4,907 research outputs found
Recent developments in bimetric theory
This review is dedicated to recent progress in the field of classical,
interacting, massive spin-2 theories, with a focus on ghost-free bimetric
theory. We will outline its history and its development as a nontrivial
extension and generalisation of nonlinear massive gravity. We present a
detailed discussion of the consistency proofs of both theories, before we
review Einstein solutions to the bimetric equations of motion in vacuum as well
as the resulting mass spectrum. We introduce couplings to matter and then
discuss the general relativity and massive gravity limits of bimetric theory,
which correspond to decoupling the massive or the massless spin-2 field from
the matter sector, respectively. More general classical solutions are reviewed
and the present status of bimetric cosmology is summarised. An interesting
corner in the bimetric parameter space which could potentially give rise to a
nonlinear theory for partially massless spin-2 fields is also discussed.
Relations to higher-curvature theories of gravity are explained and finally we
give an overview of possible extensions of the theory and review its
formulation in terms of vielbeins.Comment: 124 pages, 4 figures; minor changes, corrected typos; matches
published versio
Particular Solutions in Bimetric Theory and Their Implications
Ghost-free bimetric theory can describe gravity in the presence of an extra
spin-2 field. We study certain aspects of dynamics in this theory: (1) It is
shown that if either of the metrics is an Einstein solution then the other is
always forced to be Einstein, too. For a class of bimetric models this
constraint is stronger and as soon as one metric is Einstein, the other metric
is forced to be proportional to it. As a consequence, the models in this class
avoid a branch of pathological solutions that exhibit determinant singularities
or nonlinear ghosts. These constraints persists in a generalized form when
sources are included, but are destroyed in the massive gravity limit of the
theory. (2) For another class of bimetric models, we show the existence of
solutions that do not admit a massive gravity limit. A bimetric model that
could exhibit a nonlinear version of "partially massless" symmetry belongs to
both these classes. It is argued that if such a model exits, its symmetry will
not survive in the massive gravity limit.Comment: Latex, 18 pages. Published versio
Extended Weyl Invariance in a Bimetric Model and Partial Masslessness
We revisit a particular ghost-free bimetric model which is related to both
partial masslessness (PM) and conformal gravity. Linearly, the model propagates
six instead of seven degrees of freedom not only around de Sitter but also
around flat spacetime. Nonlinearly, the equations of motion can be recast in
the form of expansions in powers of curvatures, and exhibit a remarkable amount
of structure. In this form, the equations are shown to be invariant under
scalar gauge transformations, at least up to six orders in derivatives, the
lowest order term being a Weyl scaling of the metrics. The terms at
two-derivative order reproduce the usual PM gauge transformations on de Sitter
backgrounds. At the four-derivative order, a potential obstruction that could
destroy the symmetry is shown to vanish. This in turn guarantees the gauge
invariance to at least six-orders in derivatives. This is equivalent to adding
up to 10-derivative corrections to conformal gravity. More generally, we
outline a procedure for constructing the gauge transformations order by order
as an expansion in derivatives and comment on the validity and limitations of
the procedure. We also discuss recent arguments against the existence of a PM
gauge symmetry in bimetric theory and show that, at least in their present
form, they are evaded by the model considered here. Finally, we argue that a
bimetric approach to PM theory is more promising than one based on the
existence of a fundamental PM field.Comment: Latex, 35 pages. Matches published versio
Linear spin-2 fields in most general backgrounds
We derive the full perturbative equations of motion for the most general
background solutions in ghost-free bimetric theory in its metric formulation.
Clever field redefinitions at the level of fluctuations enable us to circumvent
the problem of varying a square-root matrix appearing in the theory. This
greatly simplifies the expressions for the linear variation of the bimetric
interaction terms. We show that these field redefinitions exist and are
uniquely invertible if and only if the variation of the square-root matrix
itself has a unique solution, which is a requirement for the linearised theory
to be well-defined. As an application of our results we examine the constraint
structure of ghost-free bimetric theory at the level of linear equations of
motion for the first time. We identify a scalar combination of equations which
is responsible for the absence of the Boulware-Deser ghost mode in the theory.
