27,197 research outputs found
Bias, redshift space distortions and primordial nongaussianity of nonlinear transformations: application to Lyman alpha forest
On large scales a nonlinear transformation of matter density field can be
viewed as a biased tracer of the density field itself. A nonlinear
transformation also modifies the redshift space distortions in the same limit,
giving rise to a velocity bias. In models with primordial nongaussianity a
nonlinear transformation generates a scale dependent bias on large scales. We
derive analytic expressions for these for a general nonlinear transformation.
These biases can be expressed entirely in terms of the one point distribution
function (PDF) of the final field and the parameters of the transformation. Our
analysis allows one to devise nonlinear transformations with nearly arbitrary
bias properties, which can be used to increase the signal in the large scale
clustering limit. We apply the results to the ionizing equilibrium model of
Lyman-alpha forest, in which Lyman-alpha flux F is related to the density
perturbation delta via a nonlinear transformation. Velocity bias can be
expressed as an average over the Lyman-alpha flux PDF. At z=2.4 we predict the
velocity bias of -0.1, compared to the observed value of -0.13 +/- 0.03. Bias
and primordial nongaussianity bias depend on the parameters of the
transformation. Measurements of bias can thus be used to constrain these
parameters, and for reasonable values of the ionizing background intensity we
can match the predictions to observations. Matching to the observed values we
predict the ratio of primordial nongaussianity bias to bias to have the
opposite sign and lower magnitude than the corresponding values for the highly
biased galaxies, but this depends on the model parameters and can also vanish
or change the sign.Comment: 18 pages, 1 figur
Deterministic dense coding and entanglement entropy
We present an analytical study of the standard two-party deterministic
dense-coding protocol, under which communication of perfectly distinguishable
messages takes place via a qudit from a pair of non-maximally entangled qudits
in pure state |S>. Our results include the following: (i) We prove that it is
possible for a state |S> with lower entanglement entropy to support the sending
of a greater number of perfectly distinguishable messages than one with higher
entanglement entropy, confirming a result suggested via numerical analysis in
Mozes et al. [Phys. Rev. A 71 012311 (2005)]. (ii) By explicit construction of
families of local unitary operators, we verify, for dimensions d = 3 and d=4, a
conjecture of Mozes et al. about the minimum entanglement entropy that supports
the sending of d + j messages, j = 2, ..., d-1; moreover, we show that the j=2
and j= d-1 cases of the conjecture are valid in all dimensions. (iii) Given
that |S> allows the sending of K messages and has the square roof of c as its
largest Schmidt coefficient, we show that the inequality c <= d/K, established
by Wu et al. [ Phys. Rev. A 73, 042311 (2006)], must actually take the form c <
d/K if K = d+1, while our constructions of local unitaries show that equality
can be realized if K = d+2 or K = 2d-1.Comment: 19 pages, 2 figures. Published versio
Parametric study of relaminarization of turbulent boundary layers on nozzle walls
By means of comparisons between theoretical predictions and experimental data, the accuracy of a boundary procedure to predict the effect of large streamwise accelerations upon initially turbulent boundary layers is assessed. The boundary layer procedure is based upon simultaneous solution of the boundary layer partial differential equations and the integral turbulence kinetic energy equation. The results of the present investigation show the ability of the procedure to accurately predict properties of boundary layers subjected to large streamwise accelerations. The procedure is used to conduct a parametric study of the effect of free stream turbulence, heat transfer, Reynolds number, acceleration, and Mach number on boundary layers in supersonic nozzles to assist in the design of a quiet tunnel. Results of the investigation show that, even in the presence of moderate free-stream turbulence levels, the boundary layer in the approach section of the quiet tunnel nozzle relaminarizes and becomes thin enough to be removed by a small slot in the nozzle wall. Furthermore, the calculations indicate that it should be possible to maintain a laminar boundary layer for the entire length of the supersonic portion of the quiet tunnel nozzle
How to measure redshift-space distortions without sample variance
We show how to use multiple tracers of large-scale density with different
biases to measure the redshift-space distortion parameter
beta=f/b=(dlnD/dlna)/b (where D is the growth rate and a the expansion factor),
to a much better precision than one could achieve with a single tracer, to an
arbitrary precision in the low noise limit. In combination with the power
spectrum of the tracers this allows a much more precise measurement of the
bias-free velocity divergence power spectrum, f^2 P_m - in fact, in the low
noise limit f^2 P_m can be measured as well as would be possible if velocity
divergence was observed directly, with rms improvement factor ~[5.2(beta^2+2
beta+2)/beta^2]^0.5 (e.g., ~10 times better than a single tracer for beta=0.4).
