1,720,969 research outputs found
Application of ODE techniques to spherical gravitational collapse: Methods and results
No full textThe final state of spherical gravitational collapse can be analyzed applying to the geodesic equations governing the behavior of light rays near the singularity relatively simple but powerful techniques of nonlinear ordinary differential equations. In this way, explicit use of exact solutions of Einstein’s field equations is not necessary, and results can be obtained for wide equations of state of the collapsing matter field
On the relative category in the brake orbits problem
Full text linkIn this paper we show how the notion of the Lusternik–Schni- relmann relative category can be used to study a multiplicity problem for brake orbits in a potential well which is homeomorphic to the N-dimen- sional unit disk. The estimate of the relative category of the set of chords with endpoints on the (N − 1)-unit sphere was shown to the third author by Fadell and Husseini while he was visiting the University of Wisconsin at Madison
Red and blue shift in spherical and axisymmetric spacetimes and astrophysical constraints
Full text linkWe compute the red and blue shifts for astrophysical and cosmological sources. In particular, we consider low, intermediate and high gravitational energy domains. Thereby, we handle the binary system Earth-Mars as low energy landscape whereas white dwarfs and neutron stars as higher energy sources. To this end, we take into account a spherical Schwarzschild-de Sitter spacetime and an axially symmetric Zipoy-Voorhees metric to model all the aforementioned systems. Feasible outcomes come from modeling neutron stars and white dwarfs with the Zipoy-Voorhees metric, where quadrupole effects are relevant, and framing solar system objects using a Schwarzschild-de Sitter spacetime. In the first case, large delta parameters seem to be favorite, leading to acceptable bounds mainly for neutron stars. In the second case, we demonstrate incompatible red and blue shifts with respect to lunar and satellite laser ranging expectations, once the cosmological constant is taken to Planck satellite's best fit. To heal this issue, we suggest coarse-grained experimental setups and propose Phobos for working out satellite laser ranging in order to get more suitable red and blue shift intervals, possibly more compatible than current experimental bounds. Implications to cosmological tensions are also debated
Late time evolution of negatively curved FLRW models
Full text linkWe study the late time evolution of negatively curved Friedmann–Lemaître–Robertson–Walker (FLRW) models with a perfect fluid matter source and a scalar field nonminimally coupled to matter. Since under mild assumptions on the potential V, it is already known—see e.g., Giambò and Miritzis (Class Quantum Grav 27:095003, 2010)—that equilibria corresponding to nonnegative local minima for V are asymptotically stable, we classify all cases where one of the energy components eventually dominates. In particular for nondegenerate minima with zero critical value, we rigorously prove that if γ, the parameter of the equation of state is larger than 2/3, then there is a transfer of energy from the fluid and the scalar field to the energy density of the scalar curvature. Thus, the scalar curvature, if present, has a dominant effect on the late evolution of the universe and eventually dominates over both the perfect fluid and the scalar field. The analysis in complemented with the case where V is exponential, and therefore the scalar field diverges to infinity
Motion of test particles in quasi anti-de Sitter regular black holes
No full textIn this paper, we explore the characteristics of two novel regular spacetimes that exhibit a nonzero vacuum energy term, under the form of a (quasi) anti-de Sitter phase. Specifically, the first metric is spherical, while the second, derived by applying the generalized Newman-Janis algorithm to the first, is axisymmetric. We show that the equations of state of the effective fluids associated with the two metrics asymptotically tend to negative values, resembling quintessence. In addition, we study test particle motions, illustrating the main discrepancies among our models and more conventional metrics exhibiting non-vanishing anti-de Sitter phase
Global Models of Collapsing Scalar Field: Endstate
No full textThe study of dynamic singularity formation in spacetime, focusing on scalar field collapse models, is analyzed. We revisit key findings regarding open spatial topologies, concentrating on minimal conditions necessary for singularity and apparent horizon formation. Moreover, we examine the stability of initial data in the dynamical system governed by Einstein’s equations, considering variations in parameters that influence naked singularity formation. We illustrate how these results apply to a family of scalar field models, concluding with a discussion on the concept of genericity in singularity studies
De Sitter-like configurations with asymptotic quintessence environment
Full text linkWe examine a spherically-symmetric class of spacetimes carrying vacuum energy, while considering the influence of an external dark energy environ- ment represented by a non-dynamical quintessence field. Our investigation focuses on a specific set of solutions affected by this field, leading to distinct kinds of spacetime deformations, resulting in regular, singular, and wormhole solutions. We thoroughly discuss the underlying physics associated with each case and demonstrate that more complex deformations are prone to instability. Ultimately, we find that our results lead to an isotropic de Sitter-like solution that behaves as a quintessence fluid. To achieve this, we investigate the nature of the corresponding fluid, showing that it cannot provide the sound speed equal to a constant equation of state parameter near the center. Consequently, we reinterpret the fluid as a slow-roll quintessence by investigating its behavior in asymptotic regimes. Further, we explore the potential implications of violat- ing the isotropy condition on the pressures and we finally compare our findings with the de Sitter and Hayward solutions, highlighting both the advantages and
disadvantages of our scenarios
Repulsive regions in Lemaître–Tolman–Bondi gravitational collapse
Full text linkWe show that in the inhomogeneous Lemaître–Tolman–Bondi space–time there are specific regions in which repulsive gravity exists. To find these regions, we use an invariant definition of repulsive gravity based upon the behavior of the curvature eigenvalues. In addition, we analyze the effects of repulsive gravity on the dynamics of the gravitational collapse. In particular, we investigate the collapse in the case of the parabolic solution for the effective scale factor of the Lemaître–Tolman–Bondi metric, corresponding to the marginally bound case. Exploring the corresponding cut-offs at which gravity becomes repulsive, we notice that black holes with dominant repulsive effects are not excluded a priori. Indeed, we demonstrate that the collapse leads, in general, to the formation of a central naked singularity; however, for particular values of the free parameters entering the model, black holes with dominant repulsive gravity can exist. We show that the expected physical process is not modified as the marginally bound condition is dropped out. Moreover, we show that this is true independently of the hypothesis that the energy–momentum tensor is built up in terms of pressureless matter. Further, we demonstrate that geodesic deviations can depend on the sign of the curvature eigenvalues. Finally, we give an astrophysical interpretation of black holes with dominant repulsive gravity. Indeed, we argue that compact objects with dominant repulsive gravity could be interpreted as progenitors of Gamma Ray Bursts
DYNAMICS AND THERMODYNAMICS OF BLACKHOLES AND NAKED SINGULARITIES (Editors of the book)
No full textConference proceedings; electronically published at
www1.mate.polimi.it/b
Constraining primordial black holes as a fraction of dark matter through accretion disk luminosity
Full text linkIn this paper, we consider the hypothesis that fractions of dark matter could
be constituted by primordial black holes (PBHs). To test this possibility, we
work out the observational properties of a static black hole embedded in the
dark matter envelope made of a PBH source. The corresponding modifications of
geometry due to such a physical system are investigated, with a particular
focus on the accretion disk luminosity in spiral galaxies. The impact of the
PBH presence is analyzed through modification of the disk luminosity and
kinematic quantities. Thus, we discuss possible constraints on the PBH
abundance in view of the most recent theoretical bounds. The results of our
study indicate that suitable PBH masses are
for PBH fractions
. In particular, a comparison with the predictions
of the exponential sphere density profile for dark matter suggests that the
best-matching configuration is achieved for and
. Consequences with respect to the current knowledge
on primordial black hole physics are discussed.Comment: 10 pages, 6 figures, Accepted for publication in Phys. Rev.
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