3,457 research outputs found
Dark Energy in vector-tensor theories of gravity
We consider a general class of vector-tensor theories of gravity and show
that solutions with accelerated expansion and a future type III singularity are
a common feature in these models. We also show that there are only six
vector-tensor theories with the same small scales behavior as General
Relativity and, in addition, only two of them can be made completely free from
instabilities. Finally, two particular models as candidates for dark energy are
proposed: on one hand, a cosmic vector that allows to alleviate the usual
naturalness and coincidence problems and, on the other hand, the
electromagnetic field is shown to give rise to an effective cosmological
constant on large scales whose value can be explained in terms of inflation at
the electroweak scale.Comment: 4 pages, 1 table. Contribution to the proceedings of Spanish
Relativity Meeting 2009, Bilbao, Spain, 7-11 September 200
Cosmic magnetic fields and dark energy in extended electromagnetism
We discuss an extended version of electromagnetism in which the usual gauge
fixing term is promoted into a physical contribution that introduces a new
scalar state in the theory. This new state can be generated from vacuum quantum
fluctuations during an inflationary era and, on super-Hubble scales, gives rise
to an effective cosmological constant. The value of such a cosmological
constant coincides with the one inferred from observations as long as inflation
took place at the electroweak scale. On the other hand, the new state also
generates an effective electric charge density on sub-Hubble scales that
produces both vorticity and magnetic fields with coherent lengths as large as
the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2010, Granada, Spain, 6-10 September 201
Diversity of economic landscapes and common land persistence in 19th century Spain: an interpretative proposal
The massive land privatization that took place over the 19th century deeply transformed the Spanish economic landscape. Nevertheless, the outcome of the process was quite different, both in pace and impact, depending on the geographic area we analyze. The explanation for this regional diversity in the persistence of common lands has been attributed to the institutional and environmental context, together with the level of market penetration that characterized the different rural societies. However, the important role that the commons themselves played in this process has been often overlooked. The aim of this paper is to complement those previous explanations by proposing a model that focuses on the collective land remaining, at any given moment, as a crucial explanatory variable and to provide an interpretative framework that would contribute to unveiling the complexity of a process that led to so many different outcomes.Common lands, privatisation, Spain, 19th century.
Metastability of non-reversible mean-field Potts model with three spins
We examine a non-reversible, mean-field Potts model with three spins on a set
with points. Without an external field, there are three
critical temperatures and five different metastable regimes. The analysis can
be extended by a perturbative argument to the case of small external fields. We
illustrate the case of large external fields with some phenomena which are not
present in the absence of external field.Comment: 34 pages, 12 figure
Infrared lessons for ultraviolet gravity: the case of massive gravity and Born-Infeld
We generalize the ultraviolet sector of gravitation via a Born-Infeld action
using lessons from massive gravity. The theory contains all of the elementary
symmetric polynomials and is treated in the Palatini formalism. We show how the
connection can be solved algebraically to be the Levi-Civita connection of an
effective metric. The non-linearity of the algebraic equations yields several
branches, one of which always reduces to General Relativity at low curvatures.
We explore in detail a {\it minimal} version of the theory, for which we study
solutions in the presence of a perfect fluid with special attention to the
cosmological evolution. In vacuum we recover Ricci-flat solutions, but also an
additional physical solution corresponding to an Einstein space. The existence
of two physical branches remains for non-vacuum solutions and, in addition, the
branch that connects to the Einstein space in vacuum is not very sensitive to
the specific value of the energy density. For the branch that connects to the
General Relativity limit we generically find three behaviours for the Hubble
function depending on the equation of state of the fluid, namely: either there
is a maximum value for the energy density that connects continuously with
vacuum, or the energy density can be arbitrarily large but the Hubble function
saturates and remains constant at high energy densities, or the energy density
is unbounded and the Hubble function grows faster than in General Relativity.
The second case is particularly interesting because it could offer an
interesting inflationary epoch even in the presence of a dust component.
Finally, we discuss the possibility of avoiding certain types of singularities
within the minimal model.Comment: 31 pages, 3 figures (Journal version, references added
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