3,471 research outputs found
Derivative Chameleons
We consider generalized chameleon models where the conformal coupling between
matter and gravitational geometries is not only a function of the chameleon
field \phi, but also of its derivatives via higher order co-ordinate
invariants. Specifically we consider the first such non-trivial conformal
factor A(\phi,X), where X is the canonical kinetic term for \phi. The
associated phenomenology is investigated and we show that such theories have a
new generic mass-altering mechanism, potentially assisting the generation of a
sufficiently large chameleon mass in dense environments. The most general
effective potential is derived for such derivative chameleon setups and
explicit examples are given. Interestingly this points us to the existence of a
purely derivative chameleon protected by a shift symmetry for \phi. We also
discuss potential ghost-like instabilities associated with mass-lifting
mechanisms and find another, mass-lowering and instability-free, branch of
solutions. This suggests that, barring fine-tuning, stable derivative models
are in fact typically anti-chameleons that suppress the field's mass in dense
environments. Furthermore we investigate modifications to the thin-shell regime
and prove a no-go theorem for chameleon effects in non-conformal geometries of
the disformal type.Comment: 28 pages, 4 figure
On Consistent Kinetic and Derivative Interactions for Gravitons
The only known fully ghost-free and consistent Lorentz-invariant kinetic term
for a graviton (or indeed for any spin-2 field) is the Einstein-Hilbert term.
Here we propose and investigate a new family of candidate kinetic interactions
and their extensions to derivative interactions involving several spin-2
fields. These new terms generically break diffeomorphism invariance(s) and as a
result can lead to the propagation of 5 degrees of freedom for a single spin-2
field - analogous to ghost-free Massive Gravity. We discuss under what
circumstances these new terms can be used to build healthy effective field
theories and in the process establish the `Jordan' and `Einstein' frame
pictures for Massive-, Bi- and Multi-Gravity.Comment: 24 pages, 8 figure
Cosmological parameter constraints for Horndeski scalar-tensor gravity
We present new cosmological parameter constraints for general Horndeski
scalar-tensor theories, using CMB, redshift space distortion, matter power
spectrum and BAO measurements from the Planck, SDSS/BOSS and 6dF surveys. We
focus on theories with cosmological gravitational waves propagating at the
speed of light, , implementing and discussing several
previously unaccounted for aspects in the constraint derivation for such
theories, that qualitatively affect the resulting constraints. In order to
ensure our conclusions are robust, we compare results for three different
parametrisations of the free functions in Horndeski scalar-tensor theories,
identifying several parametrisation-independent features of the constraints. We
also consider models, where in cosmological settings (still
allowed after GW170817 for frequency-dependent ) and show how this
affects cosmological parameter constraints.Comment: 30 pages, 9 figures, 3 table
Primordial fluctuations without scalar fields
We revisit the question of whether fluctuations in hydrodynamical,
adiabatical matter could explain the observed structures in our Universe. We
consider matter with variable equation of state w=p_0/\ep_0 and a concomitant
(under the adiabatic assumption) density dependent speed of sound, . We
find a limited range of possibilities for a set up when modes start inside the
Hubble radius, then leaving it and freezing out. For expanding Universes,
power-law w(\ep_0) models are ruled out (except when ,
requiring post-stretching the seeded fluctuations); but sharper profiles in
do solve the horizon problem. Among these, a phase transition in is
notable for leading to scale-invariant fluctuations if the initial conditions
are thermal. For contracting Universes all power-law w(\ep_0) solve the
horizon problem, but only one leads to scale-invariance: w\propto \ep_0^2 and
c_s\propto \ep_0. This model bypasses a number of problems with single scalar
field cyclic models (for which is large but constant)
Non-Gaussianity in single field models without slow-roll
We investigate non-Gaussianity in general single field models without
assuming slow-roll conditions or the exact scale-invariance of the scalar power
spectrum. The models considered include general single field inflation (e.g.
