18 research outputs found
Black hole shadows in fourth-order conformal Weyl gravity
We calculate the characteristics of the "black hole shadow" for a rotating,
neutral black hole in fourth-order conformal Weyl gravity. It is shown that the
morphology is not significantly affected by the underlying framework, except
for very large masses. Conformal gravity black hole shadows would also
significantly differ from their general relativistic counterparts if the values
of the main conformal gravity parameters, and , were increased
by several orders of magnitude. Such increased values for and
are currently ruled out by gravitational phenomenology. Therefore, it is
unlikely that these differences in black hole shadows will be detected in
future observations, carried out by the Event Horizon Telescope or other such
experiments.Comment: 21 pages, including 2 figures. Minor corrections and references
added. Final version to appear in the Canadian Journal of Physic
Constraints On Vector Unparticle Physics From Cosmic Censorship
Vector unparticle couplings to standard model fields produce repulsive corrections to gravity. From a general relativistic perspective, this leads to an effective Reissner-Nordstrom-like metric whose charge is a function of the unparticle coupling constant λ, and therefore can admit naked singularities. Requiring the system to respect cosmic censorship provides a new method of constraining the value of λ. These limits are extremely loose for stellar-mass black holes, but commensurate with existing bounds for primordial black holes. In the case of theoretical low-mass black holes, the bounds on λ are much stricter than those derived from astrophysical and accelerator phenomenology. Additional constraints on the lower limit of λ are used to estimate the mass of the smallest possible black hole MminBH that can be formed in the unparticle framework, as a function of the unparticle parameters (LambdaU;MU; dU; dBZ)
Primordial Black Hole Evaporation And Spontaneous Dimensional Reduction
Several different approaches to quantum gravity suggest the effective dimension of spacetime reduces from four to two near the Planck scale. In light of such evidence, this Letter re-examines the thermodynamics of primordial black holes (PBHs) in specific lower-dimensional gravitational models. Unlike in four dimensions, (1 + 1)-D black holes radiate with power P ∼ M2BH, while it is known no (2+1)-D (BTZ) black holes can exist in a non-anti-de Sitter universe. This has important relevance to the PBH population size and distribution, and consequently on cosmological evolution scenarios. The number of PBHs that have evaporated to present day is estimated, assuming they account for all dark matter. Entropy conservation during dimensional transition imposes additional constraints. If the cosmological constant is non-negative, no black holes can exist in the (2 + 1)-dimensional epoch, and consequently a (1 + 1)-dimensional black hole will evolve to become a new type of remnant. Although these results are conjectural and likely model-dependent, they open new questions about the viability of PBHs as dark matter candidates
Gravitationally Induced Quantum Superposition Reduction With Large Extra Dimensions
A gravity-driven mechanism (``objective reduction\u27\u27) proposed to explain quantum state reduction is analyzed in light of the possible existence of large extra dimensions in the ADD scenario. By calculating order-of-magnitude estimates for nucleon superpositions, it is shown that if the mechanism at question is correct, constraints may be placed on the number and size of extra dimensions. Hence, measurement of superposition collapse times ({\it e.g.} through diffraction or reflection experiments) could represent a new probe of extra dimensions. The influence of a time-dependent gravitational constant on the gravity-driven collapse scheme with and without the presence of extra dimensions is also discussed
Black Hole Thermodynamics in MOdified Gravity (MOG)
We analyze the thermodynamical properties of black holes in a modified theory
of gravity, which was initially proposed to obtain correct dynamics of galaxies
and galaxy clusters without dark matter. The thermodynamics of non-rotating and
rotating black hole solutions resembles similar solutions in Einstein-Maxwell
theory with the electric charge being replaced by a new mass dependent
gravitational charge . This new mass dependent charge
modifies the effective Newtonian constant from to ,
and this in turn critically affects the thermodynamics of the black holes. We
also investigate the thermodynamics of regular solutions, and explore the
limiting case when no horizons forms. So, it is possible that the modified
gravity can lead to the absence of black hole horizons in our universe.
Finally, we analyze corrections to the thermodynamics of a non-rotating black
hole and obtain the usual logarithmic correction term.Comment: Title changed slightly; new section on BH entropy corrections added;
matches version published in PL
Does Entropic Gravity Bound The Masses Of The Photon And Graviton?
If the information transfer between test particle and holographic screen in entropic gravity respects both the uncertainty principle and causality, a lower limit on the number of bits in the universe relative to its mass may be derived. Furthermore, these limits indicate particles that putatively travel at the speed of light -- the photon and/or graviton -- have a non-zero mass m≥10−68 kg. This result is found to be in excellent agreement with current experimental mass bounds on the graviton and photon, suggesting that entropic gravity may be the result of a (recent) softly-broken local symmetry. Stronger bounds emerge from consideration of ultradense matter such as neutron stars, yielding limits of m≥10−48−10−50 kg, barely within the experimental photon range and outside that of the graviton. We find that for black holes these criteria cannot be satisfied, and suggest some possible implications of this result
(1+1)-Dimensional Entropic Gravity
We consider the formulation of entropic gravity in two spacetime dimensions. The usual gravitational force law is derived even in the absence of area, as normally required by the holographic principle. A special feature of this perspective concerns the nature of temperature and entropy defined at a point. We argue that the constancy of the gravitational force in one spatial dimension implies the information contained at each point in space is an internal degree of freedom on the manifold, and furthermore is a universal constant, contrary to previous assertions that entropic gravity in one spatial dimension is ill-defined. We give some heuristic arguments for gravitation and information transfer constraints within this framework, thus adding weight to the contention that spacetime and gravitation might be emergent phenomena
Aspects of noncommutative (1+1)-dimensional black holes
We present a comprehensive analysis of the spacetime structure and
thermodynamics of dimensional black holes in a noncommutative
framework. It is shown that a wider variety of solutions are possible than the
commutative case considered previously in the literature. As expected, the
introduction of a minimal length cures singularity pathologies
that plague the standard two-dimensional general relativistic case, where the
latter solution is recovered at large length scales. Depending on the choice of
input parameters (black hole mass , cosmological constant ,
etc...), black hole solutions with zero, up to six, horizons are possible. The
associated thermodynamics allows for the either complete evaporation, or the
production of black hole remnants.Comment: 24 pages, 12 figures, some comments added, conclusions not modified,
version matching that published on PR