882 research outputs found
Constraining dark energy with Sunyaev-Zel'dovich cluster surveys
We discuss the prospects of constraining the properties of a dark energy
component, with particular reference to a time varying equation of state, using
future cluster surveys selected by their Sunyaev-Zel'dovich effect. We compute
the number of clusters expected for a given set of cosmological parameters and
propogate the errors expected from a variety of surveys. In the short term they
will constrain dark energy in conjunction with future observations of type Ia
supernovae, but may in time do so in their own right.Comment: 5 pages, 3 figures, 1 table, version accepted for publication in PR
Reconstruction Analysis of Galaxy Redshift Surveys: A Hybrid Reconstruction Method
In reconstruction analysis of galaxy redshift surveys, one works backwards
from the observed galaxy distribution to the primordial density field in the
same region, then evolves the primordial fluctuations forward in time with an
N-body code. This incorporates assumptions about the cosmological parameters,
the properties of primordial fluctuations, and the biasing relation between
galaxies and mass. These can be tested by comparing the reconstruction to the
observed galaxy distribution, and to peculiar velocity data. This paper
presents a hybrid reconstruction method that combines the `Gaussianization''
technique of Weinberg(1992) with the dynamical schemes of Nusser & Dekel(1992)
and Gramann(1993). We test the method on N-body simulations and on N-body mock
catalogs that mimic the depth and geometry of the Point Source Catalog Redshift
Survey and the Optical Redshift Survey. This method is more accurate than
Gaussianization or dynamical reconstruction alone. Matching the observed
morphology of clustering can limit the bias factor b, independent of Omega.
Matching the cluster velocity dispersions and z-space distortions of the
correlation function xi(s,mu) constrains the parameter beta=Omega^{0.6}/b.
Relative to linear or quasi-linear approximations, a fully non-linear
reconstruction makes more accurate predictions of xi(s,mu) for a given beta,
thus reducing the systematic biases of beta measurements and offering further
scope for breaking the degeneracy between Omega and b. It also circumvents the
cosmic variance noise that limits conventional analyses of xi(s,mu). It can
also improve the determination of Omega and b from joint analyses of redshift
& peculiar velocity surveys as it predicts the fully non-linear peculiar
velocity distribution at each point in z-space.Comment: 72 pages including 33 figures, submitted to Ap
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
Self-Consistency and Calibration of Cluster Number Count Surveys for Dark Energy
Cluster number counts offer sensitive probes of the dark energy if and only
if the_evolution_ of the cluster mass versus observable relation(s) is well
calibrated. We investigate the potential for internal calibration by demanding
consistency in the counts as a function of the observable. In the context of a
constant dark energy equation of state, known initial fluctuation amplitude
expected from the CMB, universal underlying mass function, and an idealized
selection, we find that the ambiguity from the normalization of the
mass-observable relationships, or an extrapolation of external mass-observable
determinations from higher masses, can be largely eliminated with a
sufficiently deep survey, even allowing for an arbitrary evolution. More
generally, number counts as a function of both the redshift and the observable
enable strong consistency tests on assumptions made in modelling the
mass-observable relations and cosmology.Comment: 4 pages, 3 figures, submitted to PRD rapid communication
Probing dark energy with cluster counts and cosmic shear power spectra: including the full covariance
(Abridged) Combining cosmic shear power spectra and cluster counts is
powerful to improve cosmological parameter constraints and/or test inherent
systematics. However they probe the same cosmic mass density field, if the two
are drawn from the same survey region, and therefore the combination may be
less powerful than first thought. We investigate the cross-covariance between
the cosmic shear power spectra and the cluster counts based on the halo model
approach, where the cross-covariance arises from the three-point correlations
of the underlying mass density field. Fully taking into account the
cross-covariance as well as non-Gaussian errors on the lensing power spectrum
covariance, we find a significant cross-correlation between the lensing power
spectrum signals at multipoles l~10^3 and the cluster counts containing halos
with masses M>10^{14}Msun. Including the cross-covariance for the combined
measurement degrades and in some cases improves the total signal-to-noise
ratios up to plus or minus 20% relative to when the two are independent. For
cosmological parameter determination, the cross-covariance has a smaller effect
as a result of working in a multi-dimensional parameter space, implying that
the two observables can be considered independent to a good approximation. We
also discuss that cluster count experiments using lensing-selected mass peaks
could be more complementary to cosmic shear tomography than mass-selected
cluster counts of the corresponding mass threshold. Using lensing selected
clusters with a realistic usable detection threshold (S/N~6 for a ground-based
survey), the uncertainty on each dark energy parameter may be roughly halved by
the combined experiments, relative to using the power spectra alone.Comment: 32 pages, 15 figures. Revised version, invited original contribution
to gravitational lensing focus issue, New Journal of Physic
On the cosmological mass function theory
This paper provides, from one side, a review of the theory of the
cosmological mass function from a theoretical point of view, starting from the
seminal paper of Press & Shechter (1974) to the last developments (Del Popolo &
Gambera (1998, 1999), Sheth & Tormen 1999 (ST), Sheth, Mo & Tormen 2001 (ST1),
Jenkins et al. 2001 (J01), Shet & Tormen 2002 (ST2), Del Popolo 2002a, Yagi et
al. 2004 (YNY)), and from another side some improvements on the multiplicity
function models in literature. ...Comment: Astronomy Reports, in prin
