185 research outputs found
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
Local conformal symmetry in non-Riemannian geometry and the origin of physical scales
We introduce an extension of the Standard Model and General Relativity built
upon the principle of local conformal invariance, which represents a
generalization of a previous work by Bars, Steinhardt and Turok. This is
naturally realized by adopting as a geometric framework a particular class of
non-Riemannian geometries, first studied by Weyl. The gravitational sector is
enriched by a scalar and a vector field. The latter has a geometric origin and
represents the novel feature of our approach. We argue that physical scales
could emerge from a theory with no dimensionful parameters, as a result of the
spontaneous breakdown of conformal and electroweak symmetries. We study the
dynamics of matter fields in this modified gravity theory and show that test
particles follow geodesics of the Levi-Civita connection, thus resolving an old
criticism raised by Einstein against Weyl's original proposal.Comment: 11 pages; v2: matches published version in EPJC; new title, includes
new sections on the coupling of matter fields to the extended gravitational
secto
A Non-singular Theory of Gravity
We present a geometrical gravitational theory which reduces to Einstein's
theory for weak gravitational potentials and which has a singularity-free
analog of the Schwarzschild metric.Comment: 8 pages, RevTeX, 3 figures included in compressed form, UTPT-94-0
Black holes in an ultraviolet complete quantum gravity
In this Letter we derive the gravity field equations by varying the action
for an ultraviolet complete quantum gravity. Then we consider the case of a
static source term and we determine an exact black hole solution. As a result
we find a regular spacetime geometry: in place of the conventional curvature
singularity extreme energy fluctuations of the gravitational field at small
length scales provide an effective cosmological constant in a region locally
described in terms of a de Sitter space. We show that the new metric coincides
with the noncommutative geometry inspired Schwarzschild black hole. Indeed, we
show that the ultraviolet complete quantum gravity, generated by ordinary
matter is the dual theory of ordinary Einstein gravity coupled to a
noncommutative smeared matter. In other words we obtain further insights about
that quantum gravity mechanism which improves Einstein gravity in the vicinity
of curvature singularities. This corroborates all the existing literature in
the physics and phenomenology of noncommutative black holes.Comment: 5 pages, 2 figures, minor corrections, version matching that
published by Physics Letters
Solving the Flatness and Quasi-flatness Problems in Brans-Dicke Cosmologies with a Varying Light Speed
We define the flatness and quasi-flatness problems in cosmological models. We
seek solutions to both problems in homogeneous and isotropic Brans-Dicke
cosmologies with varying speed of light. We formulate this theory and find
perturbative, non-perturbative, and asymptotic solutions using both numerical
and analytical methods. For a particular range of variations of the speed of
light the flatness problem can be solved. Under other conditions there exists a
late-time attractor with a constant value of \Omega that is smaller than, but
of order, unity. Thus these theories may solve the quasi-flatness problem, a
considerably more challenging problem than the flatness problem. We also
discuss the related \Lambda and quasi-\Lambda problem in these theories. We
conclude with an appraisal of the difficulties these theories may face.Comment: 21 pages, 6 figure
Cosmological Bounds on Spatial Variations of Physical Constants
We derive strong observational limits on any possible large-scale spatial
variation in the values of physical 'constants' whose space-time evolution is
driven by a scalar field. The limits are imposed by the isotropy of the
microwave background on large angular scales in theories which describe space
and time variations in the fine structure constant, the electron-proton mass
ratio, and the Newtonian gravitational constant, G. Large-scale spatial
fluctuations in the fine structure constant are bounded by 2x10^-9 and
1.2x10^-8 in the BSBM and VSL theories respectively, fluctuations in the
electron-proton mass ratio by 9x10^-5 in the BM theory and fluctuations in G by
3.6x10^-10 in Brans-Dicke theory. These derived bounds are significantly
stronger than any obtainable by direct observations of astrophysical objects at
the present time.Comment: 13 pages, 1 table, typos corrected, refs added. Published versio
Non-detection of previously reported transits of HD 97658b with MOST photometry
The radial velocity-discovered exoplanet HD 97658b was recently announced to
transit, with a derived planetary radius of 2.93 \pm 0.28 R_{Earth}. As a
transiting super-Earth orbiting a bright star, this planet would make an
attractive candidate for additional observations, including studies of its
atmospheric properties. We present and analyze follow-up photometric
observations of the HD 97658 system acquired with the MOST space telescope. Our
results show no transit with the depth and ephemeris reported in the
announcement paper. For the same ephemeris, we rule out transits for a planet
with radius larger than 2.09 R_{Earth}, corresponding to the reported 3\sigma
lower limit. We also report new radial velocity measurements which continue to
support the existence of an exoplanet with a period of 9.5 days, and obtain
improved orbital parameters.Comment: 16 pages, 4 figures; 1 Table; accepted for publication in ApJL,
includes changes made in response to the referee repor
Comments on "Note on varying speed of light theories"
In a recent note Ellis criticizes varying speed of light theories on the
grounds of a number of foundational issues. His reflections provide us with an
opportunity to clarify some fundamental matters pertaining to these theories
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