476 research outputs found
Where does the physics of extreme gravitational collapse reside?
The gravitational collapse of massive stars serves to manifest the most
severe deviations of general relativity with respect to Newtonian gravity: the
formation of horizons and spacetime singularities. Both features have proven to
be catalysts of deep physical developments, especially when combined with the
principles of quantum mechanics. Nonetheless, it is seldom remarked that it is
hardly possible to combine all these developments into a unified theoretical
model, while maintaining reasonable prospects for the independent experimental
corroboration of its different parts. In this paper we review the current
theoretical understanding of the physics of gravitational collapse in order to
highlight this tension, stating the position that the standard view on
evaporating black holes stands for. This serves as the motivation for the
discussion of a recent proposal that offers the opposite perspective,
represented by a set of geometries that regularize the classical singular
behavior and present modifications of the near-horizon Schwarzschild geometry
as the result of the propagation of non-perturbative ultraviolet effects
originated in regions of high curvature. We present an extensive exploration of
the necessary steps on the explicit construction of these geometries, and
discuss how this proposal could change our present understanding of
astrophysical black holes and even offer the possibility of detecting genuine
ultraviolet effects on future gravitational wave experiments.Comment: 43 pages, 1 figure. Review article with new results on the black to
white hole transition. Prepared for the special issue "Open Questions in
Black Hole Physics" edited by Gonzalo J. Olm
Shock-Wave Refinement of the Friedmann-Robertson-Walker Metric
The mathematics of general relativistic shock waves is introduced and
considered in a cosmological context. In particular, an expanding
Friedmann-Roberson-Walker metric is matched to a Tolman-Oppenheimer-Volkoff
metric across a spherical shock surface. This is the general relativistic
analogue of a shock-wave explosion within a static singular isothermal fluid
sphere and may be regarded as a model for the Big Bang. These shock waves are
constructed both within and beyond the Hubble radius, which corresponds to a
universe outside and inside its Schwarzschild radius respectively. Certain
self-similar perturbations of the FRW metric lead to an accelerated expansion,
even without a cosmological constant, and thus it is conjectured that such a
mechanism may account for the anomalous acceleration observed today without
recourse to dark energy
General-relativistic resistive magnetohydrodynamics in three dimensions: Formulation and tests
We present a new numerical implementation of the general-relativistic
resistive magnetohydrodynamics (MHD) equations within the Whisky code. The
numerical method adopted exploits the properties of implicit-explicit
Runge-Kutta numerical schemes to treat the stiff terms that appear in the
equations for large electrical conductivities. Using tests in one, two, and
three dimensions, we show that our implementation is robust and recovers the
ideal-MHD limit in regimes of very high conductivity. Moreover, the results
illustrate that the code is capable of describing scenarios in a very wide
range of conductivities. In addition to tests in flat spacetime, we report
simulations of magnetized nonrotating relativistic stars, both in the Cowling
approximation and in dynamical spacetimes. Finally, because of its
astrophysical relevance and because it provides a severe testbed for
general-relativistic codes with dynamical electromagnetic fields, we study the
collapse of a nonrotating star to a black hole. We show that also in this case
our results on the quasinormal mode frequencies of the excited electromagnetic
fields in the Schwarzschild background agree with the perturbative studies
within 0.7% and 5.6% for the real and the imaginary part of the l=1 mode
eigenfrequency, respectively. Finally we provide an estimate of the
electromagnetic efficiency of this process.Comment: 22 pages, 19 figure
Spacetime Singularities
We present here an overview of our basic understanding and recent
developments on spacetime singularities in the Einstein theory of gravity.
