7,404 research outputs found
Symmetron Fields: Screening Long-Range Forces Through Local Symmetry Restoration
We present a screening mechanism that allows a scalar field to mediate a long
range (~Mpc) force of gravitational strength in the cosmos while satisfying
local tests of gravity. The mechanism hinges on local symmetry restoration in
the presence of matter. In regions of sufficiently high matter density, the
field is drawn towards \phi = 0 where its coupling to matter vanishes and the
\phi-> -\phi symmetry is restored. In regions of low density, however, the
symmetry is spontaneously broken, and the field couples to matter with
gravitational strength. We predict deviations from general relativity in the
solar system that are within reach of next-generation experiments, as well as
astrophysically observable violations of the equivalence principle. The model
can be distinguished experimentally from Brans-Dicke gravity, chameleon
theories and brane-world modifications of gravity.Comment: 4 pages. v3: version appearing in PR
Systemic Risk: Simulating Local Shocks To A Global System
Using our updated model of the payment exchange system within the banking industry, we have introduced sudden local economic shocks and calculated their effect on the stability of the financial system. Our results suggest that the probability of a total banking failure, i.e., the systemic risk of the system, is insignificant unless the degree of the shock and the degree of integration between banks are very large. We find that the larger the shock, i.e., the greater the amount of loss amongst all banks, and the more isolated banks are within the payment system, the greater the likelihood of a localized or global banking system failure. However, given the current limits percentages of capitol banks can loan each other, only worldwide economic crises of cataclysmic significance would cause a collapse of the entire banking system. Hence we affirm the findings of our previous work which considered the effects of a bank failure generated by factors internal to the banking system (internal instead of internal shocks), which suggest there is minimal systemic risk in an integrated, minimally regulated, banking system.
No-Go Theorems for Generalized Chameleon Field Theories
The chameleon, or generalizations thereof, is a light scalar that couple to
matter with gravitational strength, but whose manifestation depends on the
ambient matter density. A key feature is that the screening mechanism
suppressing its effects in high-density environments is determined by the local
scalar field value. Under very general conditions, we prove two theorems
limiting its cosmological impact: i) the Compton wavelength of such a scalar
can be at most Mpc at present cosmic density, which restricts its impact to
non-linear scales; ii) the conformal factor relating Einstein- and Jordan-frame
scale factors is essentially constant over the last Hubble time, which
precludes the possibility of self-acceleration. These results imply that
chameleon-like scalar fields have a negligible effect on the linear-scale
growth history; theories that invoke a chameleon-like scalar to explain cosmic
acceleration rely on a form of dark energy rather than a genuine modified
gravity effect. Our analysis applies to a broad class of chameleon, symmetron
and dilaton theories.Comment: 4 page
Chameleon Dark Energy
Chameleons are scalar fields whose mass depends on the environment,
specifically on the ambient matter density. While nearly massless in the
cosmos, where the matter density is tiny, their mass is of order of an inverse
millimeter on Earth, where the density is high. In this note, we review how
chameleons can satisfy current experimental constraints on deviations from
General Relativity (GR). Moreover, we study the cosmological evolution with a
chameleon field and show the existence of an attractor solution, akin to the
tracker solution in quintessence models. We discuss how chameleons can
naturally drive the observed acceleration of the universeComment: 5 pages, 2 figures. To appear in the proceedings of the "Phi in the
Sky" conference, 8-10 July 2004, Porto, Portugua
Phase Control and Eclipse Avoidance in Near Rectilinear Halo Orbits
The baseline trajectory proposed for the Gateway is a southern Earth-Moon L2 Near Rectilinear Halo Orbit (NRHO). Designed to avoid eclipses, the NRHO exhibits a resonance with the lunar synodic period. The current investigation details the eclipse behavior in the baseline NRHO. Then, phase control is added to the orbit maintenance algorithm to regulate perilune passage time and maintain the eclipse-free characteristics of the Gateway reference orbit. A targeting strategy is designed to periodically target back to the long-horizon virtual reference if the orbit diverges over time in the presence of additional perturbations
Symmetron Cosmology
The symmetron is a scalar field associated with the dark sector whose
coupling to matter depends on the ambient matter density. The symmetron is
decoupled and screened in regions of high density, thereby satisfying local
constraints from tests of gravity, but couples with gravitational strength in
regions of low density, such as the cosmos. In this paper we derive the
cosmological expansion history in the presence of a symmetron field, tracking
the evolution through the inflationary, radiation- and matter-dominated epochs,
using a combination of analytical approximations and numerical integration. For
a broad range of initial conditions at the onset of inflation, the scalar field
reaches its symmetry-breaking vacuum by the present epoch, as assumed in the
local analysis of spherically-symmetric solutions and tests of gravity. For the
simplest form of the potential, the energy scale is too small for the symmetron
to act as dark energy, hence we must add a cosmological constant to drive
late-time cosmic acceleration. We briefly discuss a class of generalized,
non-renormalizable potentials that can have a greater impact on the late-time
cosmology, though cosmic acceleration requires a delicate tuning of parameters
in this case.Comment: 42 page
Classical Duals, Legendre Transforms and the Vainshtein Mechanism
We show how to generalize the classical duals found by Gabadadze {\it et al}
to a very large class of self-interacting theories. This enables one to adopt a
perturbative description beyond the scale at which classical perturbation
theory breaks down in the original theory. This is particularly relevant if we
want to test modified gravity scenarios that exhibit Vainshtein screening on
solar system scales. We recognise the duals as being related to the Legendre
transform of the original Lagrangian, and present a practical method for
finding the dual in general; our methods can also be applied to
self-interacting theories with a hierarchy of strong coupling scales, and with
multiple fields. We find the classical dual of the full quintic galileon theory
as an example.Comment: 16 page
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