30 research outputs found
Instability of the one-texture universe
The one-texture universe, introduced by Davis in 1987, is a homogeneous mapping of a scalar field with an S^3 vacuum into a closed universe. It has long been known to mathematicians that such solutions, although static, are unstable. We show by explicit construction that there are four degenerate lowest modes which are unstable, corresponding to collapse of the texture towards a single point, in the case where gravitational back reaction is neglected. We discuss the instability time scale in both static and expanding space-times; in the latter case it is of order of the present age of the universe, suggesting that, though unstable, the one-texture universe could survive to the present. The cosmic microwave background constrains the initial magnitude of this unstable perturbation to be less than âŒ10^-3
Bound and unbound substructures in Galaxy-scale Dark Matter haloes
We analyse the coarse-grained phase-space structure of the six Galaxy-scale
dark matter haloes of the Aquarius Project using a state-of-the-art 6D
substructure finder. Within r_50, we find that about 35% of the mass is in
identifiable substructures, predominantly tidal streams, but including about
14% in self-bound subhaloes. The slope of the differential substructure mass
function is close to -2, which should be compared to around -1.9 for the
population of self-bound subhaloes. Near r_50 about 60% of the mass is in
substructures, with about 30% in self-bound subhaloes. The inner 35 kpc of the
highest resolution simulation has only 0.5% of its mass in self-bound
subhaloes, but 3.3% in detected substructure, again primarily tidal streams.
The densest tidal streams near the solar position have a 3-D mass density about
1% of the local mean, and populate the high velocity tail of the velocity
distribution.Comment: Submitted to MNRAS on 12/10/2010, 11 pages, 10 figure
All-sky convolution for polarimetry experiments
We discuss all-sky convolution of the instrument beam with the sky signal in
polarimetry experiments, such as the Planck mission which will map the
temperature anisotropy and polarization of the cosmic microwave background
(CMB). To account properly for stray light (from e.g. the galaxy, sun, and
planets) in the far side-lobes of such an experiment, it is necessary to
perform the beam convolution over the full sky. We discuss this process in
multipole space for an arbitrary beam response, fully including the effects of
beam asymmetry and cross-polarization. The form of the convolution in multipole
space is such that the Wandelt-Gorski fast technique for all-sky convolution of
scalar signals (e.g. temperature) can be applied with little modification. We
further show that for the special case of a pure co-polarized, axisymmetric
beam the effect of the convolution can be described by spin-weighted window
functions. In the limits of a small angle beam and large Legendre multipoles,
the spin-weight 2 window function for the linear polarization reduces to the
usual scalar window function used in previous analyses of beam effects in CMB
polarimetry experiments. While we focus on the example of polarimetry
experiments in the context of CMB studies, we emphasise that the formalism we
develop is applicable to anisotropic filtering of arbitrary tensor fields on
the sphere.Comment: 8 pages, 1 figure; Minor changes to match version accepted by Phys.
Rev.
Dark Matter implications of the Fermi-LAT measurement of anisotropies in the diffuse gamma-ray background: status report
For the first time, the Fermi-LAT measured the angular power spectrum (APS)
of anisotropies in the diffuse gamma-ray background. The data is found to be
broadly compatible with a model with contributions from the point sources in
the 1-year catalog, the galactic diffuse background, and the extragalactic
isotropic emission; however deviations are present at both large and small
angular scales. In this study, we complement the model with a contribution from
Dark Matter (DM) whose distribution is modeled exploiting the results of the
most recent N-body simulations, considering the contribution of extragalactic
halos and subhalos (from Millennium-II) and of galactic substructures (from
Aquarius). With the use of the Fermi Science Tools, these simulations serve as
templates to produce mock gamma-ray count maps for DM gamma-ray emission, both
in the case of an annihilating and a decaying DM candidate. The APS will then
be computed and compared with the Fermi-LAT results to derive constraints on
the DM particle physics properties. The possible systematic due to an imperfect
model of the galactic foreground is also studied and taken into account
properly. The present paper reports on the status of the project.Comment: Proceeding for the RICAP2011 conferenc
Generalized Analysis of Weakly-Interacting Massive Particle Searches
We perform a generalized analysis of data from WIMP search experiments for
point-like WIMPs of arbitrary spin and general Lorenz-invariant WIMP-nucleus
interaction. We show that in the non-relativistic limit only spin-independent
(SI) and spin-dependent (SD) WIMP-nucleon interactions survive, which can be
parameterized by only five independent parameters. We explore this
five-dimensional parameter space to determine whether the annual modulation
observed in the DAMA experiment can be consistent with all other experiments.
