358 research outputs found
Chern-Simons Modified Gravity as a Torsion Theory and its Interaction with Fermions
We study the tetrad formulation of Chern-Simons (CS) modified gravity, which
adds a Pontryagin term to the Einstein-Hilbert action with a
spacetime-dependent coupling field. We first verify that CS modified gravity
leads to a theory with torsion, where this tensor is given by an antisymmetric
product of the Riemann tensor and derivatives of the CS coupling. We then
calculate the torsion in the far field of a weakly gravitating source within
the parameterized post-Newtonian formalism, and specialize the result to Earth.
We find that CS torsion vanishes only if the coupling vanishes, thus
generically leading to a modification of gyroscopic precession, irrespective of
the coupling choice. Perhaps most interestingly, we couple fermions to CS
modified gravity via the standard Dirac action and find that these further
correct the torsion tensor. Such a correction leads to two new results: (i) a
generic enhancement of CS modified gravity by the Dirac equation and axial
fermion currents; (ii) a new two-fermion interactions, mediated by an axial
current and the CS correction. We conclude with a discussion of the
consequences of these results in particle detectors and realistic astrophysical
systems.Comment: 11 pages, submitted to Phys. Rev.
Cycle threshold values are inversely associated with poorer outcomes in hospitalised patients with Covid-19:a prospective, observational cohort study conducted at a UK tertiary hospital
ABSTRACT: This single-centre observational study demonstrated that lower cycle threshold (Ct) values (indicating higher viral loads) on admission to hospital were associated with poorer outcomes in unvaccinated, hospitalized patients with coronavirus disease 2019 (COVID-19). Demographic and outcome data were collected prospectively for all adult patients who tested positive for severe acute respiratory syndrome coronavirus-2 on admission to the University Hospitals North Midlands NHS Trust between 1 February and 1 July 2020. Nasopharyngeal swab samples were obtained, and a valid Ct value was determined for all patients using the Viasure reverse transcription polymerase chain reaction assay, validated by Public Health England, on admission to hospital. Multi-variable logistic regression results based on data from 618 individuals demonstrated a significant inverse relationship between the odds of death and Ct values (adjusted odds ratio 0.95, 95% confidence interval 0.92â0.98, P=0.001). The association remained highly significant after adjusting for known clinical risk factors for COVID-1
Brane Worlds and the Cosmic Coincidence Problem
Brane world models with `large' extra dimensions with radii in the r_l ~
0.01- 0.1 mm range and smaller ones at r_s < (1 TeV)^(-1) have the potential to
solve the cosmic coincidence problem, i.e. the apparently fortuitous equality
between dark matter and dark energy components today. The main ingredient is
the assumption of a stabilization mechanism fixing the total volume of the
compact submanifold, but allowing for shape deformations. The latter are
associated with phenomenologically safe ultra-light scalar fields. Bulk fields
Casimir energy naturally plays the role of dark energy, which decreases in time
because of expanding r_l. Stable Kaluza Klein states may play the role of dark
matter with increasing, O(1/r_s), mass. The cosmological equations exhibit
attractor solutions in which the global equation of state is negative, the
ratio between dark energy and dark matter is constant and the observed value of
the ratio is obtained for two large extra dimensions. Experimental searches of
large extra dimensions should take into account that, due to the strong
coupling between dark matter and radii dynamics, the size of the large extra
dimensions inside the galactic halo may be smaller than the average value.Comment: 6 pages, enlarged discussion on the compact volume stabilization
mechanism. Version to appear on Phys. Rev.
Loop-Generated Bounds on Changes to the Graviton Dispersion Relation
We identify the effective theory appropriate to the propagation of massless
bulk fields in brane-world scenarios, to show that the dominant low-energy
effect of asymmetric warping in the bulk is to modify the dispersion relation
of the effective 4-dimensional modes. We show how such changes to the graviton
dispersion relation may be bounded through the effects they imply, through
loops, for the propagation of standard model particles. We compute these bounds
and show that they provide, in some cases, the strongest constraints on
nonstandard gravitational dispersions. The bounds obtained in this way are the
strongest for the fewest extra dimensions and when the extra-dimensional Planck
mass is the smallest. Although the best bounds come for warped 5-D scenarios,
for which the 5D Planck Mass is O(TeV), even in 4 dimensions the graviton loop
can lead to a bound on the graviton speed which is comparable with other
constraints.Comment: 18 pages, LaTeX, 4 figures, uses revte
Boundary Term in Metric f(R) Gravity: Field Equations in the Metric Formalism
The main goal of this paper is to get in a straightforward form the field
equations in metric f(R) gravity, using elementary variational principles and
adding a boundary term in the action, instead of the usual treatment in an
equivalent scalar-tensor approach. We start with a brief review of the
Einstein-Hilbert action, together with the Gibbons-York-Hawking boundary term,
which is mentioned in some literature, but is generally missing. Next we
present in detail the field equations in metric f(R) gravity, including the
discussion about boundaries, and we compare with the Gibbons-York-Hawking term
in General Relativity. We notice that this boundary term is necessary in order
to have a well defined extremal action principle under metric variation.Comment: 12 pages, title changes by referee recommendation. Accepted for
publication in General Relativity and Gravitation. Matches with the accepted
versio
Could dark energy be vector-like?
