5,892 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
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
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
Bayesian Reasoning in Eternal Inflation: A Solution to the Measure Problem
Probabilities in eternal inflation are traditionally defined as limiting
frequency distributions, but a unique and unambiguous probability measure
remains elusive. In this paper, we present a different approach, based on
Bayesian reasoning. Our starting point is the master equation governing vacuum
dynamics, which describes a random walk on the network of vacua. Our
probabilities require two pieces of prior information, both pertaining to
initial conditions: a prior density for the time of nucleation, and a
prior probability for the ancestral vacuum. For ancestral vacua, we
advocate the uniform prior as a conservative choice, though our conclusions are
fairly insensitive to this choice. For the time of nucleation, we argue that a
uniform prior is consistent with the time-translational invariance of the
master equation and represents the minimally-informative choice. The resulting
predictive probabilities coincide with Bousso's "holographic" prior
probabilities and are closely related to Garriga and Vilenkin's "comoving"
probabilities. Despite making the least informative priors, these probabilities
are surprisingly predictive. They favor vacua whose surrounding landscape
topography is that of a deep funnel, akin to the folding funnels of
naturally-occurring proteins. They predict that we exist during the approach to
near-equilibrium, much earlier than the mixing time for the landscape. We also
consider a volume-weighted , which amounts to weighing vacua by
physical volume. The predictive probabilities in this case coincide with the
GSVW measure. The Bayesian framework allows us to compare the plausibility of
the uniform-time and volume-weighted hypotheses to explain our data by
computing the Bayesian evidence for each. We argue, under general and plausible
assumptions, that posterior odds overwhelmingly favor the uniform-time
hypothesis.Comment: 34 pages, 1 figur
Early-Time Measure in Eternal Inflation
In a situation like eternal inflation, where our data is replicated at
infinitely-many other space-time events, it is necessary to make a prior
assumption about our location to extract predictions. The principle of
mediocrity entails that we live at asymptotic late times, when the occupational
probabilities of vacua has settled to a near-equilibrium distribution. In this
paper we further develop the idea that we instead exist during the approach to
equilibrium, much earlier than the exponentially-long mixing time. In this case
we are most likely to reside in vacua that are easily accessed dynamically.
Using first-passage statistics, we prove that vacua that maximize their
space-time volume at early times have: 1. maximal ever-hitting probability; 2.
minimal mean first-passage time; and 3. minimal decay rate. These requirements
are succinctly captured by an early-time measure. The idea that we live at
early times is a predictive guiding principle, with many phenomenological
implications. First, our vacuum should lie deep in a funneled region, akin to
folding energy landscapes of proteins. Second, optimal landscape regions are
characterized by relatively short-lived vacua, with lifetime of order the de
Sitter Page time. For our vacuum, this lifetime is ~years, which
is consistent with the Standard Model estimate due to Higgs metastability.
Third, the measure favors vacua with small, positive vacuum energy. This can
address the cosmological constant problem, provided there are sufficiently many
vacua in the entire ensemble of funnels. As a concrete example, we study the
Bousso-Polchinski lattice of flux vacua, and find that the early-time measure
favors lattices with the fewest number of flux dimensions. This favors
compactifications with a large hierarchy between the lightest modulus and all
other K\"ahler and complex structure moduli.Comment: 34 pages, 3 figure
Implications of the Weak Gravity Conjecture for Tidal Love Numbers of Black Holes
The Weak Gravity Conjecture indicates that extremal black holes in the low
energy effective field theory should be able to decay. This criterion gives
rise to non-trivial constraints on the coefficients of higher-order derivative
corrections to gravity. In this paper, we investigate the tidal deformability
of neutral black holes due to higher-order derivative corrections. As a proof
of concept, we consider a correction of cubic order in the Riemann curvature
tensor. The tidal Love numbers of neutral black holes receive leading-order
corrections from higher-order derivative terms, since black holes in pure
General Relativity have vanishing tidal Love number. We conclude that the
interplay between the tidal deformability of black holes and the Weak Gravity
Conjecture provides useful information about the effective field theory.Comment: 23 pages, 2 figures. v2: matching published versio
Conformal Symmetries of Adiabatic Modes in Cosmology
We remark on the existence of non-linearly realized conformal symmetries for
scalar adiabatic perturbations in cosmology. These conformal symmetries are
present for any cosmological background, beyond any slow-roll or quasi-de
Sitter approximation. The dilatation transformation shifts the curvature
perturbation by a constant, and corresponds to the well-known symmetry under
spatial rescaling. We argue that the scalar sector is also invariant under
special conformal transformations, which shift the curvature perturbation by a
term linear in the spatial coordinates. We discuss whether these conformal
symmetries can be extended to include tensor perturbations. Tensor modes
introduce their own set of non-linearly realized symmetries. We identify an
infinite set of large gauge transformations which maintain the transverse,
traceless gauge condition, while shifting the tensor mode non-trivially.Comment: 16 page
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