103 research outputs found
Spherically Symmetric Solutions to Fourth-Order Theories of Gravity
Gravitational theories generated from Lagrangians of the form f(R) are
considered. The spherically symmetric solutions to these equations are
discussed, paying particular attention to features that differ from the
standard Schwarzschild solution. The asymptotic form of solutions is described,
as is the lack of validity of Birkhoff's theorem. Exact solutions are presented
which illustrate these points and their stability and geodesics are
investigated.Comment: 10 pages, published versio
Post-Newtonian cosmological modelling
V. A. A. S. and T. C. both acknowledge support from the STF
Parametrizing Theories of Gravity on Large and Small Scales in Cosmology
5 pages, 1 figure5 pages, 1 figure5 pages, 1 figure5 pages, 1 figureSCOAP
The Parameterised Post-Newtonian Limit of Fourth-Order Theories of Gravity
We determine the full post-Newtonian limit of theories of gravity that extend
general relativity by replacing the Ricci scalar, R, in the generating
Lagrangian by some analytic function, f(R). We restrict ourselves to theories
that admit Minkowski space as a suitable background, and perform a perturbative
expansion in the manner prescribed by the parameterised post-Newtonian
formalism. Extra potentials are found to be present that are not accounted for
in the usual treatment, and a discussion is provided on how they may be used to
observationally distinguished these theories from general relativity at the
post-Newtonian level.Comment: 12 page
On the luminosity distance and the epoch of acceleration
Standard cosmological models based on general relativity (GR) with dark
energy predict that the Universe underwent a transition from decelerating to
accelerating expansion at a moderate redshift . Clearly, it
is of great interest to directly measure this transition in a model-independent
way, without the assumption that GR is the correct theory of gravity. We
explore to what extent supernova (SN) luminosity distance measurements provide
evidence for such a transition: we show that, contrary to intuition, the
well-known "turnover" in the SN distance residuals relative to an
empty (Milne) model does not give firm evidence for such a transition within
the redshift range spanned by SN data. The observed turnover in that diagram is
predominantly due to the negative curvature in the Milne model, {\em not} the
deceleration predicted by CDM and relatives. We show that there are
several advantages in plotting distance residuals against a flat,
non-accelerating model , and also remapping the axis to ; we outline a number of useful and intuitive properties of this
presentation. We conclude that there are significant complementarities between
SNe and baryon acoustic oscillations (BAOs): SNe offer high precision at low
redshifts and give good constraints on the net {\em amount} of acceleration
since , but are weak at constraining ; while radial BAO
measurements are probably superior for placing direct constraints on .Comment: Latex, 13 pages, 7 figures. Accepted by MNRAS. For the busy reader,
Figs 4 and 6 are the main result
Perturbation theory for cosmologies with nonlinear structure
The next generation of cosmological surveys will operate over unprecedented
scales, and will therefore provide exciting new opportunities for testing
general relativity. The standard method for modelling the structures that these
surveys will observe is to use cosmological perturbation theory for linear
structures on horizon-sized scales, and Newtonian gravity for non-linear
structures on much smaller scales. We propose a two-parameter formalism that
generalizes this approach, thereby allowing interactions between large and
small scales to be studied in a self-consistent and well-defined way. This uses
both post-Newtonian gravity and cosmological perturbation theory, and can be
used to model realistic cosmological scenarios including matter, radiation and
a cosmological constant. We find that the resulting field equations can be
written as a hierarchical set of perturbation equations. At leading-order,
these equations allow us to recover a standard set of Friedmann equations, as
well as a Newton-Poisson equation for the inhomogeneous part of the Newtonian
energy density in an expanding background. For the perturbations in the
large-scale cosmology, however, we find that the field equations are sourced by
both non-linear and mode-mixing terms, due to the existence of small-scale
structures. These extra terms should be expected to give rise to new
gravitational effects, through the mixing of gravitational modes on small and
large scales - effects that are beyond the scope of standard linear
cosmological perturbation theory. We expect our formalism to be useful for
accurately modelling gravitational physics in universes that contain non-linear
structures, and for investigating the effects of non-linear gravity in the era
of ultra-large-scale surveys.Comment: "21 pages, 2 appendices. Equations (29) and (80) have been corrected
from the published version.
