283 research outputs found
Group-Theoretic Evidence for SO(10) Grand Unification
The hypercharges of the fermions are not uniquely determined in SO(10) grand
unification, but rather depend upon which linear combination of the two U(1)
subgroups of SO(10) > SU(3) X SU(2) X U(1) X U(1) remains unbroken. We show
that, in general, a given hypercharge assignment can be obtained only with very
high-dimensional Higgs representations. The observation that the standard model
is obtained with low-dimensional Higgs representations can therefore be
regarded as further evidence for SO(10) grand unification. This evidence is
independent of the fact that SO(10) > SU(5).Comment: 6 pages, Late
Polarization Diffusion from Spacetime Uncertainty
A model of Lorentz invariant random fluctuations in photon polarization is
presented. The effects are frequency dependent and affect the polarization of
photons as they propagate through space. We test for this effect by confronting
the model with the latest measurements of polarization of Cosmic Microwave
Background (CMB) photons.Comment: 4 pages, 1 figur
{\sc CosmoNet}: fast cosmological parameter estimation in non-flat models using neural networks
We present a further development of a method for accelerating the calculation
of CMB power spectra, matter power spectra and likelihood functions for use in
cosmological Bayesian inference. The algorithm, called {\sc CosmoNet}, is based
on training a multilayer perceptron neural network. We compute CMB power
spectra (up to ) and matter transfer functions over a hypercube in
parameter space encompassing the confidence region of a selection of
CMB (WMAP + high resolution experiments) and large scale structure surveys (2dF
and SDSS). We work in the framework of a generic 7 parameter non-flat
cosmology. Additionally we use {\sc CosmoNet} to compute the WMAP 3-year, 2dF
and SDSS likelihoods over the same region. We find that the average error in
the power spectra is typically well below cosmic variance for spectra, and
experimental likelihoods calculated to within a fraction of a log unit. We
demonstrate that marginalised posteriors generated with {\sc CosmoNet} spectra
agree to within a few percent of those generated by {\sc CAMB} parallelised
over 4 CPUs, but are obtained 2-3 times faster on just a \emph{single}
processor. Furthermore posteriors generated directly via {\sc CosmoNet}
likelihoods can be obtained in less than 30 minutes on a single processor,
corresponding to a speed up of a factor of . We also demonstrate the
capabilities of {\sc CosmoNet} by extending the CMB power spectra and matter
transfer function training to a more generic 10 parameter cosmological model,
including tensor modes, a varying equation of state of dark energy and massive
neutrinos. {\sc CosmoNet} and interfaces to both {\sc CosmoMC} and {\sc
Bayesys} are publically available at {\tt
www.mrao.cam.ac.uk/software/cosmonet}.Comment: 8 pages, submitted to MNRA
Relating gravitational wave constraints from primordial nucleosynthesis, pulsar timing, laser interferometers, and the CMB: implications for the early universe
We derive a general master equation relating the gravitational-wave
observables r and Omega_gw(f). Here r is the tensor-to-scalar ratio,
constrained by cosmic-microwave-background (CMB) experiments; and Omega_gw(f)
is the energy spectrum of primordial gravitational-waves, constrained e.g. by
pulsar-timing measurements, laser-interferometer experiments, and Big Bang
Nucleosynthesis (BBN). Differentiating the master equation yields a new
expression for the tilt d(ln Omega_gw(f))/d(ln f). The relationship between r
and Omega_gw(f) depends sensitively on the uncertain physics of the early
universe, and we show that this uncertainty may be encapsulated (in a
model-independent way) by two quantities: w_hat(f) and nt_hat(f), where
nt_hat(f) is a certain logarithmic average over nt(k) (the primordial tensor
spectral index); and w_hat(f) is a certain logarithmic average over w_tilde(a)
(the effective equation-of-state in the early universe, after horizon
re-entry). Here the effective equation-of-state parameter w_tilde(a) is a
combination of the ordinary equation-of-state parameter w(a) and the bulk
viscosity zeta(a). Thus, by comparing constraints on r and Omega_gw(f), one can
obtain (remarkably tight) constraints in the [w_hat(f), nt_hat(f)] plane. In
particular, this is the best way to constrain (or detect) the presence of a
``stiff'' energy component (with w > 1/3) in the early universe, prior to BBN.
