147 research outputs found

### Conformal classes of Brans-Dicke gravity

A classification of Brans-Dicke theories of gravitation, based on the
behaviour of the dimensionless gravitational coupling constant, is given. It is
noted that the discussion takes place in the current literature, about which of
the two distinguished conformal frames in which scalar-tensor theories of
gravity can be formulated: the Jordan frame and the Einstein frame, is the
physical one, may, in most cases, be meaningless for both frames may belong to
the same conformal class. It is also noted that the Jordan frame formulation of
Brans-Dicke gravity with ordinary matter nonminimally coupled, that is shown to
be just the Jordan frame formulation of general relativity, is scale-invariant,
unlike the situation with the Jordan frame representation of Brans-Dicke
gravity with matter minimally coupled (the original formulation of Brans-Dicke
theory), where the presence of nonzero mass ordinary matter breaks the
scale-invariance of the theory.Comment: 3 pages, LaTeX, a crucial error on violation of the WEP was
corrected, minor changes adde

### On the physical consequences of a Weyl invariant theory of gravity

In this paper we explore the physical consequences of assuming Weyl
invariance of the laws of gravity from the classical standpoint exclusively.
Actual Weyl invariance requires to replace the underlying Riemannian
geometrical structure of the background spacetimes by Weyl integrable geometry
(WIG). We show that gauge freedom, a distinctive feature of Weyl invariant
theories of gravity, leads to very unusual consequences. For instance, within
the cosmological setting in a WIG-based conformal invariant gravity theory,
also known as conformal general relativity (CGR), a static universe is
physically equivalent to a universe undergoing de Sitter expansion. It happens
also that spherically symmetric black holes are physically equivalent to
singularity-free wormholes. Another outstanding consequence of gauge freedom in
the framework of CGR is that inflation is not required to explain the flatness,
horizon and relict particle abundances, among other puzzles that arise in
standard GR-based cosmology. Besides, the cosmological constant and the mass
hierarchy problems do not arise neither in this setup.Comment: 31 pages, 2 figures. After a 6-years period draft rewritten to a
large extent. Aims and scope slightly modified, title changed. Improved
physical discussion including the physical implications of gauge freedom.
Figures added for better understanding. arXiv admin note: text overlap with
arXiv:1811.0245

### Symmetry relating Gravity with Antigravity: A possible resolution of the Cosmological Constant Problem?

I discuss possible implications a symmetry relating gravity with antigravity
might have for smoothing out of the cosmological constant puzzle. For this
purpose, a very simple model with spontaneous symmetry breaking is explored,
that is based on Einstein-Hilbert gravity with two self-interacting scalar
fields. The second (exotic) scalar particle with negative energy density, could
be interpreted, alternatively, as an antigravitating particle with positive
energy.Comment: 3 pages, no figures, revte

### Scale invariance and broken electroweak symmetry may coexist together

Here we show that local scale invariance -- invariance under Weyl rescalings
-- may safely coexist with broken electroweak symmetry if assume the Weyl
geometric theory to govern the affine structure of spacetime. We find that
within the resulting scale invariant theory of gravity cosmological inflation
is enough to explain the large hierarchy between the Higgs and Planck masses.Comment: 5 pages, no figures. Sustantial improvements: (i) the discussion on
generation of mass hierarchy complemented with necessary statements about the
measurable quantities in Weyl spaces, and (ii) unnecessary discussion on the
coosmological constant, not relevant to the main focus of the paper remove

### Spontaneous orbifold symmetry breaking and generation of mass hierarchy

A very simple mechanism is proposed that stabilizes the orbifold geometry
within the context of the Randall-Sundrum proposal for solving the hierarchy
problem. The electroweak TeV scale is generated from the Planck scale by
spontaneous breaking of the orbifold symmetryComment: 10 pages, Revtex, no figure

### Is the cosmological singularity really unavoidable in general relativity?

The initial singularity problem in standard general relativity is treated on
the light of a viewpoint asserting that this formulation of Einstein's theory
and its conformal formulations are physically equivalent. We show that flat
Friedmann-Robertson-Walker universes and open dust-filled and radiation-filled
universes are singularity free when described in terms of the formulation of
general relativity conformal to the canonical one.Comment: this paper has been withdrawn by the author because it significantly
overlaps with gr-qc/000207

### Time-like vs Space-like Extra Dimensions

Higher-dimensional theories with time-like and space-like extra dimensions
are compared both from the conceptual and from the phenomenological points of
view. In this context causality and unitarity are discussed. It is shown that
additional time-like dimensions allow to recover four-dimensional phenomenology
without invoking neither Kaluza-Klein compactification procedure nor brane
world construct. It is shown, also, that contrary to scenarios with space-like
extra dimensions, in higher-dimensional space-times with additional time-like
dimensions the cosmological constant problem can be safely solved.Comment: 5 pages, no figure

### Scale invariance: fake appearances

In this paper we shall show that, unless the affine geometrical structure of
the underlying spacetime manifold is specified, there is an ambiguity in the
understanding of the scale invariance -- also Weyl invariance -- of the given
theory of gravity. In this regard we cast doubts on several well-known theories
which are claimed to be scale-invariant setups. We shall show that in a
consistent scale invariant theory not only the action but also the geometrical
laws which govern the affine structure of the underlying spacetimes are
invariant under the Weyl gauge transformations.Comment: 12 pages, no figures. An important clarifying footnote and
bibliographic references added. Discussion on the singularity issue improved.
arXiv admin note: text overlap with arXiv:1401.264

### Scale Invariant Kaluza-Klein Theory and the Fate of the Gravitational Collapse

Pushing forward the similitudes between the gravitational collapse and the
expansion of the universe (in the reversed sense of time), it should be
expected that, during the collapse, eventually, a spacetime domain would be
reached where attained energy scales are very high. In consequence some of the
compactified extra dimensions may be decompactified and some presently broken
symmetries may be restored. A more fundamental theory (of which Einstein's
theory is a symmetry broken phase) is then expected to take account of further
description of the collapse. I propose a simple (classical) model for the
description of the late stages of the gravitational collapse: A non-Riemannian,
scale-invariant version of 5-dimensional Kaluza-Klein theory in which the
standard Riemann structure of the higher-dimensional manifold is replaced by a
Weyl-integrable one. A class of solutions, that generalize the "soliton" one by
Gross and Perry and Davidson and Owen, is found. This class contains both naked
singularities and wormhole solutions. On physical grounds it is argued that a
wormhole is the most reasonable destiny of the gravitational collapse.Comment: 10 pages, latex, no figure

### On a symmetry relating gravity with antigravity

I investigate the impact of a "would be" fundamental symmetry of the laws of
nature under the interchange of gravity and antigravity, on the understanding
of negative energies in general relativity. For this purpose a toy model that
is based on Einstein-Hilbert gravity with two minimally coupled
self-interacting scalar fields is explored, where the second (exotic) scalar
field with negative energy density may be regarded, alternatively, as an
antigravitating field with positive energy. Spontaneous breakdown of reflection
symmetry is then considered in order to discuss the implications the proposed
"would be" fundamental symmetry might have for the vanishing of the
cosmological constant. A possible connection of the gravity-antigravity
symmetry with the so called quintom field is also explored.Comment: 9 pages, no figures. This is a sequel to the earlier paper
arXiv:gr-qc/0411064. A section on the normalized gravitational charge added.
Physical discussion enriche

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