18,109 research outputs found
Why Not Consider Closed Universes?
We consider structure formation and CMB anisotropies in a closed universe,
both with and without a cosmological constant. The CMB angular power spectrum
and the matter transfer function are presented, along with a discussion of
their relative normalization. This represents the first full numerical
evolution of density perturbations and anisotropies in a spherical geometry. We
extend the likelihood function vs. Omega from the COBE 2-year data to Omega>=1.
For large Omega the presence of a very steep rise in the spectrum towards low
ell allows us to put an upper limit of Omega<=1.5 (95%CL) for primordial
spectra with n<=1. This compares favorably with existing limits on Omega. We
show that there are a range of closed models which are consistent with
observational constraints while being even older than the currently popular
flat models with a cosmological constant. Future constraints from degree scale
CMB data may soon probe this region of parameter space. A derivation of the
perturbed Einstein, fluid and Boltzmann equations for open and closed
geometries is presented in an appendix.Comment: 24 pages, including 13 figures in a uuencoded self-unpacking shell
script. Submitted to Ap
Perturbative analysis on infrared aspects of noncommutative QED on R^4
Here we examine the noncommutative counterpart of QED, which is called as
noncommutative QED. The theory is obtained by examining the consistent minimal
coupling to noncommutative U(1) gauge field. The *-product admits the coupling
of the matter with only three varieties of charges, i.e., 0, +1 and -1.
Ultraviolet divergence can be absorbed into the rescaling of the fields and the
parameters at least at one loop level. To examine the infrared aspect of the
theory the anomalous magnetic dipole moment is calculated. The dependence on
the direction of photon momentum reflects the Lorentz symmetry violation of the
system. The explicit calculation of the finite part of the photon vacuum
polarization shows the singularity ln({q C^TC q}) (C^{\mu\nu} is a
noncommutative parameter.) in the infrared side which also exists in
noncommutative Yang-Mills theory. It is associated with the ultraviolet
behavior of the theory. We also consider the extension to chiral gauge theory
in the present context, but the requirement of anomaly cancellation allows only
noncommutative QED.Comment: 10 pages, LaTEX2e, A part of results changed, reference adde
A Characteristic Scale on the Cosmic Microwave Sky
The current suite of results from Cosmic Microwave Background anisotropy
experiments is fulfilling the promise of providing extraordinary levels of
discrimination between cosmological models. We calculate a binned anisotropy
power spectrum, which we tabulate, along with error bars and bin-to-bin
correlations, so that it can be easily used for constraining models. The
resulting power spectrum is flat at large angles, with a gradual rise to a
prominent peak at around 0.5 degrees and a decrease thereafter. This is
precisely the shape predicted by inflationary-inspired adiabatic models. Within
that class of cosmologies, this characteristic scale imprinted on the CMB sky
can be used to infer that the geometry of the Universe is very close to flat.
The next wave of CMB results should add fuel to the debate about whether or not
the Universe once inflated, as well as beginning in earnest the task of
measuring cosmological parameters.Comment: 6 pages, 1 figure. A less technical article based on the same work
has appeared in Science Perspectives under the title "How Flat is the
Universe?" (Science, Mar 24, 2000, 2171-2172
Topology and Dark Energy: Testing Gravity in Voids
Modified gravity has garnered interest as a backstop against dark matter and
dark energy (DE). As one possible modification, the graviton can become
massive, which introduces a new scalar field - here with a Galileon-type
symmetry. The field can lead to a nontrivial equation of state (EOS) of DE
which is density-and-scale-dependent. Tension between Type Ia supernovae and
Planck could be reduced. In voids the scalar field dramatically alters the EOS
of DE, induces a soon-observable gravitational slip between the two metric
potentials, and develops a topological defect (domain wall) due to a nontrivial
vacuum structure for the field.Comment: Revised version, added detail, conclusions unchanged, matches PRL
published version in content. 4 pages, 2 figure
"Sample Variance" in Small-Scale CMB Anisotropy Experiments
We discuss the effects of finite sky coverage and the uncertainty in
extracting information about the power spectrum from experiments on small
angular scales. In general the cosmic variance is enhanced by a factor of
, where is the solid angle sampled by the experiment. As a rough
guide, an experiment with sensitivity peaking at the th multipole has to
cover \simgt\ell independent patches to have a lower ``sample variance'' than
for a whole-sky measurement of the quadrupole. Our approach gives a relatively
simple way of attaching an error bar to the theoretical prediction for a
particular experiment, and thereby comparing theories with experimental
results, without the need for computationally-intensive Maximum Likelihood or
Monte Carlo calculations.Comment: 6 pages in Plain TeX with 1 postscript figure appende
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