Redshift space correlations and scale-dependent stochastic biasing of density peaks
We calculate the redshift space correlation function and the power spectrum
of density peaks of a Gaussian random field. In the linear regime k < 0.1
h/Mpc, the redshift space power spectrum is
P^s_{pk}(k,u) = exp(-f^2 s_{vel}^2 k^2 u^2) * [b_{pk}(k) + b_{vel}(k) f
u^2]^2 * P_m(k), where u is the angle with respect to the line of sight,
s_{vel} is the one-dimensional velocity dispersion, f is the growth rate, and
b_{pk}(k) and b_{vel}(k) are k-dependent linear spatial and velocity bias
factors. For peaks, the value of s_{vel} depends upon the functional form of
b_{vel}. The peaks model is remarkable because it has unbiased velocities --
peak motions are driven by dark matter flows -- but, in order to achieve this,
b_{vel} is k-dependent. We speculate that this is true in general: k-dependence
of the spatial bias will lead to k-dependence of b_{vel} even if the biased
tracers flow with the dark matter. Because of the k-dependence of the linear
bias parameters, standard manipulations applied to the peak model will lead to
k-dependent estimates of the growth factor that could erroneously be
interpreted as a signature of modified dark energy or gravity. We use the
Fisher formalism to show that the constraint on the growth rate f is degraded
by a factor of two if one allows for a k-dependent velocity bias of the peak
type. We discuss a simple estimate of nonlinear evolution and illustrate the
effect of the peak bias on the redshift space multipoles. For k < 0.1 h/Mpc,
the peak bias is deterministic but k-dependent, so the configuration space bias
is stochastic and scale dependent, both in real and redshift space. We provide
expressions for this stochasticity and its evolution (abridged).Comment: 24 pages, 7 figures (v3): references added (v4): added
figure+appendix. In press in PR
Footprints of Statistical Anisotropies
We propose and develop a formalism to describe and constrain statistically
anisotropic primordial perturbations. Starting from a decomposition of the
primordial power spectrum in spherical harmonics, we find how the temperature
fluctuations observed in the CMB sky are directly related to the coefficients
in this harmonic expansion. Although the angular power spectrum does not
discriminate between statistically isotropic and anisotropic perturbations, it
is possible to define analogous quadratic estimators that are direct measures
of statistical anisotropy. As a simple illustration of our formalism we test
for the existence of a preferred direction in the primordial perturbations
using full-sky CMB maps. We do not find significant evidence supporting the
existence of a dipole component in the primordial spectrum.Comment: 26 pages, 5 double figures. Uses RevTeX
Planck intermediate results. XXIX. All-sky dust modelling with Planck, IRAS, and WISE observations
We present all-sky modelling of the high resolution Planck, IRAS, and WISE
infrared (IR) observations using the physical dust model presented by Draine
and Li in 2007 (DL). We study the performance and results of this model, and
discuss implications for future dust modelling. The present work extends the DL
dust modelling carried out on nearby galaxies using Herschel and Spitzer data
to Galactic dust emission. We employ the DL dust model to generate maps of the
dust mass surface density, the optical extinction Av, and the starlight
intensity parametrized by Umin. The DL model reproduces the observed spectral
energy distribution (SED) satisfactorily over most of the sky, with small
deviations in the inner Galactic disk and in low ecliptic latitude areas. We
compare the DL optical extinction Av for the diffuse interstellar medium with
optical estimates for 2 10^5 quasi-stellar objects (QSOs) observed in the Sloan
digital sky survey. The DL Av estimates are larger than those determined
towards QSOs by a factor of about 2, which depends on Umin. The DL fitting
parameter Umin, effectively determined by the wavelength where the SED peaks,
appears to trace variations in the far-IR opacity of the dust grains per unit
Av, and not only in the starlight intensity. To circumvent the model
deficiency, we propose an empirical renormalization of the DL Av estimate,
dependent of Umin, which compensates for the systematic differences found with
QSO observations. This renormalization also brings into agreement the DL Av
estimates with those derived for molecular clouds from the near-IR colours of
stars in the 2 micron all sky survey. The DL model and the QSOs data are used
to compress the spectral information in the Planck and IRAS observations for
the diffuse ISM to a family of 20 SEDs normalized per Av, parameterized by
Umin, which may be used to test and empirically calibrate dust models.Comment: Final version that has appeared in A&
Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
Using precise full-sky observations from Planck, and applying several methods
of component separation, we identify and characterize the emission from the
Galactic "haze" at microwave wavelengths. The haze is a distinct component of
diffuse Galactic emission, roughly centered on the Galactic centre, and extends
to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining
the Planck data with observations from the WMAP we are able to determine the
spectrum of this emission to high accuracy, unhindered by the large systematic
biases present in previous analyses. The derived spectrum is consistent with
power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding
free-free emission as the source and instead favouring hard-spectrum
synchrotron radiation from an electron population with a spectrum (number
density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the
microwave haze morphology is consistent with that of the Fermi gamma-ray "haze"
or "bubbles," indicating that we have a multi-wavelength view of a distinct
component of our Galaxy. Given both the very hard spectrum and the extended
nature of the emission, it is highly unlikely that the haze electrons result
from supernova shocks in the Galactic disk. Instead, a new mechanism for
cosmic-ray acceleration in the centre of our Galaxy is implied.Comment: 15 pages, 9 figures, submitted to Astronomy and Astrophysic
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