Finally, although most of our analysis does not assume inflation, we point out
that if CMB experiments detect a non-zero value for r, then we will immediately
obtain (as a free by-product) a new upper bound w_hat < 0.55 on the
logarithmically averaged effective equation-of-state parameter during the
``primordial dark age'' between the end of inflation and the start of BBN.Comment: v1: 12 + 6 pages (main text + appendices), 7 figures; v2: fonts fixed
in figure
Interstellar dust in the BOOMERanG maps
Interstellar dust (ISD) emission is present in the mm-wave maps obtained by the BOOMERanG experiment at intermediate and high Galactic latitudes. We find that, while being sub-dominant at the lower frequencies (90,150, 240 GHz), thermal emission from ISD is dominant at 410 GHz, and is well correlated with the IRAS map at 100 ”m. We find also that the angular power spectrum of ISD fluctuations at 410 GHz is a power law, and its level is negligible with respect to the angular power spectrum of the Cosmic Microwave Background (CMB) at 90 and 150 GHz
Cosmic Microwave Background Polarization
Cosmic microwave background (CMB) anisotropy is our richest source of
cosmological information; the standard cosmological model was largely
established thanks to study of the temperature anisotropies. By the end of the
decade, the Planck satellite will close this important chapter and move us
deeper into the new frontier of polarization measurements. Numerous
ground--based and balloon--borne experiments are already forging into this new
territory. Besides providing new and independent information on the primordial
density perturbations and cosmological parameters, polarization measurements
offer the potential to detect primordial gravity waves, constrain dark energy
and measure the neutrino mass scale. A vigorous experimental program is
underway worldwide and heading towards a new satellite mission dedicated to CMB
polarization.Comment: Review given at TAUP 2005; References added; Additional reference
Sky maps without anisotropies in the cosmic microwave background are a better fit to WMAP's uncalibrated time ordered data than the official sky maps
The purpose of this reanalysis of the WMAP uncalibrated time ordered data
(TOD) was two fold. The first was to reassess the reliability of the detection
of the anisotropies in the official WMAP sky maps of the cosmic microwave
background (CMB). The second was to assess the performance of a proposed
criterion in avoiding systematic error in detecting a signal of interest. The
criterion was implemented by testing the null hypothesis that the uncalibrated
TOD was consistent with no anisotropies when WMAP's hourly calibration
parameters were allowed to vary. It was shown independently for all 20 WMAP
channels that sky maps with no anisotropies were a better fit to the TOD than
those from the official analysis. The recently launched Planck satellite should
help sort out this perplexing result.Comment: 11 pages with 1 figure and 2 tables. Extensively rewritten to explain
the research bette
Foregrounds in the BOOMERANG-LDB data: a preliminary rms analysis
We present a preliminary analysis of the BOOMERanG LDB maps, focused on
foregrounds. BOOMERanG detects dust emission at moderately low galactic
latitudes () in bands centered at 90, 150, 240, 410 GHz. At higher
Galactic latitudes, we use the BOOMERanG data to set conservative upper limits
on the level of contamination at 90 and 150 GHz. We find that the mean square
signal correlated with the IRAS/DIRBE dust template is less than 3% of the mean
square signal due to CMB anisotropy
Instrumental and Analytic Methods for Bolometric Polarimetry
We discuss instrumental and analytic methods that have been developed for the
first generation of bolometric cosmic microwave background (CMB) polarimeters.
The design, characterization, and analysis of data obtained using Polarization
Sensitive Bolometers (PSBs) are described in detail. This is followed by a
brief study of the effect of various polarization modulation techniques on the
recovery of sky polarization from scanning polarimeter data. Having been
successfully implemented on the sub-orbital Boomerang experiment, PSBs are
currently operational in two terrestrial CMB polarization experiments (QUaD and
the Robinson Telescope). We investigate two approaches to the analysis of data
from these experiments, using realistic simulations of time ordered data to
illustrate the impact of instrumental effects on the fidelity of the recovered
polarization signal. We find that the analysis of difference time streams takes
full advantage of the high degree of common mode rejection afforded by the PSB
design. In addition to the observational efforts currently underway, this
discussion is directly applicable to the PSBs that constitute the polarized
capability of the Planck HFI instrument.Comment: 23 pages, 11 figures. for submission to A&
Mapping the CMB Sky: The BOOMERANG experiment
We describe the BOOMERanG experiment, a stratospheric balloon telescope
intended to measure the Cosmic Microwave Background anisotropy at angular
scales between a few degrees and ten arcminutes. The experiment has been
optimized for a long duration (7 to 14 days) flight circumnavigating Antarctica
at the end of 1998. A test flight was performed on Aug.30, 1997 in Texas. The
level of performance achieved in the test flight was satisfactory and
compatible with the requirements for the long duration flight.Comment: 11 pages, 6 figure
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