This would allow a high precision determination of f D as a function of
redshift with an error as low as 0.1%. We find up to two orders of magnitude
improvement in Figure of Merit for the Dark Energy equation of state relative
to Stage II, a factor of several better than other proposed Stage IV Dark
Energy surveys. The ratio b_2/b_1 will be determined with an even greater
precision than beta, producing, when measured as a function of scale, an
exquisitely sensitive probe of the onset of non-linear bias. We also extend in
more detail previous work on the use of the same technique to measure
non-Gaussianity. Currently planned redshift surveys are typically designed with
signal to noise of unity on scales of interest, and are not optimized for this
technique. Our results suggest that this strategy may need to be revisited as
there are large gains to be achieved from surveys with higher number densities
of galaxies.Comment: 22 pages, 13 figure
Standard and non-standard primordial neutrinos
The standard cosmological model predicts the existence of a cosmic neutrino
background with a present density of about 110 cm^{-3} per flavour, which
affects big-bang nucleosynthesis, cosmic microwave background anisotropies, and
the evolution of large scale structures. We report on a precision calculation
of the cosmic neutrino background properties including the modification
introduced by neutrino oscillations. The role of a possible
neutrino-antineutrino asymmetry and the impact of non-standard
neutrino-electron interactions on the relic neutrinos are also briefly
discussed.Comment: 4 pages, no figures. Contribution to the proceedings of SNOW 2006,
Stockholm, May 2-6, 2006. Typos corrected, updated reference
Kaleidoscope laser
We report the first calculations of mode patterns of unstable-cavity lasers with truly two-dimensional transverse geometries. A detailed account of numerical techniques, incorporating a nonorthogonal beam-propagation method, and results for cavities with a range of transverse symmetries, such as regular polygonal and rhomboid, are presented. In view of the beautiful complexity of the eigenmodes predicted, a novel kaleidoscope laser is proposed
Large-scale Correlation of Mass and Galaxies with the Lyman-alpha Forest Transmitted Flux
We present predictions of the correlation between the Lyman-alpha forest
absorption in quasar spectra and the mass within \sim 5 Mpc/h (comoving) of the
line of sight, using fully hydrodynamic and hydro-PM numerical simulations of
the cold dark matter model supported by present observations. The observed
correlation based on galaxies and the Lya forest can be directly compared to
our theoretical results, assuming that galaxies are linearly biased on large
scales. Specifically, we predict the average value of the mass fluctuation,
, conditioned to a fixed value of the Lya forest transmitted flux
delta_F, after they have been smoothed over a 10 Mpc/h cube and line of sight
interval, respectively. We find that /sigma_m as a function of
delta_F/sigma_F has a slope of 0.6 at this smoothing scale, where sigma_m and
sigma_F are the rms dispersions (this slope should decrease with the smoothing
scale). We show that this value is largely insensitive to the cosmological
model and other Lya forest parameters. Comparison of our predictions to
observations should provide a fundamental test of our ideas on the nature of
the Lya forest and the distribution of galaxies, and can yield a measurement of
the bias factor of any type of galaxies that are observed in the vicinity of
Lya forest lines of sight.Comment: Submitted to ApJ, 41 page
Further development of a method for computing three-dimensional subsonic viscous flows in turbofan lobe mixers
