326 research outputs found
Generation of Circular Polarization of the Cosmic Microwave Background
The standard cosmological model, which includes only Compton scattering
photon interactions at energy scales near recombination, results in zero
primordial circular polarization of the cosmic microwave background. In this
paper we consider a particular renormalizable and gauge-invariant standard
model extension coupling photons to an external vector field via a Chern-Simons
term, which arises as a radiative correction if gravitational torsion couples
to fermions. We compute the transport equations for polarized photons from a
Boltzmann-like equation, showing that such a coupling will source circular
polarization of the microwave background. For the particular coupling
considered here, the circular polarization effect is always negligible compared
to the rotation of the linear polarization orientation, also derived using the
same formalism. We note the possibility that limits on microwave background
circular polarization may probe other photon interactions and related
fundamental effects such as violations of Lorentz invariance.Comment: 20 pages. Revised version includes an explicit calculation of gauge
invariance. Text reorganized to improve clarity, and references adde
The Sunyaev-Zeldovich Effect from Quasar Feedback
The observed relationship between X-ray luminosity and temperature of the
diffuse intercluster medium clearly shows the effect of nongravitational
heating on the formation of galaxy clusters. Quasar feedback into the
intergalactic medium can potentially be an important source of heating, and can
have significant impact on structure formation. This feedback process is a
source of thermal Sunyaev-Zel'dovich distortions of the cosmic microwave
background. Using a simple one-dimensional Sedov-Taylor model of energy
outflow, we calculate the angular power spectrum of the temperature distortion,
which has an amplitude on the order of one micro-Kelvin. This signal will be at
the noise limit of upcoming arcminute-scale microwave background experiments,
including the Atacama Cosmology Telescope and the South Pole Telescope, but
will be directly detectable with deep exposures by the Atacama Large Millimeter
Array or by stacking many microwave images.Comment: The discussion of detectability is expanded. Matches the ApJ Letters
accepted versio
A Limit on the Polarized Anisotropy of the Cosmic Microwave Background at Subdegree Angular Scales
A ground-based polarimeter, PIQUE, operating at 90 GHz has set a new limit on
the magnitude of any polarized anisotropy in the cosmic microwave background.
The combination of the scan strategy and full width half maximum beam of 0.235
degrees gives broad window functions with average multipoles, l = 211+294-146
and l = 212+229-135 for the E- and B-mode window functions, respectively. A
joint likelihood analysis yields simultaneous 95% confidence level flat band
power limits of 14 and 13 microkelvin on the amplitudes of the E- and B-mode
angular power spectra, respectively. Assuming no B-modes, a 95% confidence
limit of 10 microkelvin is placed on the amplitude of the E-mode angular power
spectrum alone.Comment: 4 pages, 3 figures, submitted to Astrophysical Journal Letter
Green's function for gravitational waves in FRW spacetimes
A method for calculating the retarded Green's function for the gravitational
wave equation in Friedmann-Roberson-Walker spacetimes, within the formalism of
linearized Einstein gravity is developed. Hadamard's general solution to
Cauchy's problem for second-order, linear partial differential equations is
applied to the FRW gravitational wave equation. The retarded Green's function
may be calculated for any FRW spacetime, with curved or flat spatial sections,
for which the functional form of the Ricci scalar curvature is known. The
retarded Green's function for gravitational waves propagating through a
cosmological fluid composed of both radiation and dust is calculated
analytically for the first time. It is also shown that for all FRW spacetimes
in which the Ricci scalar curvatures does not vanish, , the Green's
function violates Huygens' principle; the Green's function has support inside
the light-cone due to the scatter of gravitational waves off the background
curvature.Comment: 9 pages, FERMILAB-Pub-93/189-
Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters
Future high-resolution microwave background measurements hold the promise of
detecting galaxy clusters throughout our Hubble volume through their
Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number
density of galaxy clusters is highly sensitive to cluster mass through
fluctuations in the matter power spectrum, as well as redshift through the
comoving volume and the growth factor. This sensitivity in principle allows
tight constraints on such quantities as the equation of state of dark energy
and the neutrino mass. We evaluate the ability of future cluster surveys to
measure these quantities simultaneously when combined with PLANCK-like CMB
data. Using a simple effective model for uncertainties in the cluster mass-SZ
flux relation, we evaluate systematic shifts in cosmological constraints from
