us to borrow the results over the past two decades pertaining to the study of the renormalization group improvement of Einstein's equations, which was based on the possibility that quantum Einstein gravity may be non-perturbatively renormalizable and asymptotically safe because of the presence of interacting (non-Gaussian) ultraviolet fixed points. The particular areal-radial function that eliminates the interior of a black hole, and furnishes a truly static metric solution everywhere, is used to establish the desired energy-scale relation k = k( r), which is obtained from the k (energy) dependent modifications to the running Newtonian coupling G( k), cosmological constant Λ( k), and space-time metric gij,( k)( x). (Anti) de Sitter - Schwarzschild metrics are also explored as examples. We conclude with a discussion of the role that asymptotic safety may have in the geometry of phase spaces (cotangent bundles of space-time) (i.e., in establishing a quantum space-time geometry or classical phase geometry correspondence g ij,( k)( x) ↔ gij( x, E)). [ABSTRACT FROM AUTHOR]

Particle collisions are considered within the context of f( R) gravity described by , where R stands for the Ricci scalar and α is a nonzero constant. The center of mass energy of head-on colliding particles moving in opposite radial directions near the naked singularity or horizon are considered. Collision of particles in the same direction near the event horizon yields finite energy while the energy of oppositely moving particles grows unbounded. Addition of a cosmological constant does not change this feature. Collision of a massless outgoing photon with an infalling particle and collision of two oppositely moving photons following null-geodesics are also taken into account. [ABSTRACT FROM AUTHOR]

It was shown that with the combination of three Liouville-type dilaton potentials, one can derive dilaton black holes in the background of anti-de-Sitter (AdS) spaces. In this paper, we further extend the study on the dilaton AdS black holes by investigating their thermodynamic instability through a geometry approach. First, we review thermodynamic quantities of the solutions and check the validity of the first law of thermodynamics. Then, we investigate phase transitions and stability of the solutions. In particular, we disclose the effects of the dilaton field on the stability of the black holes. We also employ the geometrical approach toward thermodynamical behavior of the system and find that the divergencies in the Ricci scalar coincide with roots and divergencies in the heat capacity. We find that the behavior of the Ricci scalar around divergence points depends on the type of the phase transition. [ABSTRACT FROM AUTHOR]

In this paper we use a dynamic form of modified Newtonian gravity to reformulate the equations for gravitational time dilation. Here we introduce the generic equations for gravitational time dilation. It is shown that these equations agree exactly with gravitational time dilation in satellite navigation systems. The equations are also in agreement with a reanalysis of observations of gravitational red shifts in black hole accretion discs. Using these equations, we translate the time dilation into a finite value at the black hole event horizon. Thus this reformulation resolves the difficulties of the existence of black hole singularities. Importantly these dynamic gravitational equations provide testable predictions in the vicinity of black holes. [ABSTRACT FROM AUTHOR]

Abstract: We review recent progress in numerical relativity simulations of black-hole (BH) spacetimes. Following a brief summary of the methods employed in the modeling, we summarize the key results in two major areas of BH physics: (i) BHs as sources of gravitational waves (GWs) and (ii) astrophysical systems involving BHs. We conclude with a list of the most urgent tasks for numerical relativity in these areas. [Copyright &y& Elsevier]

Abstract: Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GRB sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes. [Copyright &y& Elsevier]

Abstract: Open questions in GRB physics are summarized as of 2011, including classification, progenitor, central engine, ejecta composition, energy dissipation and particle acceleration mechanism, radiation mechanism, long term engine activity, external shock afterglow physics, origin of high energy emission, and cosmological setting. Prospects of addressing some of these problems with the upcoming Chinese–French GRB mission, SVOM, are outlined. [Copyright &y& Elsevier]

Abstract: The center of our galaxy hosts the nearest super-massive black hole to the solar system, identified with the compact radio source Sgr A*. High energy experiments have tried in the past to detect the X/gamma-ray emission expected from the accretion of the surrounding material into this super-massive black hole. Only recently, however, thanks to the new generation of telescopes, it has been possible to reveal high energy radiation associated with Sgr A* or its close environment. I will review and discuss in particular the results on the Sgr A* X-ray flares discovered with Chandra and XMM-Newton, on the central soft gamma-ray source detected with INTEGRAL, and on the galactic center TeV emission revealed by HESS. To cite this article: A. Goldwurm, C. R. Physique 8 (2007). [Copyright &y& Elsevier]

Abstract: Several physical and astrophysical problems related to accretion onto black holes and neutron stars are briefly reviewed. I discuss the observed differences between these two types of compact objects in quiescent Soft X-ray Transients. Then I review the status of various non-standard objects suggested as an alternative to black holes. Finally, I present new results and a suggestion about the nature of the jet activity in Active Galactic Nuclei. To cite this article: J.-P. Lasota, C. R. Physique 8 (2007). [Copyright &y& Elsevier]

Abstract: We now know that our Galaxy harbors at its centre a supermassive black hole. This result came after more than 2 decades of infrared studies of the Galactic Centre and important instrumental developments in infrared detectors and in high spatial resolution techniques. Adaptive optics, which allows diffraction-limited infrared observations and enhanced sensitivity, was actually the major breakthrough in this respect. We introduce in the first section of this article what was our knowledge of the Galactic Centre before the advent of adaptive optics. In the second section, after a reminder of the first adaptive optics observations of this region, we highlight the specificities of Galactic Centre adaptive optics observations. In the third and fourth sections, we present the major results obtained from adaptive optics observations of the Galactic Centre: the case of the supermassive black hole and the paradox of youth. In the fifth section, we introduce two main future facilities that will provide even higher spatial resolutions, Gravity—a second generation VLTI instrument—and Extremely Large Telescopes, and the improvements that we expect with these new instruments in our knowledge of the Galactic Centre region. We conclude in the last section. To cite this article: Y. Clénet et al., C. R. Physique 8 (2007). [Copyright &y& Elsevier]

Abstract: Over the past decade we have learned that probably all ellipticals and bulges of galaxies contain central supermassive black holes (SMBH). SMBH masses correlate well with the luminosities, and in turn the stellar masses of the bulges harboring them, with about 0.15% of the bulge mass being found in the SMBH. Pure disk galaxies, on the other hand, do not, in general, seem to contain SMBHs. Here we review the best cases for SMBH detection in galaxies, discuss methods and associated uncertainties, summarize correlations between SMBH masses and host galaxy properties, and finally address possible future developments. To cite this article: R. Bender, R.P. Saglia, C. R. Physique 8 (2007). [Copyright &y& Elsevier]

We consider the nonlinear Klein–Gordon equation □u+m2u+λ&z.sfnc;u&z.sfnc;2u=0, with λ⩾0, outside a Kerr black hole. We solve the global Cauchy problem for large data with minimum regularity. Then, using a Penrose compactification, we prove, in the massless case, the existence of smooth asymptotic profiles and Sommerfeld radiation conditions, at the horizon and at null infinity, for smooth solutions. [Copyright &y& Elsevier]