2,684 research outputs found
Generalized multi-Proca fields
We extend previous results on healthy derivative self-interactions for a
Proca field to the case of a set of massive vector fields. We obtain non-gauge
invariant derivative self-interactions for the vector fields that maintain the
appropriate number of propagating degrees of freedom. In view of the potential
cosmological applications, we restrict to interactions with an internal
rotational symmetry. We provide a systematical construction order by order in
derivatives of the fields and making use of the antisymmetric Levi-Civita
tensor. We then compare with the one single vector field case and show that the
interactions can be broadly divided into two groups, namely the ones obtained
from a direct extension of the generalized Proca terms and genuine multi-Proca
interactions with no correspondence in the single Proca case. We also discuss
the curved spacetime version of the interactions to include the necessary
non-minimal couplings to gravity. Finally, we explore the cosmological
applications and show that there are three different vector fields
configurations giving rise to isotropic solutions. Two of them have already
been considered in the literature and the third one, representing a combination
of the first two, is new and offers unexplored cosmological scenarios.Comment: 13 page
Ghosts in metric-affine higher order curvature gravity
We disprove the widespread belief that higher order curvature theories of
gravity in the metric-affine formalism are generally ghost-free. This is
clarified by considering a sub-class of theories constructed only with the
Ricci tensor and showing that the non-projectively invariant sector propagates
ghost-like degrees of freedom. We also explain how these pathologies can be
avoided either by imposing a projective symmetry or additional constraints in
the gravity sector. Our results put forward that higher order curvature gravity
theories generally remain pathological in the metric-affine (and hybrid)
formalisms and highlight the key importance of the projective symmetry and/or
additional constraints for their physical viability and, by extension, of
general metric-affine theories.Comment: 7 page
Spacetimes with vector distortion: Inflation from generalised Weyl geometry
Spacetime with general linear vector distortion is introduced. Thus, the
torsion and the nonmetricity of the affine connection are assumed to be
proportional to a vector field (and not its derivatives). The resulting
two-parameter family of non-Riemannian geometries generalises the conformal
Weyl geometry and some other interesting special cases. Taking into account the
leading order quadratic curvature correction to the Einstein-Hilbert action
results uniquely in the one-parameter extension of the Starobinsky inflation
known as the alpha-attractor. The most general quadratic curvature action
introduces, in addition to the canonical vector kinetic term, novel ghost-free
vector-tensor interactions.Comment: 5 pages, 1 table, 0 figures. V2: Added the general quadratic theor
Limits on the anomalous speed of gravitational waves from binary pulsars
A large class of modified theories of gravity used as models for dark energy
predict a propagation speed for gravitational waves which can differ from the
speed of light. This difference of propagations speeds for photons and
gravitons has an impact in the emission of gravitational waves by binary
systems. Thus, we revisit the usual quadrupolar emission of binary system for
an arbitrary propagation speed of gravitational waves and obtain the
corresponding period decay formula. We then use timing data from the
Hulse-Taylor binary pulsar and obtain that the speed of gravitational waves can
only differ from the speed of light at the percentage level. This bound places
tight constraints on dark energy models featuring an anomalous propagations
speed for the gravitational waves.Comment: 6 pages, 1 figure, Prepared for the IWARA2016 proceeding
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
A cosmic vector for dark energy
In this work we show that the presence of a vector field on cosmological
scales could explain the present phase of accelerated expansion of the
universe. The proposed theory contains no dimensional parameters nor potential
terms and does not require unnatural initial conditions in the early universe,
thus avoiding the so called cosmic coincidence problem. In addition, it fits
the data from high-redshift supernovae with excellent precision, making
definite predictions for cosmological parameters. Upcoming observations will be
able to clearly discriminate this model from standard cosmology with
cosmological constant.Comment: 5 pages, 3 figures, 1 table. New comments and references included.
Final version to appear in Phys. Rev.
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
Cosmology with a Continuous Tower of Scalar Fields
We study the cosmological evolution for a universe in the presence of a
continuous tower of massive scalar fields which can drive the current phase of
accelerated expansion of the universe and, in addition, can contribute as a
dark matter component. The tower consists of a continuous set of massive scalar
fields with a gaussian mass distribution. We show that, in a certain region of
the parameter space, the {\it heavy} modes of the tower (those with masses much
larger than the Hubble expansion rate) dominate at early times and make the
tower behave like the usual single scalar field whose coherent oscillations
around the minimum of the potential give a matter-like contribution. On the
other hand, at late times, the {\it light} modes (those with masses much
smaller than the Hubble expansion rate) overcome the energy density of the
tower and they behave like a perfect fluid with equation of state ranging from
0 to -1, depending on the spectral index of the initial spectrum. This is a
distinctive feature of the tower with respect to the case of quintessence
fields, since a massive scalar field can only give acceleration with equation
of state close to -1. Such unique property is the result of a synergy effect
between the different mass modes. Interestingly, we find that, for some choices
of the spectral index, the tower tracks the matter component at high redshifts
(or it can even play the role of the dark matter) and eventually becomes the
dominant component of the universe and give rise to an accelerated expansion.Comment: 13 pages, 8 figures. V2: minor changes to match published versio
The canonical frame of purified gravity
In the recently introduced gauge theory of translations, dubbed Coincident
General Relativity, gravity is described with neither torsion nor curvature in
the spacetime affine geometry. The action of the theory enjoys an enhanced
symmetry and avoids the second derivatives that appear in the conventional
Einstein-Hilbert action. While it implies the equivalent classical dynamics,
the improved action principle can make a difference in considerations of
energetics, thermodynamics, and quantum theory. This essay reports on possible
progress in those three aspects of gravity theory. In the so-called purified
gravity, 1) energy-momentum is described locally by a conserved, symmetric
tensor, 2) the Euclidean path integral is convergent without the addition of
boundary or regulating terms and 3) it is possible to identify a canonical
frame for quantisation.Comment: 7 pages, no figure
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