Procedure for computing subsonic, turbulent flow in turbofan lobe mixers was extended to allow consideration of flow fields in which a swirl component of velocity may be present. Additional, an optional k-lambda turbulence model was added to the procedure. The method of specifying the initial flow field was also modified, allowing parametric specification or radial secondary flow velocities, and making it possible to consider initial flow fields which have significant inlet secondary flow vorticity. A series of example calculations was performed which demonstrate the various capabilities of the modified code. These calculations demonstrate the effects of initial secondary flows of various magnitudes, the effects of swirl, and the effects of turbulence model on the mixing process. The results of these calculations indicate that the initial secondary flows, presumed to be generated within the lobes, play a dominant role in the mixing process, and that the predicted results are relatively insensitive to the turbulence model used
A very deep Chandra observation of Abell 1795: The Cold Front and Cooling Wake
We present a new analysis of very deep Chandra observations of the galaxy
cluster Abell 1795. Utilizing nearly 750 ks of net ACIS imaging, we are able to
resolve the thermodynamic structure of the Intracluster Medium (ICM) on length
scales of ~ 1 kpc near the cool core. We find several previously unresolved
structures, including a high pressure feature to the north of the BCG that
appears to arise from the bulk motion of Abell 1795's cool core. To the south
of the cool core, we find low temperature (~ 3 keV), diffuse ICM gas extending
for distances of ~ 50 kpc spatially coincident with previously identified
filaments of H-alpha emission. Gas at similar temperatures is also detected in
adjacent regions without any H-alpha emission. The X-ray gas coincident with
the H-alpha filament has been measured to be cooling spectroscopically at a
rate of ~ 1 Solar Masses/ yr, consistent with measurements of the star
formation rate in this region as inferred from UV observations, suggesting that
the star formation in this filament as inferred by its H and UV
emission can trace its origin to the rapid cooling of dense, X-ray emitting
gas. The H-alpha filament is not a unique site of cooler ICM, however, as ICM
at similar temperatures and even higher metallicities not cospatial with
H emission is observed just to the west of the H-alpha filament,
suggesting that it may have been uplifted by Abell 1795's central active
galaxy. Further simulations of cool core sloshing and AGN feedback operating in
concert with one another will be necessary to understand how such a dynamic
cool core region may have originated and why the H-alpha emission is so
localized with respect to the cool X-ray gas despite the evidence for a
catastrophic cooling flow.Comment: 14 Pages, 10 Figures, Resubmitted to ApJ after first referee report,
Higher Resolution Figures available upon reques
Anomalous Thermoelectric power of over-doped Bi2Sr2CaCu2O8 superconductor
Temperature dependence of thermoelectric power S(T) of three differently
processed Bi2Sr2CaCu2O8 (Bi2212) samples, viz. as-processed melt quenched
(Bi2212-MQ), 6000C N2-annealed (Bi2212-N2) and 6000C O2-annealed (Bi2212-O2) is
reported here. All the samples possess single-phase character and their
superconducting transition temperatures (TcR=0) are 85 K, 90 K and 72 K
respectively for Bi2212-MQ, Bi2212-N2 and Bi2212-O2. While Bi2212-MQ and
Bi2212-N2 samples are in near optimum doping regime, Bi2212-O2 is an over-doped
sample. TcS=0 values obtained through S(T) data are also in line with those
deduced from the temperature dependence of resistance and DC magnetization.
Interestingly, S(T) behaviour of the optimally-doped Bi2212-MQ and Bi2212-N2
samples is seen to be positive in whole temperature range, it is found negative
for the over-doped Bi2212-O2 sample above TcS=0. These results have been seen
in the light of the recent band structure calculations and the ensuing split
Fermi surface as determined by angle-resolved photoelectron spectroscopy
(ARPES).Comment: 11 Pages Text + Figs: comments welcome ([email protected]
- âŠ