DBI and canonical inflation) as well as bimetric models. We compute the full
non-Gaussian amplitude, its size fnl, its shape, and the running with scale
n_{NG}. In doing so we show that observational constraints allow significant
violations of slow roll conditions and we derive explicit bounds on slow-roll
parameters for fast-roll single field scenarios. A variety of new observational
signatures is found for models respecting these bounds. We also explicitly
construct concrete model implementations giving rise to this new phenomenology.Comment: 28 pages + appendices, 6 figures, References and minor comments added
in revised versio
New massive bigravity cosmologies with double matter coupling
We study a previously largely unexplored branch of homogeneous and isotropic
background solutions in ghost-free massive bigravity with consistent double
matter coupling. For a certain family of parameters we find `self-inflated'
FLRW cosmologies, i.e. solutions with an accelerated early-time period during
the radiation-dominated era. In addition, these solutions also display an
accelerated late-time period closely mimicking GR with a cosmological constant.
Interestingly, within this family, the particular case of
gives bouncing cosmologies, where there is an infinite contracting past, a
non-zero minimum value of the scale factor at the bounce, and an infinite
expanding future.Comment: Last version includes minor changes to text and reference
Radiative stability and observational constraints on dark energy and modified gravity
Radiative stability places strong constraints on general dark energy and
modified gravity theories. We consider Horndeski scalar-tensor theories with
luminally propagating gravitational waves (as extensively discussed in the wake
of GW170817) and show that generically there is a tension between obtaining
observable deviations from General Relativity (GR) in cosmology and the
requirement of radiative stability. Using this as a constraint, we discuss the
subsets of theories that are capable of yielding observable, radiatively stable
departures from GR. A key consequence are significantly tightened cosmological
parameter constraints on dark energy and modified gravity parameters, which we
explicitly compute using data from the Planck, SDSS/BOSS and 6dF surveys.Comment: 7 pages, 2 figure
The decoupling limit of Multi-Gravity: Multi-Galileons, Dualities and More
In this paper we investigate the decoupling limit of a particular class of
multi-gravity theories, i.e. of theories of interacting spin-2 fields. We
explicitly compute the interactions of helicity-0 modes in this limit, showing
that they take on the form of multi-Galileons and dual forms. In the process we
extend the recently discovered Galileon dualities, deriving a set of new
multi-Galileon dualities. These are also intrinsically connected to healthy,
but higher-derivative, multi-scalar field theories akin to `beyond Horndeski'
models.Comment: 41 pages, 2 figure
On scale-free extensions of massive (bi-)gravity
We discuss a scale-free model of bigravity, in which the mass parameter of
the standard bigravity potential is promoted to a dynamical scalar field. This
modification retains the ghost-free bigravity structure, in particular it
remains free of the Boulware-Deser ghost. We investigate the theory's
interaction structure, focusing on its consistent scaling limits and strong
coupling scales. Furthermore we explore the model's quadratic action, both
around generic background configurations and paying special attention to
cosmological backgrounds and to the associated background evolution. Finally we
consider the possibility of realizing a phase of late-time acceleration as well
as a quasi-de Sitter inflationary stage at early times, when the promoted "mass
scalar" becomes the inflaton.Comment: 36 pages; v2 clarifying comments added, references updated, results
unchange
Strong-coupling scales and the graph structure of multi-gravity theories
In this paper we consider how the strong-coupling scale, or perturbative
cutoff, in a multi-gravity theory depends upon the presence and structure of
interactions between the different fields. This can elegantly be rephrased in
terms of the size and structure of the `theory graph' which depicts the
interactions in a given theory. We show that the question can be answered in
terms of the properties of various graph-theoretical matrices, affording an
efficient way to estimate and place bounds on the strong-coupling scale of a
given theory. In light of this we also consider the problem of relating a given
theory graph to a discretised higher dimensional theory, a la dimensional
deconstruction.Comment: 23 pages, 7 figures; v2: additional references included, and minor
typos corrected; version published in JHE
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