Several issues related to physical significance and implications of
singularities are discussed. The nature and existence of singularities are
considered which indicate the formation of super ultra-dense regions in the
universe as predicted by the general theory of relativity. Such singularities
develop during the gravitational collapse of massive stars and in cosmology at
the origin of the universe. Possible astrophysical implications of the
occurrence of singularities in the spacetime universe are indicated. We discuss
in some detail the profound and key fundamental issues that the singularities
give rise to, such as the cosmic censorship and predictability in the universe,
naked singularities in gravitational collapse and their relevance in black hole
physics today, and their astrophysical implications in modern relativistic
astrophysics and cosmology.Comment: 45 pages, LaTex; Invited Review article for the `Springer Handbook of
Spacetime' (eds A. Ashtekar and V. Petkov
Recent developments in gravitational collapse and spacetime singularities
It is now known that when a massive star collapses under the force of its own gravity,
the final fate of such a continual gravitational collapse will be either a black hole or a
naked singularity under a wide variety of physically reasonable circumstances within the
framework of general theory of relativity. The research of recent years has provided con-
siderable clarity and insight on stellar collapse, black holes and the nature and structure
of spacetime singularities. We discuss several of these developments here. There are also important fundamental questions that remain unanswered on the final fate of collapse of a massive matter cloud in gravitation theory, especially on naked singularities which are hypothetical astrophysical objects and on the nature of cosmic censorship hypothesis. These issues have key implications for our understanding on black hole physics today, its astrophysical applications, and for certain basic questions in cosmology and possible quantum theories of gravity. We consider these issues here and summarize recent results and current progress in these directions. The emerging astrophysical and observational perspectives and implications are dicussed, with particular reference to the properties of accretion discs around black holes and naked singularities, which may provide charac-teristic signatures and could help distinguish these object
Recent developments in gravitational collapse and spacetime singularities
It is now known that when a massive star collapses under the force of its own gravity,
the final fate of such a continual gravitational collapse will be either a black hole or a
naked singularity under a wide variety of physically reasonable circumstances within the
framework of general theory of relativity. The research of recent years has provided con-
siderable clarity and insight on stellar collapse, black holes and the nature and structure
of spacetime singularities. We discuss several of these developments here. There are also important fundamental questions that remain unanswered on the final fate of collapse of a massive matter cloud in gravitation theory, especially on naked singularities which are hypothetical astrophysical objects and on the nature of cosmic censorship hypothesis. These issues have key implications for our understanding on black hole physics today, its astrophysical applications, and for certain basic questions in cosmology and possible quantum theories of gravity. We consider these issues here and summarize recent results and current progress in these directions. The emerging astrophysical and observational perspectives and implications are dicussed, with particular reference to the properties of accretion discs around black holes and naked singularities, which may provide charac-teristic signatures and could help distinguish these object
Gravitational-wave research as an emerging field in the Max Planck Society. The long roots of GEO600 and of the Albert Einstein Institute
On the occasion of the 50th anniversary since the beginning of the search for
gravitational waves at the Max Planck Society, and in coincidence with the 25th
anniversary of the foundation of the Albert Einstein Institute, we explore the
interplay between the renaissance of general relativity and the advent of
relativistic astrophysics following the German early involvement in
gravitational-wave research, to the point when gravitational-wave detection
became established by the appearance of full-scale detectors and international
collaborations. On the background of the spectacular astrophysical discoveries
of the 1960s and the growing role of relativistic astrophysics, Ludwig Biermann
and his collaborators at the Max Planck Institute for Astrophysics in Munich
became deeply involved in research related to such new horizons. At the end of
the 1960s, Joseph Weber's announcements claiming detection of gravitational
waves sparked the decisive entry of this group into the field, in parallel with
the appointment of the renowned relativist Juergen Ehlers. The Munich area
group of Max Planck institutes provided the fertile ground for acquiring a
leading position in the 1970s, facilitating the experimental transition from
resonant bars towards laser interferometry and its innovation at increasingly
large scales, eventually moving to a dedicated site in Hannover in the early
1990s. The Hannover group emphasized perfecting experimental systems at pilot
scales, and never developed a full-sized detector, rather joining the LIGO
Scientific Collaboration at the end of the century. In parallel, the Max Planck
Institute for Gravitational Physics (Albert Einstein Institute) had been
founded in Potsdam, and both sites, in Hannover and Potsdam, became a unified
entity in the early 2000s and were central contributors to the first detection
of gravitational waves in 2015.Comment: 94 pages. Enlarged version including new results from further
archival research. A previous version appears as a chapter in the volume The
Renaissance of General Relativity in Context, edited by A. Blum, R. Lalli and
J. Renn (Boston: Birkhauser, 2020
Mechanics and Equilibrium Geometry of Black Holes, Membranes, and Strings
This course is designed to give a mathematically coherent introduction to the
classical thory of black holes and also of strings and membranes (which are
like the horizon of a black hole in being examples of physical systems based on
a dynamically evolving world sheet) giving particular attention given to the
study of the geometry of their equilibrium states.Comment: 93 page latex file (with typo corrections and redrafted C.P.
diagrams) of contribution to BLACK HOLE PHYSICS (NATO ASI C364) ed. V. de
Sabbata, Z. Zhang (Kluwer, Dordrecht, 1992) 283-35
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