In this paper I explore whether a vector field can be the origin of the
present stage of cosmic acceleration. In order to avoid violations of isotropy,
the vector has be part of a ``cosmic triad'', that is, a set of three identical
vectors pointing in mutually orthogonal spatial directions. A triad is indeed
able to drive a stage of late accelerated expansion in the universe, and there
exist tracking attractors that render cosmic evolution insensitive to initial
conditions. However, as in most other models, the onset of cosmic acceleration
is determined by a parameter that has to be tuned to reproduce current
observations. The triad equation of state can be sufficiently close to minus
one today, and for tachyonic models it might be even less than that. I briefly
analyze linear cosmological perturbation theory in the presence of a triad. It
turns out that the existence of non-vanishing spatial vectors invalidates the
decomposition theorem, i.e. scalar, vector and tensor perturbations do not
decouple from each other. In a simplified case it is possible to analytically
study the stability of the triad along the different cosmological attractors.
The triad is classically stable during inflation, radiation and matter
domination, but it is unstable during (late-time) cosmic acceleration. I argue
that this instability is not likely to have a significant impact at present.Comment: 28 pages, 6 figures. Uses RevTeX4. v2: Discussion about relation to
phantoms added and additional references cite
Local Gravity Constraints and Power Law f(R) Theories
There is a conformal equivalence between power law theories and scalar
field theories in which the scalar degree of freedom evolves under the action
of an exponential potential function. In the scalar field representation there
is a strong coupling of the scalar field with the matter sector due to the
conformal transformation. We use chameleon mechanism to implement constraints
on the potential function of the scalar field in order that the resulting model
be in accord with Solar System experiments. Investigation of these constraints
reveals that there may be no possibility to distinguish between a power law
function and the usual Einstein-Hilbert Lagrangian density.Comment: 11 Pages, no figure. To appear in Gravitation and Cosmolog
A sub-horizon framework for probing the relationship between the cosmological matter distribution and metric perturbations
The relationship between the metric and nonrelativistic matter distribution
depends on the theory of gravity and additional fields, providing a possible
way of distinguishing competing theories. With the assumption that the geometry
and kinematics of the homogeneous universe have been measured to sufficient
accuracy, we present a procedure for understanding and testing the relationship
between the cosmological matter distribution and metric perturbations (along
with their respective evolution) using the ratio of the physical size of the
perturbation to the size of the horizon as our small expansion parameter. We
expand around Newtonian gravity on linear, subhorizon scales with coefficient
functions in front of the expansion parameter. Our framework relies on an
ansatz which ensures that (i) the Poisson equation is recovered on small scales
(ii) the metric variables (and any additional fields) are generated and
supported by the nonrelativistic matter overdensity. The scales for which our
framework is intended are small enough so that cosmic variance does not
significantly limit the accuracy of the measurements and large enough to avoid
complications from nonlinear effects and baryon cooling. The coefficient
functions provide a general framework for contrasting the consequences of
Lambda CDM and its alternatives. We calculate the coefficient functions for
general relativity with a cosmological constant and dark matter, GR with dark
matter and quintessence, scalar-tensor theories, f(R) gravity and braneworld
models. We identify a possibly unique signature of braneworld models.
Constraining the coefficient functions provides a streamlined approach for
testing gravity in a scale dependent manner. We briefly discuss the
observations best suited for an application of our framework.Comment: Updated references and minor changes to match the published version
in MNRA
f(R) Gravity and scalar-tensor theory
In the present paper we will investigate the relation between scalar-tensor
theory and theories of gravity. Such studies have been performed in the
past for the metric formalism of gravity; here we will consider mainly
the Palatini formalism, where the metric and the connections are treated as
independent quantities. We will try to investigate under which circumstances
theories of gravity are equivalent to scalar-tensor theory and examine
the implications of this equivalence, when it exists.Comment: minor changes to match published version, references adde
The Laser Astrometric Test of Relativity Mission
This paper discusses new fundamental physics experiment to test relativistic
gravity at the accuracy better than the effects of the 2nd order in the
gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR)
mission uses laser interferometry between two micro-spacecraft whose lines of
sight pass close by the Sun to accurately measure deflection of light in the
solar gravity. The key element of the experimental design is a redundant
geometry optical truss provided by a long-baseline (100 m) multi-channel
stellar optical interferometer placed on the International Space Station. The
geometric redundancy enables LATOR to measure the departure from Euclidean
geometry caused by the solar gravity field to a very high accuracy. LATOR will
not only improve the value of the parameterized post-Newtonian (PPN) parameter
gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach
ability to measure effects of the next post-Newtonian order (1/c^4) of light
deflection resulting from gravity's intrinsic non-linearity. The solar
quadrupole moment parameter, J2, will be measured with high precision, as well
as a variety of other relativistic. LATOR will lead to very robust advances in
the tests of fundamental physics: this mission could discover a violation or
extension of general relativity, or reveal the presence of an additional long
range interaction in the physical law. There are no analogs to the LATOR
experiment; it is unique and is a natural culmination of solar system gravity
experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International
Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12
December 2003, Washington, D
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