262 research outputs found
An Environmental Variation of Constants
Models of modified gravity, whereby local tests of gravity are evaded thanks
to a screening mechanism of the chameleon or Damour-Polyakov types, lead to a
spatial variation of the particle masses and the fine structure constant. This
is triggered by the environmental dependence of the value of the scalar field
whose presence modifies gravity. In dense media, the field settles at a density
dependent value while in sparse environments it takes the background
cosmological value. We estimate that the maximal deviation of constants from
their present values is constrained by local tests of gravity, and must be less
than .Comment: 14 pages, 7 figures, published versio
Screening fifth forces in k-essence and DBI models
New fifth forces have not yet been detected in the laboratory or in the solar
system, hence it is typically difficult to introduce new light scalar fields
that would mediate such forces. In recent years it has been shown that a number
of non-linear scalar field theories allow for a dynamical mechanism, such as
the Vainshtein and chameleon ones, that suppresses the strength of the scalar
fifth force in experimental environments. This is known as screening, however
it is unclear how common screening is within non-linear scalar field theories.
k-essence models are commonly studied examples of non-linear models, with DBI
as the best motivated example, and so we ask whether these non-linearities are
able to screen a scalar fifth force. We find that a Vainshtein-like screening
mechanism exists for such models although with limited applicability. For
instance, we cannot find a screening mechanism for DBI models. On the other
hand, we construct a large class of k-essence models which lead to the
acceleration of the Universe in the recent past for which the fifth force
mediated by the scalar can be screened.Comment: 26 page
The effective field theory of K-mouflage
We describe K-mouflage models of modified gravity using the effective field
theory of dark energy. We show how the Lagrangian density defining the
K-mouflage models appears in the effective field theory framework, at both the
exact fully nonlinear level and at the quadratic order of the effective action.
We find that K-mouflage scenarios only generate the operator at each order . We also reverse engineer K-mouflage models
by reconstructing the whole effective field theory, and the full cosmological
behaviour, from two functions of the Jordan-frame scale factor in a tomographic
manner. This parameterisation is directly related to the implementation of the
K-mouflage screening mechanism: screening occurs when is large in a dense
environment such as the deep matter and radiation eras. In this way, K-mouflage
can be easily implemented as a calculable subclass of models described by the
effective field theory of dark energy which could be probed by future surveys.Comment: 36 pages, 8 figure
Gauge/Cosmology Brane-to-Brane Duality
We introduce a duality relation between two distinct branes, a cosmological
brane with macroscopic matter and a holographic brane with microscopic gauge
fields. Using brane-world cosmology with a single brane in a 5-dimensional AdS5
background, we find an explicit time-dependent holographic correspondence
between the bulk metric surrounding the cosmological brane and the N=4 gauge
field theory living on the boundary of the Z2-symmetric mirror bulk, identified
with the holographic brane. We then relate the cosmic acceleration on the
cosmological brane to the conformal anomaly of the gauge theory on the
holographic brane. This leads to a dual microscopic interpretation of the
number of e-foldings of the cosmological eras on the cosmological brane.Comment: 21 pages, 1 figur
Chameleon Fragmentation
A scalar field dark energy candidate could couple to ordinary matter and
photons, enabling its detection in laboratory experiments. Here we study the
quantum properties of the chameleon field, one such dark energy candidate, in
an "afterglow" experiment designed to produce, trap, and detect chameleon
particles. In particular, we investigate the possible fragmentation of a beam
of chameleon particles into multiple particle states due to the highly
non-linear interaction terms in the chameleon Lagrangian. Fragmentation could
weaken the constraints of an afterglow experiment by reducing the energy of the
regenerated photons, but this energy reduction also provides a unique signature
which could be detected by a properly-designed experiment. We show that
constraints from the CHASE experiment are essentially unaffected by
fragmentation for and potentials, but are weakened for
steeper potentials, and we discuss possible future afterglow experiments.Comment: 27 pages, 7 figure
K-mouflage Cosmology: the Background Evolution
We study the cosmology of K-mouflage theories at the background level. We
show that the effects of the scalar field are suppressed at high matter density
in the early Universe and only play a role in the late time Universe where the
deviations of the Hubble rate from its -CDM counterpart can be of the
order five percent for redshifts . Similarly, we find
that the equation of state can cross the phantom divide in the recent past and
even diverge when the effective scalar energy density goes negative and
subdominant compared to matter, preserving the positivity of the squared Hubble
rate. These features are present in models for which Big Bang Nucleosynthesis
is not affected. We analyze the fate of K-mouflage when the nonlinear kinetic
terms give rise to ghosts, particle excitations with negative energy. In this
case, we find that the K-mouflage theories can only be considered as an
effective description of the Universe at low energy below keV. In the safe
ghost-free models, we find that the equation of state always diverges in the
past and changes significantly by a few percent since .Comment: 18 page
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