The pure SI interaction is ruled out except for very small region of parameter
space with the WIMP mass close to 50 GeV and the ratio of the WIMP-neutron to
WIMP-proton SI couplings . For the predominantly SD
interaction, we find an upper limit to the WIMP mass of about 18 GeV, which can
only be weakened if the constraint stemming from null searches for energetic
neutrinos from WIMP annihilation the Sun is evaded. None of the regions of the
parameter space that can reconcile all WIMP search results can be easily
accommodated in the minimal supersymmetric extension of the standard model.Comment: 27 pages, 3 figure
Hydrodynamic Detonation Instability in Electroweak and QCD Phase Transitions
The hydrodynamic stability of deflagration and detonation bubbles for a first
order electroweak and QCD phase transition has been discussed recently with the
suggestion that detonations are stable. We examine here the case of a
detonation more carefully. We find that in front of the bubble wall
perturbations do not grow with time, but behind the wall modes exist which grow
exponentially. We briefly discuss the possible meaning of this instability.Comment: 12 pages, 3 figures available on request, Latex,
FERMILAB--PUB--93/098--
Corrections to Bino Annihilation I: Sfermion Mixing
We consider corrections to bino annihilation due to sfermion mixing.Comment: 11 pages in LaTex plus 4 postscript figures (included),
CfPA--93--th--21, UMN--TH--1205/9
Power spectrum for the small-scale Universe
The first objects to arise in a cold dark matter universe present a daunting
challenge for models of structure formation. In the ultra small-scale limit,
CDM structures form nearly simultaneously across a wide range of scales.
Hierarchical clustering no longer provides a guiding principle for theoretical
analyses and the computation time required to carry out credible simulations
becomes prohibitively high. To gain insight into this problem, we perform
high-resolution (N=720^3 - 1584^3) simulations of an Einstein-de Sitter
cosmology where the initial power spectrum is P(k) propto k^n, with -2.5 < n <
-1. Self-similar scaling is established for n=-1 and n=-2 more convincingly
than in previous, lower-resolution simulations and for the first time,
self-similar scaling is established for an n=-2.25 simulation. However, finite
box-size effects induce departures from self-similar scaling in our n=-2.5
simulation. We compare our results with the predictions for the power spectrum
from (one-loop) perturbation theory and demonstrate that the renormalization
group approach suggested by McDonald improves perturbation theory's ability to
predict the power spectrum in the quasilinear regime. In the nonlinear regime,
our power spectra differ significantly from the widely used fitting formulae of
Peacock & Dodds and Smith et al. and a new fitting formula is presented.
Implications of our results for the stable clustering hypothesis vs. halo model
debate are discussed. Our power spectra are inconsistent with predictions of
the stable clustering hypothesis in the high-k limit and lend credence to the
halo model. Nevertheless, the fitting formula advocated in this paper is purely
empirical and not derived from a specific formulation of the halo model.Comment: 30 pages including 10 figures; accepted for publication in MNRA
Universality and Critical Phenomena in String Defect Statistics
The idea of biased symmetries to avoid or alleviate cosmological problems
caused by the appearance of some topological defects is familiar in the context
of domain walls, where the defect statistics lend themselves naturally to a
percolation theory description, and for cosmic strings, where the proportion of
infinite strings can be varied or disappear entirely depending on the bias in
the symmetry. In this paper we measure the initial configurational statistics
of a network of string defects after a symmetry-breaking phase transition with
initial bias in the symmetry of the ground state. Using an improved algorithm,
which is useful for a more general class of self-interacting walks on an
infinite lattice, we extend the work in \cite{MHKS} to better statistics and a
different ground state manifold, namely , and explore various different
discretisations. Within the statistical errors, the critical exponents of the
Hagedorn transition are found to be quite possibly universal and identical to
the critical exponents of three-dimensional bond or site percolation. This
improves our understanding of the percolation theory description of defect
statistics after a biased phase transition, as proposed in \cite{MHKS}. We also
find strong evidence that the existence of infinite strings in the Vachaspati
Vilenkin algorithm is generic to all (string-bearing) vacuum manifolds, all
discretisations thereof, and all regular three-dimensional lattices.Comment: 62 pages, plain LaTeX, macro mathsymb.sty included, figures included.
also available on
http://starsky.pcss.maps.susx.ac.uk/groups/pt/preprints/96/96011.ps.g