1,920 research outputs found
String Evolution with Friction
We study the effects of friction on the scaling evolution of string networks
in condensed matter and cosmological contexts. We derive a generalized
`one-scale' model with the string correlation length and velocity as
dynamical variables. In non-relativistic systems, we obtain a well-known
law, showing that loop production is important. For
electroweak cosmic strings, we show transient damped epoch scaling with
(or, in the matter era, ). A low initial
density implies an earlier period with . For GUT strings, the
approach to linear scaling is faster than previously estimated.Comment: 8 pages, uuencoded gziped .ps file. Paper submitted to Phys. Rev.
Let
The shape of primordial non-Gaussianity and the CMB bispectrum
We present a set of formalisms for comparing, evolving and constraining
primordial non-Gaussian models through the CMB bispectrum. We describe improved
methods for efficient computation of the full CMB bispectrum for any general
(non-separable) primordial bispectrum, incorporating a flat sky approximation
and a new cubic interpolation. We review all the primordial non-Gaussian models
in the present literature and calculate the CMB bispectrum up to l <2000 for
each different model. This allows us to determine the observational
independence of these models by calculating the cross-correlation of their CMB
bispectra. We are able to identify several distinct classes of primordial
shapes - including equilateral, local, warm, flat and feature (non-scale
invariant) - which should be distinguishable given a significant detection of
CMB non-Gaussianity. We demonstrate that a simple shape correlator provides a
fast and reliable method for determining whether or not CMB shapes are well
correlated. We use an eigenmode decomposition of the primordial shape to
characterise and understand model independence. Finally, we advocate a
standardised normalisation method for based on the shape
autocorrelator, so that observational limits and errors can be consistently
compared for different models.Comment: 32 pages, 20 figure
Cosmic String Evolution in Higher Dimensions
We obtain the equations of motion for cosmic strings in extensions of the 3+1
FRW model with extra dimensions. From these we derive a generalisation of the
Velocity-dependent One-Scale (VOS) model for cosmic string network evolution
which we apply, first, to a higher-dimensional isotropic FRW model and,
second, to a 3+1 FRW model with static flat extra dimensions. In the former
case the string network does not achieve a scaling regime because of the
diminishing rate of string intersections (), but this can be avoided in
the latter case by considering compact, small extra dimensions, for which there
is a reduced but still appreciable string intercommuting probability. We note
that the velocity components lying in the three expanding dimensions are
Hubble-damped, whereas those in the static extra dimensions are only very
weakly damped. This leads to the pathological possibility, in principle, that
string motion in the three infinite dimensions can come to a halt preventing
the strings from intersecting, with the result that scaling is not achieved and
the strings irreversibly dominate the early universe. We note criteria by which
this can be avoided, notably if the spatial structure of the network becomes
essentially three-dimensional, as is expected for string networks produced in
brane inflation. Applying our model to a brane inflation setting, we find
scaling solutions in which the effective 3D string motion does not necessarily
stop, but it is slowed down because of the excitations trapped in the extra
dimensions. These effects are likely to influence cosmic string network
evolution for a long period after formation and we discuss their more general
implications.Comment: 23 pages, 8 figures. Minor updates and notational clarification
Primordial non-Gaussianity and the CMB bispectrum
We present a new formalism, together with efficient numerical methods, to
directly calculate the CMB bispectrum today from a given primordial bispectrum
using the full linear radiation transfer functions. Unlike previous analyses
which have assumed simple separable ansatze for the bispectrum, this work
applies to a primordial bispectrum of almost arbitrary functional form, for
which there may have been both horizon-crossing and superhorizon contributions.
We employ adaptive methods on a hierarchical triangular grid and we establish
their accuracy by direct comparison with an exact analytic solution, valid on
large angular scales. We demonstrate that we can calculate the full CMB
bispectrum to greater than 1% precision out to multipoles l<1800 on reasonable
computational timescales. We plot the bispectrum for both the superhorizon
('local') and horizon-crossing ('equilateral') asymptotic limits, illustrating
its oscillatory nature which is analogous to the CMB power spectrum
Contribution of domain wall networks to the CMB power spectrum
We use three domain wall simulations from the radiation era to the late time
dark energy domination era based on the PRS algorithm to calculate the
energy-momentum tensor components of domain wall networks in an expanding
universe. Unequal time correlators in the radiation, matter and cosmological
constant epochs are calculated using the scaling regime of each of the
simulations. The CMB power spectrum of a network of domain walls is determined.
The first ever quantitative constraint for the domain wall surface tension is
obtained using a Markov chain Monte Carlo method; an energy scale of domain
walls of 0.93 MeV, which is close but below the Zel'dovich bound, is
determined.Comment: Submitted to Physics Letters
Primordial non-Gaussianity and Bispectrum Measurements in the Cosmic Microwave Background and Large-Scale Structure
The most direct probe of non-Gaussian initial conditions has come from
bispectrum measurements of temperature fluctuations in the Cosmic Microwave
Background and of the matter and galaxy distribution at large scales. Such
bispectrum estimators are expected to continue to provide the best constraints
on the non-Gaussian parameters in future observations. We review and compare
the theoretical and observational problems, current results and future
prospects for the detection of a non-vanishing primordial component in the
bispectrum of the Cosmic Microwave Background and large-scale structure, and
the relation to specific predictions from different inflationary models.Comment: 82 pages, 23 figures; Invited Review for the special issue "Testing
the Gaussianity and Statistical Isotropy of the Universe" for Advances in
Astronom
General CMB and Primordial Bispectrum Estimation I: Mode Expansion, Map-Making and Measures of f_NL
We present a detailed implementation of two bispectrum estimation methods
which can be applied to general non-separable primordial and CMB bispectra. The
method exploits bispectrum mode decompositions on the domain of allowed
wavenumber or multipole values. Concrete mode examples constructed from
symmetrised tetrahedral polynomials are given, demonstrating rapid convergence
for known bispectra. We use these modes to generate simulated CMB maps of high
resolution (l > 2000) given an arbitrary primordial power spectrum and
bispectrum or an arbitrary late-time CMB angular power spectrum and bispectrum.
By extracting coefficients for the same separable basis functions from an
observational map, we are able to present an efficient and general f_NL
estimator for a given theoretical model. The estimator has two versions
comparing theoretical and observed coefficients at either primordial or late
times, thus encompassing a wider range of models, including secondary
anisotropies, lensing and cosmic strings. We provide examples and validation of
both f_NL estimation methods by direct comparison with simulations in a
WMAP-realistic context. In addition, we show how the full bispectrum can be
extracted from observational maps using these mode expansions, irrespective of
the theoretical model under study. We also propose a universal definition of
the bispectrum parameter F_NL for more consistent comparison between
theoretical models. We obtain WMAP5 estimates of f_NL for the equilateral model
from both our primordial and late-time estimators which are consistent with
each other, as well as with results already published in the literature. These
general bispectrum estimation methods should prove useful for the analysis of
nonGaussianity in the Planck satellite data, as well as in other contexts.Comment: 41 pages, 17 figure
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