The bimetric scalar constraint is in general not manifestly covariant in its
nature. However, in the massive gravity limit the constraint assumes a
covariant form when one of the interaction parameters is set to zero. For that
case our analysis provides an alternative and almost trivial proof of the
absence of the Boulware-Deser ghost. Our findings generalise previous results
in the metric formulation of massive gravity and also agree with studies of its
vielbein version.Comment: Latex, 20 pages. Matches published versio
New Species of Marsupial Frog (Hemiphractidae: \u3ci\u3eGastrotheca\u3c/i\u3e) from an Isolated Montane Forest in Southern Peru
We describe a new species of marsupial frog (genus Gastrotheca) from an isolated patch of cloud forest in the upper reaches of the Pachachaca River, a tributary of the Apurimac River in southern Peru (Apurimac Region). The new species is small with males less than 30 mm and a single female 35.3 mm in snout–vent length. A juvenile was collected from inside an epiphytic bromeliad at ~3 m, suggesting that this species is arboreal or uses bromeliad tanks as microhabitats. We describe the advertisement call, which has a dominant frequency above 2.2 kHz. Based on morphology and advertisement call, the new species is most similar to Gastrotheca piperata described from the Yungas of Bolivia. The new species differs from G. piperata by having smaller size, no dermal fold on the tarsus, a bluntly rounded to vertical snout in profile, gray coloration on dorsal surfaces of Fingers I and II and by producing advertisement calls that have a higher dominant frequency and a smaller number of pulses
Heavy spin-2 Dark Matter
We provide further details on a recent proposal addressing the nature of the
dark sectors in cosmology and demonstrate that all current observations related
to Dark Matter can be explained by the presence of a heavy spin-2 particle.
Massive spin-2 fields and their gravitational interactions are uniquely
described by ghost-free bimetric theory, which is a minimal and natural
extension of General Relativity. In this setup, the largeness of the physical
Planck mass is naturally related to extremely weak couplings of the heavy
spin-2 field to baryonic matter and therefore explains the absence of signals
in experiments dedicated to Dark Matter searches. It also ensures the
phenomenological viability of our model as we confirm by comparing it with
cosmological and local tests of gravity. At the same time, the spin-2 field
possesses standard gravitational interactions and it decays universally into
all Standard Model fields but not into massless gravitons. Matching the
measured DM abundance together with the requirement of stability constrains the
spin-2 mass to be in the 1 to 100 TeV range.Comment: Latex, 46 pages, 4 figures. Matches published versio
Gravitational Origin of Dark Matter
Observational evidence for the existence of Dark Matter is limited to its
gravitational effects. The extensive program for dedicated searches has yielded
null results so far, challenging the most popular models. Here we propose that
this is the case because the very existence of cold Dark Matter is a
manifestation of gravity itself. The consistent bimetric theory of gravity, the
only known ghost-free extension of General Relativity involving a massless and
a massive spin-2 field, automatically contains a perfect Dark Matter candidate.
We demonstrate that the massive spin-2 particle can be heavy, stable on
cosmological scales, and that it interacts with matter only through a
gravitational type of coupling. Remarkably, these features persist in the same
region of parameter space where bimetric theory satisfies the current gravity
tests. We show that the observed Dark Matter abundance can be generated via
freeze-in and suggest possible particle physics and gravitational signatures of
our bimetric Dark Matter model.Comment: Latex, 5 page
A new species of Noblella (Amphibia, Anura, Craugastoridae) from the humid montane forests of Cusco, Peru.
A new species of Noblella is described from the humid montane forest of the Región Cusco in Peru. Specimens were collected at 2330-2370 m elevation in Madre Selva, near Santa Ana, in the province of La Convención. The new species is readily distinguished from all other species of Noblella by having a broad, irregularly shaped, white mark on black background on chest and belly. The new species further differs from known Peruvian species of Noblella by the combination of the following characters: tympanic membrane absent, small tubercles on the upper eyelid and on dorsum, tarsal tubercles or folds absent, tips of digits not expanded, no circumferential grooves on digits, dark brown facial mask and lateral band extending from the tip of the snout to the inguinal region. The new species has a snout-to-vent length of 15.6 mm in one adult male and 17.6 mm in one adult female. Like other recently described species in the genus, this new Noblella inhabits high-elevation forests in the Andes and likely has a restricted geographic distribution
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