1,414 research outputs found
Anisotropy of the Microwave Sky at 90 GHz: Results from Python II
We report on additional observations of degree scale anisotropy at 90~GHz
from the Amundsen-Scott South Pole Station in Antarctica. Observations during
the first season with the Python instrument yielded a statistically significant
sky signal; in this paper we report the confirmation of that signal with data
taken in the second year, and on results from an interleaving set of fields.Comment: 10 pages, plus 2 figures. Postscript and uufiles versions available
via anonymous ftp at ftp://astro.uchicago.edu/pub/astro/ruhl/pyI
Three-dimensional Gross-Pitaevskii solitary waves in optical lattices: stabilization using the artificial quartic kinetic energy induced by lattice shaking
In this Letter, we show that a three-dimensional Bose-Einstein solitary wave
can become stable if the dispersion law is changed from quadratic to quartic.
We suggest a way to realize the quartic dispersion, using shaken optical
lattices. Estimates show that the resulting solitary waves can occupy as little
as -th of the Brillouin zone in each of the three directions and
contain as many as atoms, thus representing a \textit{fully
mobile} macroscopic three-dimensional object.Comment: 8 pages, 1 figure, accepted in Phys. Lett.
A Signature of Cosmic Strings Wakes in the CMB Polarization
We calculate a signature of cosmic strings in the polarization of the cosmic
microwave background (CMB). We find that ionization in the wakes behind moving
strings gives rise to extra polarization in a set of rectangular patches in the
sky whose length distribution is scale-invariant. The length of an individual
patch is set by the co-moving Hubble radius at the time the string is
perturbing the CMB. The polarization signal is largest for string wakes
produced at the earliest post-recombination time, and for an alignment in which
the photons cross the wake close to the time the wake is created. The maximal
amplitude of the polarization relative to the temperature quadrupole is set by
the overdensity of free electrons inside a wake which depends on the ionization
fraction inside the wake. The signal can be as high as
in degree scale polarization for a string at high redshift (near recombination)
and a string tension given by .Comment: 8 pages, 3 figure
Primordial helium recombination. I. Feedback, line transfer, and continuum opacity
Precision measurements of the cosmic microwave background temperature anisotropy on scales ℓ>500 will be available in the near future. Successful interpretation of these data is dependent on a detailed understanding of the damping tail and cosmological recombination of both hydrogen and helium. This paper and two companion papers are devoted to a precise calculation of helium recombination. We discuss several aspects of the standard recombination picture, and then include feedback, radiative transfer in He i lines with partial redistribution, and continuum opacity from H i photoionization. In agreement with past calculations, we find that He ii recombination proceeds in Saha equilibrium, whereas He i recombination is delayed relative to Saha due to the low rates connecting excited states of He i to the ground state. However, we find that at z<2200 the continuum absorption by the rapidly increasing H i population becomes effective at destroying photons in the He i 21Po-11S line, causing He i recombination to finish around z≃1800, much earlier than previously estimated
Brueckner-Goldstone perturbation theory for the half-filled Hubbard model in infinite dimensions
We use Brueckner-Goldstone perturbation theory to calculate the ground-state
energy of the half-filled Hubbard model in infinite dimensions up to fourth
order in the Hubbard interaction. We obtain the momentum distribution as a
functional derivative of the ground-state energy with respect to the bare
dispersion relation. The resulting expressions agree with those from
Rayleigh-Schroedinger perturbation theory. Our results for the momentum
distribution and the quasi-particle weight agree very well with those obtained
earlier from Feynman-Dyson perturbation theory for the single-particle
self-energy. We give the correct fourth-order coefficient in the ground-state
energy which was not calculated accurately enough from Feynman-Dyson theory due
to the insufficient accuracy of the data for the self-energy, and find a good
agreement with recent estimates from Quantum Monte-Carlo calculations.Comment: 15 pages, 8 fugures, submitted to JSTA
Strong-coupling approach to the Mott--Hubbard insulator on a Bethe lattice in Dynamical Mean-Field Theory
We calculate the Hubbard bands for the half-filled Hubbard model on a Bethe
lattice with infinite coordination number up to and including third order in
the inverse Hubbard interaction. We employ the Kato--Takahashi perturbation
theory to solve the self-consistency equation of the Dynamical Mean-Field
Theory analytically for the single-impurity Anderson model in multi-chain
geometry. The weight of the secondary Hubbard sub-bands is of fourth order so
that the two-chain geometry is sufficient for our study. Even close to the
Mott--Hubbard transition, our results for the Mott--Hubbard gap agree very well
with those from numerical Dynamical Density-Matrix Renormalization Group
(DDMRG) calculations. The density of states of the lower Hubbard band also
agrees very well with DDMRG data, apart from a resonance contribution at the
upper band edge which cannot be reproduced in low-order perturbation theory.Comment: 40 pages, 7 figure
The Feynman propagator for spin foam quantum gravity
We link the notion causality with the orientation of the 2-complex on which
spin foam models are based. We show that all current spin foam models are
orientation-independent, pointing out the mathematical structure behind this
independence. Using the technology of evolution kernels for quantum
fields/particles on Lie groups/homogeneous spaces, we construct a generalised
version of spin foam models, introducing an extra proper time variable and
prove that different ranges of integration for this variable lead to different
classes of spin foam models: the usual ones, interpreted as the quantum gravity
analogue of the Hadamard function of QFT or as a covariant definition of the
inner product between quantum gravity states; and a new class of causal models,
corresponding to the quantum gravity analogue of the Feynman propagator in QFT,
non-trivial function of the orientation data, and implying a notion of
''timeless ordering''.Comment: RevTex, 5 pages, no figures; v2-3:minor typos correcte
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