Inhomogeneous Gravity
We study the inhomogeneous cosmological evolution of the Newtonian
gravitational 'constant' G in the framework of scalar-tensor theories. We
investigate the differences that arise between the evolution of G in the
background universes and in local inhomogeneities that have separated out from
the global expansion. Exact inhomogeneous solutions are found which describe
the effects of masses embedded in an expanding FRW Brans-Dicke universe. These
are used to discuss possible spatial variations of G in different regions. We
develop the technique of matching different scalar-tensor cosmologies of
different spatial curvature at a boundary. This provides a model for the linear
and non-linear evolution of spherical overdensities and inhomogeneities in G.
This allows us to compare the evolution of G and \dot{G} that occurs inside a
collapsing overdense cluster with that in the background universe. We develop a
simple virialisation criterion and apply the method to a realistic lambda-CDM
cosmology containing spherical overdensities. Typically, far slower evolution
of \dot{G} will be found in the bound virialised cluster than in the
cosmological background. We consider the behaviour that occurs in Brans-Dicke
theory and in some other representative scalar-tensor theories.Comment: 15 pages, 15 figures. Submitted to MNRAS. References adde
Cosmological viability of f(R)-gravity as an ideal fluid and its compatibility with a matter dominated phase
We show that f(R)-gravity can, in general, give rise to cosmological viable
models compatible with a matter-dominated epoch evolving into a late
accelerated phase. We discuss the various representations of f(R)-gravity as an
ideal fluid or a scalar-tensor gravity theory, taking into account conformal
transformations. We point out that mathematical equivalence does not
correspond, in several cases, to the physical equivalence of Jordan frame and
Einstein frame. Finally, we show that wide classes of f(R)-gravity models,
including matter and accelerated phases, can be phenomenologically
reconstructed by means of observational data. In principle, any popular
quintessence models could be "reframed" as an f(R)-gravity model.Comment: 11 pages, 1 figur
Parameterized post-Newtonian cosmology
Einstein's theory of gravity has been extensively tested on solar system
scales, and for isolated astrophysical systems, using the perturbative
framework known as the parameterized post-Newtonian (PPN) formalism. This
framework is designed for use in the weak-field and slow-motion limit of
gravity, and can be used to constrain a large class of metric theories of
gravity with data collected from the aforementioned systems. Given the
potential of future surveys to probe cosmological scales to high precision, it
is a topic of much contemporary interest to construct a similar framework to
link Einstein's theory of gravity and its alternatives to observations on
cosmological scales. Our approach to this problem is to adapt and extend the
existing PPN formalism for use in cosmology. We derive a set of equations that
use the same parameters to consistently model both weak fields and cosmology.
This allows us to parameterize a large class of modified theories of gravity
and dark energy models on cosmological scales, using just four functions of
time. These four functions can be directly linked to the background expansion
of the universe, first-order cosmological perturbations, and the weak-field
limit of the theory. They also reduce to the standard PPN parameters on solar
system scales. We illustrate how dark energy models and scalar-tensor and
vector-tensor theories of gravity fit into this framework, which we refer to as
"parameterized post-Newtonian cosmology" (PPNC).Comment: 30 pages, no figures, v2: matches published versio
The Parkes Multibeam Pulsar Survey: PSR J1811-1736 - a pulsar in a highly eccentric binary system
We are undertaking a high-frequency survey of the Galactic plane for radio
pulsars, using the 13-element multibeam receiver on the 64-m Parkes radio
telescope. We describe briefly the survey system and some of the initial
results. PSR J1811-1736, one of the first pulsars discovered with this system,
has a rotation period of 104 ms. Subsequent timing observations using the 76-m
radio telescope at Jodrell Bank show that it is in an 18.8-day,
highly-eccentric binary orbit. We have measured the rate of advance of
periastron which indicates a total system mass of 2.6 +- 0.9 Msun, and the
minimum companion mass is about 0.7 Msun. This, the high orbital eccentricity
and the recycled nature of the pulsar suggests that this system is composed of
two neutron stars, only the fourth or fifth such system known in the disk of
the Galaxy.Comment: 6 pages, 3 embedded EPS figures, to be published in MNRA
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