106 research outputs found
Power spectrum and correlation function errors: Poisson vs. Gaussian shot noise
Poisson distributed shot noise is normally considered in the Gaussian limit
in cosmology. However, if the shot noise is large enough and the correlation
function/power spectrum conspires, the Gaussian approximation mis-estimates the
errors and their covariance significantly. The power spectrum, even for
initially Gaussian densities,acquires cross correlations which can be large,
while the change in the correlation function error matrix is diagonal except at
zero separation. Two and three dimensional power law correlation function and
power spectrum examples are given. These corrections appear to have a large
effect when applied to galaxy clusters, e.g. for SZ selected galaxy clusters in
2 dimensions. This can increase the error estimates for cosmological parameter
estimation and consequently affect survey strategies, as the corrections are
minimized for surveys which are deep and narrow rather than wide and shallow.
In addition, a rewriting of the error matrix for the power spectrum/correlation
function is given which eliminates most of the Bessel function dependence (in
two dimensions) and all of it (in three dimensions), which makes the
calculation of the error matrix more tractable. This applies even when the shot
noise is in the (usual) Gaussian limit.Comment: 22 pages, 4 figures, 3 equations corrected/figures updated, results
unchange
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
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
Late-time cosmology in (phantom) scalar-tensor theory: dark energy and the cosmic speed-up
We consider late-time cosmology in a (phantom) scalar-tensor theory with an
exponential potential, as a dark energy model with equation of state parameter
close to -1 (a bit above or below this value). Scalar (and also other kinds of)
matter can be easily taken into account. An exact spatially-flat FRW cosmology
is constructed for such theory, which admits (eternal or transient)
acceleration phases for the current universe, in correspondence with
observational results. Some remarks on the possible origin of the phantom,
starting from a more fundamental theory, are also made. It is shown that
quantum gravity effects may prevent (or, at least, delay or soften) the cosmic
doomsday catastrophe associated with the phantom, i.e. the otherwise
unavoidable finite-time future singularity (Big Rip). A novel dark energy model
(higher-derivative scalar-tensor theory) is introduced and it is shown to admit
an effective phantom/quintessence description with a transient acceleration
phase. In this case, gravity favors that an initially insignificant portion of
dark energy becomes dominant over the standard matter/radiation components in
the evolution process.Comment: LaTeX file, 48 pages, discussion of Big Rip is enlarged, a reference
is adde
Dynamics of liquid He-4 in confined geometries from Time-Dependent Density Functional calculations
We present numerical results obtained from Time-Dependent Density Functional
calculations of the dynamics of liquid He-4 in different environments
characterized by geometrical confinement. The time-dependent density profile
and velocity field of He-4 are obtained by means of direct numerical
integration of the non-linear Schrodinger equation associated with a
phenomenological energy functional which describes accurately both the static
and dynamic properties of bulk liquid He-4. Our implementation allows for a
general solution in 3-D (i.e. no symmetries are assumed in order to simplify
the calculations). We apply our method to study the real-time dynamics of pure
and alkali-doped clusters, of a monolayer film on a weakly attractive surface
and a nano-droplet spreading on a solid surface.Comment: q 1 tex file + 9 Ps figure
On Physical Equivalence between Nonlinear Gravity Theories
We argue that in a nonlinear gravity theory, which according to well-known
results is dynamically equivalent to a self-gravitating scalar field in General
Relativity, the true physical variables are exactly those which describe the
equivalent general-relativistic model (these variables are known as Einstein
frame). Whenever such variables cannot be defined, there are strong indications
that the original theory is unphysical. We explicitly show how to map, in the
presence of matter, the Jordan frame to the Einstein one and backwards. We
study energetics for asymptotically flat solutions. This is based on the
second-order dynamics obtained, without changing the metric, by the use of a
Helmholtz Lagrangian. We prove for a large class of these Lagrangians that the
ADM energy is positive for solutions close to flat space. The proof of this
Positive Energy Theorem relies on the existence of the Einstein frame, since in
the (Helmholtz--)Jordan frame the Dominant Energy Condition does not hold and
the field variables are unrelated to the total energy of the system.Comment: 37 pp., TO-JLL-P 3/93 Dec 199
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
String Universality
If there is a single underlying "theory of everything" which in some limits
of its "moduli space" reduces to the five weakly coupled string theories in
10D, and 11D SUGRA, then it is possible that all six of them have some common
domain of validity and that they are in the same universality class, in the
sense that the 4D low energy physics of the different theories is the same. We
call this notion String Universality. This suggests that the true vacuum of
string theory is in a region of moduli space equally far (in some sense) from
all perturbative theories, most likely around the self-dual point with respect
to duality symmetries connecting them. We estimate stringy non-perturbative
effects from wrapped brane instantons in each perturbative theory, show how
they are related by dualities, and argue that they are likely to lead to moduli
stabilization only around the self-dual point. We argue that moduli
stabilization should occur near the string scale, and SUSY breaking should
occur at a much lower intermediate scale, and that it originates from different
sources. We discuss the problems of moduli stabilization and SUSY breaking in
currently popular scenarios, explain why these problems are generic, and
discuss how our scenario can evade them. We show that String Universality is
not inconsistent with phenomenology but that it is in conflict with some
popular versions of brane world scenarios.Comment: 48 pages, 1 figure; one reference adde
Suppression of Coulomb-nuclear interference in the near-barrier elastic scattering of 17 Ne from 208 Pb
The proton drip-line nucleus 17Ne is considered a good candidate for a Borromean two-proton halo with a 15O + p+ pstructure. Angular distributions of the elastic scattering and inclusive 15O production for a 136 MeV 17Ne beam incident on a 208Pb target were measured for the first time at the SPIRAL1 facility, GANIL. Use of the GLORIA detector array allowed high-resolution data over a wide angular range from 20âŠup to 95âŠin the laboratory frame to be obtained. The elastic scattering angular distribution shows similarities with those for both 6He and 20Ne at equivalent collision energies with respect to the corresponding Coulomb barriers, exhibiting the suppression of the Coulomb rainbow peak characteristic of strong coupling. Optical model and coupled channel fits suggest that this is due to a combination of coupling to low-lying quadrupole resonances and Coulomb dipole coupling to the low-lying continuum, although their relative importance depends on the relevant B(E2)values which remain to be firmly determined
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
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