4,376 research outputs found
Nonlocal Cosmology
We explore nonlocally modified models of gravity, inspired by quantum loop
corrections, as a mechanism for explaining current cosmic acceleration. These
theories enjoy two major advantages: they allow a delayed response to cosmic
events, here the transition from radiation to matter dominance, and they avoid
the usual level of fine tuning; instead, emulating Dirac's dictum, the required
large numbers come from the large time scales involved. Their solar system
effects are safely negligible, and they may even prove useful to the black hole
information problem.Comment: Expanded(!) version, to appear in Phys. Rev. Letter
Neutron Stars in f(R) Gravity with Perturbative Constraints
We study the structure of neutron stars in f(R) gravity theories with
perturbative constraints. We derive the modified Tolman-Oppenheimer-Volkov
equations and solve them for a polytropic equation of state. We investigate the
resulting modifications to the masses and radii of neutron stars and show that
observations of surface phenomena alone cannot break the degeneracy between
altering the theory of gravity versus choosing a different equation of state of
neutron-star matter. On the other hand, observations of neutron-star cooling,
which depends on the density of matter at the stellar interior, can place
significant constraints on the parameters of the theory.Comment: Discussion extended, typos corrected, figures revised. Accepted for
publication in PR
A Simple Operator Check of the Effective Fermion Mode Function during Inflation
We present a relatively simple operator formalism which reproduces the
leading infrared logarithm of the one loop quantum gravitational correction to
the fermion mode function on a locally de Sitter background. This rule may
serve as the basis for an eventual stochastic formulation of quantum gravity
during inflation. Such a formalism would not only effect a vast simplification
in obtaining the leading powers of at fixed loop orders, it would also
permit us to sum the series of leading logarithms. A potentially important
point is that our rule does not seem to be consistent with any simple infrared
truncation of the fields. Our analysis also highlights the importance of spin
as a gravitational interaction that persists even when kinetic energy has
redshifted to zero.Comment: 39 pages, no figuire.(1) New version has clarified the ultimate
motivation by adding sentences to the abstract and to the penultimate
paragraph of the introduction. (2) By combining a number of references and
equations we have managed to reduce the length by 2 page
Effect of Agricultural Activity on River Water Quality: A Case Study for the Lower Colorado River Basin
This case study investigates the effect of a change in cropping pattern involving expanded acres of crops for biofuel feedstock, on the discharge of nutrients to rivers. Annual data from 1968-2008 on stream flow, cropped acres, and precipitation for Wharton County, Texas are used. A positive impact of increased corn acreage over this period on river discharge is identified.Biofuels, Stream Flow, Discharge, Production Economics, Resource /Energy Economics and Policy,
Civil Practice and Procedure
This article summarizes the major developments in Virginia civil practice and procedure over the past two years, specifically covering significant decisions by the Supreme Court of Virginiaand changes to the Rules of the Supreme Court of Virginia dating from April 22, 2005, to April 20, 2007. The article also addresses legislative enactments by the General Assembly in its 2005 and 2006 sessions
One Loop Graviton Self-Energy In A Locally De Sitter Background
The graviton tadpole has recently been computed at two loops in a locally de
Sitter background. We apply intermediate results of this work to exhibit the
graviton self-energy at one loop. This quantity is interesting both to check
the accuracy of the first calculation and to understand the relaxation effect
it reveals. In the former context we show that the self-energy obeys the
appropriate Ward identity. We also show that its flat space limit agrees with
the flat space result obtained by Capper in what should be the same gauge.Comment: 35 pages, plain TeX, 4 Postscript files, uses psfig.sty, revised June
1996 for publication in Physical Review
The Parameterized Post-Friedmann Framework for Theories of Modified Gravity: Concepts, Formalism and Examples
A unified framework for theories of modified gravity will be an essential
tool for interpreting the forthcoming deluge of cosmological data. We present
such a formalism, the Parameterized Post-Friedmann framework (PPF), which
parameterizes the cosmological perturbation theory of a wide variety of
modified gravity models. PPF is able to handle spin-0 degrees of freedom from
new scalar, vector and tensor fields, meaning that it is not restricted to
simple models based solely on cosmological scalar fields. A direct
correspondence is maintained between the parameterization and the underlying
space of theories, which allows us to build up a `dictionary' of modified
gravity theories and their PPF correspondences. In this paper we describe the
construction of the parameterization and demonstrate its use through a number
of worked examples relevant to the current literature. We indicate how the
formalism will be implemented numerically, so that the dictionary of modified
gravity can be pitted against forthcoming observations.Comment: 24 pages, updated to match version published in PRD. Discussion of
section 4 extended. Suggestions for the busy reader are given at the end of
section
Primordial Gravitational Waves Enhancement
We reconsider the enhancement of primordial gravitational waves that arises
from a quantum gravitational model of inflation. A distinctive feature of this
model is that the end of inflation witnesses a brief phase during which the
Hubble parameter oscillates in sign, changing the usual Hubble friction to
anti-friction. An earlier analysis of this model was based on numerically
evolving the graviton mode functions after guessing their initial conditions
near the end of inflation. The current study is based on an equation which
directly evolves the normalized square of the magnitude. We are also able to
make a very reliable estimate for the initial condition using a rapidly
converging expansion for the sub-horizon regime. Results are obtained for the
energy density per logarithmic wave number as a fraction of the critical
density. These results exhibit how the enhanced signal depends upon the number
of oscillatory periods; they also show the resonant effects associated with
particular wave numbers.Comment: 25 pages, 14 figure
Does Physical Activity Influence Semantic Memory Activation in Amnestic Mild Cognitive Impairment?
The effect of physical activity (PA) on functional brain activation for semantic memory in amnestic mild cognitive impairment (aMCI) was examined using event-related functional magnetic resonance imaging during fame discrimination. Significantly greater semantic memory activation occurred in the left caudate of High- versus Low-PA patients, (P=0.03), suggesting PA may enhance memory-related caudate activation in aMCI
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