6,578 research outputs found
Detecting Baryon Acoustic Oscillations
Baryon Acoustic Oscillations are a feature imprinted in the galaxy
distribution by acoustic waves traveling in the plasma of the early universe.
Their detection at the expected scale in large-scale structures strongly
supports current cosmological models with a nearly linear evolution from
redshift approximately 1000, and the existence of dark energy. Besides, BAOs
provide a standard ruler for studying cosmic expansion. In this paper we focus
on methods for BAO detection using the correlation function measurement. For
each method, we want to understand the tested hypothesis (the hypothesis H0 to
be rejected) and the underlying assumptions. We first present wavelet methods
which are mildly model-dependent and mostly sensitive to the BAO feature. Then
we turn to fully model-dependent methods. We present the most often used method
based on the chi^2 statistic, but we find it has limitations. In general the
assumptions of the chi^2 method are not verified, and it only gives a rough
estimate of the significance. The estimate can become very wrong when
considering more realistic hypotheses, where the covariance matrix of the
measurement depends on cosmological parameters. Instead we propose to use a new
method based on two modifications: we modify the procedure for computing the
significance and make it rigorous, and we modify the statistic to obtain better
results in the case of varying covariance matrix. We verify with simulations
that correct significances are different from the ones obtained using the
classical chi^2 procedure. We also test a simple example of varying covariance
matrix. In this case we find that our modified statistic outperforms the
classical chi^2 statistic when both significances are correctly computed.
Finally we find that taking into account variations of the covariance matrix
can change both BAO detection levels and cosmological parameter constraints
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
A New Statistic for Analyzing Baryon Acoustic Oscillations
We introduce a new statistic omega_l for measuring and analyzing large-scale
structure and particularly the baryon acoustic oscillations. omega_l is a
band-filtered, configuration space statistic that is easily implemented and has
advantages over the traditional power spectrum and correlation function
estimators. Unlike these estimators, omega_l can localize most of the acoustic
information into a single dip at the acoustic scale while also avoiding
sensitivity to the poorly constrained large scale power (i.e., the integral
constraint) through the use of a localized and compensated filter. It is also
sensitive to anisotropic clustering through pair counting and does not require
any binning. We measure the shift in the acoustic peak due to nonlinear effects
using the monopole omega_0 derived from subsampled dark matter catalogues as
well as from mock galaxy catalogues created via halo occupation distribution
(HOD) modeling. All of these are drawn from 44 realizations of 1024^3 particle
dark matter simulations in a 1h^{-1}Gpc box at z=1. We compare these shifts
with those obtained from the power spectrum and conclude that the results
agree. This indicates that any distance measurements obtained from omega_0 and
P(k) will be consistent with each other. We also show that it is possible to
extract the same amount of acoustic information using either omega_0 or P(k)
from equal volume surveys.Comment: 12 pages, 7 figures. ApJ accepted. Edit: Now updated with final
accepted versio
Inédit : Rayon et Gnôle [Essai de définition de la carence idéologique dans le domaine de la forme]
Bilayer Quantum Hall Systems at nuT = 1: Coulomb Drag and the Transition from Weak to Strong Interlayer Coupling
Measurements revealing anomalously large frictional drag at the transition between the weakly and strongly coupled regimes of a bilayer two-dimensional electron system at total Landau level filling factor nuT = 1 are reported. This result suggests the existence of fluctuations, either static or dynamic, near the phase boundary separating the quantized Hall state at small layer separations from the compressible state at larger separations. Interestingly, the anomalies in drag seem to persist to larger layer separations than does interlayer phase coherence as detected in tunneling
Double layer two-dimensional electron systems: Probing the transition from weak to strong coupling with Coulomb drag
Frictional drag measurements revealing anomalously large dissipation at the
transition between the weakly- and strongly-coupled regimes of a bilayer
two-dimensional electron system at total Landau level filling factor
are reported. This result suggests the existence of fluctuations, either static
or dynamic, near the phase boundary separating the quantized Hall state at
small layer separations from the compressible state at larger separations.
Interestingly, the anomalies in drag seem to persist to larger layer
separations than does interlayer phase coherence as detected in tunneling.Comment: 4 pages, 4 figure
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