114 research outputs found
General Relativity as an Attractor in Scalar-Tensor Stochastic Inflation
Quantum fluctuations of scalar fields during inflation could determine the
very large-scale structure of the universe. In the case of general
scalar-tensor gravity theories these fluctuations lead to the diffusion of
fundamental constants like the Planck mass and the effective Brans--Dicke
parameter, . In the particular case of Brans--Dicke gravity, where
is constant, this leads to runaway solutions with infinitely large
values of the Planck mass. However, in a theory with variable we find
stationary probability distributions with a finite value of the Planck mass
peaked at exponentially large values of after inflation. We conclude
that general relativity is an attractor during the quantum diffusion of the
fields.Comment: LaTeX (with RevTex) 11 pages, 2 uuencoded figures appended, also
available on WWW via http://star.maps.susx.ac.uk/index.htm
STATIONARY SOLUTIONS IN BRANS-DICKE STOCHASTIC INFLATIONARY COSMOLOGY
In Brans-Dicke theory the Universe becomes divided after inflation into many
exponentially large domains with different values of the effective
gravitational constant. Such a process can be described by diffusion equations
for the probability of finding a certain value of the inflaton and dilaton
fields in a physical volume of the Universe. For a typical chaotic inflation
potential, the solutions for the probability distribution never become
stationary but grow forever towards larger values of the fields. We show here
that a non-minimal conformal coupling of the inflaton to the curvature scalar,
as well as radiative corrections to the effective potential, may provide a
dynamical cutoff and generate stationary solutions. We also analyze the
possibility of large nonperturbative jumps of the fluctuating inflaton scalar
field, which was recently revealed in the context of the Einstein theory. We
find that in the Brans--Dicke theory the amplitude of such jumps is strongly
suppressed.Comment: 19 pages, LaTe
Phase-plane analysis of Friedmann-Robertson-Walker cosmologies in Brans-Dicke gravity
We present an autonomous phase-plane describing the evolution of
Friedmann-Robertson-Walker models containing a perfect fluid (with barotropic
index gamma) in Brans-Dicke gravity (with Brans-Dicke parameter omega). We find
self-similar fixed points corresponding to Nariai's power-law solutions for
spatially flat models and curvature-scaling solutions for curved models. At
infinite values of the phase-plane variables we recover O'Hanlon and Tupper's
vacuum solutions for spatially flat models and the Milne universe for negative
spatial curvature. We find conditions for the existence and stability of these
critical points and describe the qualitative evolution in all regions of the
(omega,gamma) parameter space for 0-3/2. We show that the
condition for inflation in Brans-Dicke gravity is always stronger than the
general relativistic condition, gamma<2/3.Comment: 24 pages, including 9 figures, LaTe
Constraints from Inflation on Scalar-Tensor Gravity Theories
We show how observations of the perturbation spectra produced during
inflation may be used to constrain the parameters of general scalar-tensor
theories of gravity, which include both an inflaton and dilaton field. An
interesting feature of these models is the possibility that the curvature
perturbations on super-horizon scales may not be constant due to non-adiabatic
perturbations of the two fields. Within a given model, the tilt and relative
amplitude of the scalar and tensor perturbation spectra gives constraints on
the parameters of the gravity theory, which may be comparable with those from
primordial nucleosynthesis and post-Newtonian experiments.Comment: LaTeX (with RevTex) 19 pages, 8 uuencoded figures appended, also
available on WWW via http://star.maps.susx.ac.uk/index.htm
The Behaviour Of Cosmological Models With Varying-G
We provide a detailed analysis of Friedmann-Robertson-Walker universes in a
wide range of scalar-tensor theories of gravity. We apply solution-generating
methods to three parametrised classes of scalar-tensor theory which lead
naturally to general relativity in the weak-field limit. We restrict the
parameters which specify these theories by the requirements imposed by the
weak-field tests of gravitation theories in the solar system and by the
requirement that viable cosmological solutions be obtained. We construct a
range of exact solutions for open, closed, and flat isotropic universes
containing matter with equation of state and in vacuum.