cluster SZ surveys. We find that a systematic bias of 10% in cluster mass
measurements can give rise to shifts in cosmological parameter estimates at
levels larger than the statistical errors. Systematic errors are
unlikely to be detected from the mass and redshift dependence of cluster number
counts alone; increasing survey size has only a marginal effect. Implications
for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted
versio
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X-Ray Study of Pressure-Collapsed Fullerite
X-ray-diffraction studies are described for a new phase of carbon called collapsed fullerite (CF) that was produced by application of high pressure to fullerite (C). At 20 GPa there is an irreversible transition to a phase that has neither the (111) Bragg peak of diamond nor any of the Bragg peaks associated with the fcc phase of C. The spectrum of CF is flat and featureless in the range of study.Engineering and Applied Science
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Evidence of Annealing Effects on a High-Density Si/SiO2 Interfacial Layer
Thermally grown Si(001)/SiO samples were studied by x-ray reflectivity. Fits of model electron density profiles to the data reveal the existence of an interfacial layer at the Si/SiO interface up to 15-Ă
-thick, with density higher than either the crystalline Si or the main oxide layer. This density of the layer is reduced by a postoxidation anneal.Engineering and Applied Science
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X-Ray Grazing Incidence Diffraction from Alkylsiloxane Monolayers on Silicon Wafers
Xâray reflection (both specular and offâspecular) and grazing incidence diffraction (GID) have been used to study the structure of alkylsiloxane monolayers () formed by selfâassembly from solution on silicon wafers. GID studies of complete monolayers reveal a single ring of scattering associated with the monolayer. The Lorentzian line shape of this ring indicates that the film is characterized by liquidlike order, with a typical translational correlation length of about 45 Ă
. The thermal coefficient of expansion of the monolayer, as determined from the GID peak position, is approximately equal to the value for liquid nâalkanes. Upon either heating or cooling, the monolayer correlation lengths decrease, suggesting that the differential thermalâexpansion coefficients of the film and substrate figure prominently in thermal changes of the molecular ordering. GID data for incomplete monolayers also reveal a single ring of scattering associated with the monolayer. While both the translational correlation lengths and integrated peak areas are significantly reduced relative to complete monolayers, the peak positions of the incomplete monolayers are comparable to those of complete monolayers. Given the lower average areal density of incomplete monolayers, this finding implies that incomplete monolayers are inhomogeneous.Engineering and Applied Science
Mass Models for Spiral Galaxies from 2-D Velocity Maps
We model the mass distributions of 40 high surface brightness spiral galaxies
inside their optical radii, deriving parameters of mass models by matching the
predicted velocities to observed velocity maps. We use constant mass-to-light
disk and bulge models, and we have tried fits with no halo and with three
different halo density profiles. The data require a halo in most, but not all,
cases, while in others the best fit occurs with negligible mass in the luminous
component, which we regard as unphysical. All three adopted halo profiles lead
to fits of about the same quality, and our data therefore do not constrain the
functional form of the halo profile. The halo parameters display large
degeneracies for two of the three adopted halo functions, but the separate
luminous and dark masses are better constrained. However, the fitted disk and
halo masses vary substantially between the adopted halo models, indicating that
even high quality 2-D optical velocity maps do not provide significant
constraints on the dark matter content of a galaxy. We demonstrate that data
from longslit observations are likely to provide still weaker constraints. We
conclude that additional information is needed in order to constrain the
separate disk and halo masses in a galaxy.Comment: 41 pages, 13 figures, accepted for publication in A
Production of Gravitational Waves in the nMSSM
During a strongly first-order phase transition gravitational waves are
produced by bubble collisions and turbulent plasma motion. We analyze the
relevant characteristics of the electroweak phase transition in the nMSSM to
determine the generated gravitational wave signal. Additionally, we comment on
correlations between the production of gravitational waves and baryogenesis. We
conclude that the gravitational wave relic density in this model is generically
too small to be detected in the near future by the LISA experiment. We also
consider the case of a "Standard Model" with dimension-six Higgs potential,
which leads to a slightly stronger signal of gravitational waves.Comment: 29 pages, 7 figures; published version, some comments adde
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