12 research outputs found
Cosmic String Loop Microlensing
Cosmic superstring loops within the galaxy microlens background point sources
lying close to the observer-string line of sight. For suitable alignments,
multiple paths coexist and the (achromatic) flux enhancement is a factor of
two. We explore this unique type of lensing by numerically solving for
geodesics that extend from source to observer as they pass near an oscillating
string. We characterize the duration of the flux doubling and the scale of the
image splitting. We probe and confirm the existence of a variety of fundamental
effects predicted from previous analyses of the static infinite straight
string: the deficit angle, the Kaiser-Stebbins effect, and the scale of the
impact parameter required to produce microlensing. Our quantitative results for
dynamical loops vary by O(1) factors with respect to estimates based on
infinite straight strings for a given impact parameter. A number of new
features are identified in the computed microlensing solutions. Our results
suggest that optical microlensing can offer a new and potentially powerful
methodology for searches for superstring loop relics of the inflationary era.Comment: 20 pages, 19 figure
Gravitational Interactions in a General Multibrane Model
The gravitational interactions of the four-dimensional effective theory
describing a general -brane model in five dimensions without radion
stabilization are analyzed. Both uncompactified and orbifolded models are
considered. The parameter space is constrained by requiring that there be no
ghost modes in the theory, and that the Eddington parameterized post-Newtonian
parameter be consistent with observations. We show that we must reside
on the brane on which the warp factor is maximized. The resultant theory
contains radion modes in a nonlinear sigma model, with the target space
being a subset of hyperbolic space. Imposing observational constraints on the
relative strengths of gravitational interactions of dark and visible matter
shows that at least 99.8% of the dark matter must live on our brane in this
model.Comment: 18 pages, 4 figures. Version 2 (submitted to PRD) adds analysis on
orbifold
Shape dependence of Vainshtein screening
Scalar field theories that possess a Vainshtein mechanism are able to dynamically suppress the associated fifth forces in the presence of massive sources through derivative nonlinearities. The resulting equations of motion for the scalar are highly nonlinear, and therefore very few analytic solutions are known. Here, we present a brief investigation of the structure of Vainshtein screening in symmetrical configurations, focusing in particular on the spherical, cylindrical and planar solutions that are relevant for observations of the cosmic web. We consider Vainshtein screening in both the Galileon model, where the nonlinear terms involve second derivatives of the scalar, and a k-essence theory, where the nonlinear terms involve only first derivatives of the scalar. We find that screening, and consequently the suppression of the scalar force, is most efficient around spherical sources, weaker around cylindrical sources and can be absent altogether around planar sources
A Class of Effective Field Theory Models of Cosmic Acceleration
We explore a class of effective field theory models of cosmic acceleration
involving a metric and a single scalar field. These models can be obtained by
starting with a set of ultralight pseudo-Nambu-Goldstone bosons whose couplings
to matter satisfy the weak equivalence principle, assuming that one boson is
lighter than all the others, and integrating out the heavier fields. The result
is a quintessence model with matter coupling, together with a series of
correction terms in the action in a covariant derivative expansion, with
specific scalings for the coefficients. After eliminating higher derivative
terms and exploiting the field redefinition freedom, we show that the resulting
theory contains nine independent free functions of the scalar field when
truncated at four derivatives. This is in contrast to the four free functions
found in similar theories of single-field inflation, where matter is not
present. We discuss several different representations of the theory that can be
obtained using the field redefinition freedom. For perturbations to the
quintessence field today on subhorizon lengthscales larger than the Compton
wavelength of the heavy fields, the theory is weakly coupled and natural in the
sense of t'Hooft. The theory admits a regime where the perturbations become
modestly nonlinear, but very strong nonlinearities lie outside its domain of
validity.Comment: 43 pages, 2 figures; Version 3 publication versio
Simple implementation of general dark energy models
We present a formalism for the numerical implementation of general theories of dark energy, combining the computational simplicity of the equation of state for perturbations approach with the generality of the effective field theory approach. An effective fluid description is employed, based on a general action describing single-scalar field models. The formalism is developed from first principles, and constructed keeping the goal of a simple implementation into CAMB in mind. Benefits of this approach include its straightforward implementation, the generality of the underlying theory, the fact that the evolved variables are physical quantities, and that model-independent phenomenological descriptions may be straightforwardly investigated. We hope this formulation will provide a powerful tool for the comparison of theoretical models of dark energy with observational data