We study the range of early and late-time behaviours displayed, examine when
there is a `bounce' at early times, and expansion maxima in closed models.Comment: 58 pages LaTeX, 6 postscript figures, uses eps
Preheating in Supersymmetric Hybrid Inflation
We study preheating in a general class of supersymmetric hybrid inflation
model. Supersymmetry leads to only one coupling constant in the potential and
thus only one natural frequency of oscillation for the homogeneous fields,
whose classical evolution consequently differs from that of a general
(non-supersymmetric) hybrid model. We emphasise the importance of mixing
effects in these models which can significantly change the rate of production
of particles. We perform a general study of the rate of production of the
particles associated with the homogeneous fields, and show how preheating is
efficient in producing these quanta. Preheating of other particle species will
be model dependent, and in order to investigate this we consider a realistic
working model of supersymmetric hybrid inflation which solves the strong-CP
problem via an approximate Peccei-Quinn symmetry, which was proposed by us
previously. We study axion production in this model and show that properly
taking into account the mixing between the fields suppresses the axion
production, yet enhances the production of other particles. Finally we
demonstrate the importance of backreaction effects in this model which have the
effect of shutting off axion production, leaving the axion safely within
experimental bounds.Comment: 37 pages, Latex, 11 eps figures, 14 ps (colour) figure
One-Loop Superstring Cosmology and the Non-Singular Universe
We study the cosmological implications of the one-loop terms in the string
expansion. In particular, we find non-singular solutions which interpolate
between a contracting universe and an expanding universe, and show that these
solutions provide a mechanism for removing the initial conditions problem
peculiar to spatially closed FRW cosmologies. In addition, we perform numerical
calculations to show that the non-singular cosmologies do not require a careful
choice of initial conditions, and estimate the likely magnitude of higher order
terms in the string expansion.Comment: 19 pages, 3 figures (postscript), Latex2e, discussion of curvature
invariants added. To appear in Phys. Rev.
Conditions for Successful Extended Inflation
We investigate, in a model-independent way, the conditions required to obtain
a satisfactory model of extended inflation in which inflation is brought to an
end by a first-order phase transition. The constraints are that the correct
present strength of the gravitational coupling is obtained, that the present
theory of gravity is satisfactorily close to general relativity, that the
perturbation spectra from inflation are compatible with large scale structure
observations and that the bubble spectrum produced at the phase transition
doesn't conflict with the observed level of microwave background anisotropies.
We demonstrate that these constraints can be summarized in terms of the
behaviour in the conformally related Einstein frame, and can be compactly
illustrated graphically. We confirm the failure of existing models including
the original extended inflation model, and construct models, albeit rather
contrived ones, which satisfy all existing constraints.Comment: 8 pages RevTeX file with one figure incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/infcos_papers.html; Revised to include
extra references, results unchanged, to appear Phys Rev
The PAU Survey: Narrow-band image photometry
PAUCam is an innovative optical narrow-band imager mounted at the William
Herschel Telescope built for the Physics of the Accelerating Universe Survey
(PAUS). Its set of 40 filters results in images that are complex to calibrate,
with specific instrumental signatures that cannot be processed with traditional
data reduction techniques. In this paper we present two pipelines developed by
the PAUS data management team with the objective of producing science-ready
catalogues from the uncalibrated raw images. The Nightly pipeline takes care of
all image processing, with bespoke algorithms for photometric calibration and
scatter-light correction. The Multi-Epoch and Multi-Band Analysis (MEMBA)
pipeline performs forced photometry over a reference catalogue to optimize the
photometric redshift performance. We verify against spectroscopic observations
that the current approach delivers an inter-band photometric calibration of
0.8% across the 40 narrow-band set. The large volume of data produced every
night and the rapid survey strategy feedback constraints require operating both
pipelines in the Port d'Informaci\'o Cientifica data centre with intense
parallelization. While alternative algorithms for further improvements in
photo-z performance are under investigation, the image calibration and
photometry presented in this work already enable state-of-the-art photometric
redshifts down to iAB=23.0.Comment: 32 pages, 26 figures, MNRAS in pres
The Physics of the Accelerating Universe Survey: narrow-band image photometry
PAUCam is an innovative optical narrow-band imager mounted at the William Herschel Telescope built for the Physics of the Accelerating Universe Survey (PAUS). Its set of 40 filters results in images that are complex to calibrate, with specific instrumental signatures that cannot be processed with traditional data reduction techniques. In this paper, we present two pipelines developed by the PAUS data management team with the objective of producing science-ready catalogues from the uncalibrated raw images. The NIGHTLY pipeline takes care of entire image processing, with bespoke algorithms for photometric calibration and scatter-light correction. The Multi-Epoch and Multi-Band Analysis pipeline performs forced photometry over a reference catalogue to optimize the photometric redshift (photo-z) performance. We verify against spectroscopic observations that the current approach delivers an inter-band photometric calibration of 0.8 per cent across the 40 narrow-band set. The large volume of data produced every night and the rapid survey strategy feedback constraints require operating both pipelines in the Port d’Informació Cientifica data centre with intense parallelization. While alternative algorithms for further improvements in photo-z performance are under investigation, the image calibration and photometry presented in this work already enable state-of-the-art photo-z down to iAB = 23.0
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