Home  About us  Contact
 

Density Perturbations (density + perturbation)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Stability of surface waves at magnetized plasma , metal interface

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 7 2003
Yu. A. Akimov
Abstract The stability of high-frequency potential surface waves at a dense magnetized plasma , metal interface with respect to a low-frequency plasma density modulation is studied in the point of view of the surface waves control. The discussion is addressed to the situation, when an external steady magnetic field is directed perpendicularly to the interface. The nonlinear interaction process of the high-frequency surface wave, its satellites and the low-frequency plasma density perturbation is investigated. It is shown that the low-frequency plasma density perturbation can be represented as a superposition of forced waves of surface and volume types and can lead to an additional attenuation of the surface waves. This attenuation arises when the surface wave amplitude exceeds the threshold value. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The potential of X-ray cluster surveys to constrain primordial non-Gaussianity

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
B. Sartoris
ABSTRACT We present forecasts for constraints on deviations from Gaussian distribution of primordial density perturbations from future high-sensitivity X-ray surveys of galaxy clusters. Our analysis is based on computing the Fisher matrix for number counts and large-scale power spectrum of clusters. The surveys that we consider have high sensitivity and wide area to detect about 2.5 × 105 extended sources, and to provide reliable measurements of robust mass proxies for about 2 × 104 clusters. Based on the so-called self-calibration approach, and including Planck priors in our analysis, we constrain at once nine cosmological parameters and four nuisance parameters, which define the relation between cluster mass and X-ray flux. Because of the scale dependence of large-scale bias induced by local-shape non-Gaussianity, we find that the power spectrum provides strong constraints on the non-Gaussianity fNL parameter, which complement the stringent constraints on the power spectrum normalization, ,8, from the number counts. To quantify the joint constraints on the two parameters, ,8 and fNL, that specify the timing of structure formation for a fixed background expansion, we define the figure of merit . We find that our surveys constrain deviations from Gaussianity with a precision of ,fNL, 10 at 1, confidence level, with FoMSFT, 39. We point out that constraints on fNL are weakly sensitive to the uncertainties in the knowledge of the nuisance parameters. As an application of non-Gaussian constraints from available data, we analyse the impact of positive skewness on the occurrence of XMMU-J2235, a massive distant cluster recently discovered at z, 1.4. We confirm that in a WMAP -7 Gaussian ,CDM cosmology, within the survey volume, , 5 × 10,3 objects like this are expected to be found. To increase the probability of finding such a cluster by a factor of at least 10, one needs to evade either the available constraints on fNL or on the power-spectrum normalization ,8. [source]

The 2dF-SDSS LRG and QSO survey: QSO clustering and the L,z degeneracy

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
J. DaÂngela
ABSTRACT We combine the quasi-stellar object (QSO) samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-Sloan Digital Sky Survey luminous red galaxy (LRG) and QSO Survey (2dF-SDSS LRG and QSO, hereafter 2SLAQ) in order to investigate the clustering of z, 1.5 QSOs and measure the correlation function (,). The clustering signal in redshift-space and projected along the sky direction is similar to that previously obtained from the 2QZ sample alone. By fitting functional forms for ,(,, ,), the correlation function measured along and across the line of sight, we find, as expected, that ,, the dynamical infall parameter and ,0m, the cosmological density parameter, are degenerate. However, this degeneracy can be lifted by using linear theory predictions under different cosmological scenarios. Using the combination of the 2QZ and 2SLAQ QSO data, we obtain: ,QSO(z= 1.5) = 0.60+0.14,0.11, ,0m= 0.25+0.09,0.07 which imply a value for the QSO bias, b(z= 1.4) = 1.5 ± 0.2. The combination of the 2QZ with the fainter 2SLAQ QSO sample further reveals that QSO clustering does not depend strongly on luminosity at fixed redshift. This result is inconsistent with the expectation of simple ,high peaks' biasing models where more luminous, rare QSOs are assumed to inhabit higher mass haloes. The data are more consistent with models which predict that QSOs of different luminosities reside in haloes of similar mass. By assuming ellipsoidal models for the collapse of density perturbations, we estimate the mass of the dark matter haloes which the QSOs inhabit as ,3 × 1012 h,1 M,. We find that this halo mass does not evolve strongly with redshift nor depend on QSO luminosity. Assuming a range of relations which relate halo to black hole mass, we investigate how black hole mass correlates with luminosity and redshift, and ascertain the relation between Eddington efficiency and black hole mass. Our results suggest that QSOs of different luminosities may contain black holes of similar mass. [source]

The cluster abundance in cosmic string models for structure formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
P. P. Avelino
We use the present observed number density of large X-ray clusters to constrain the amplitude of matter density perturbations induced by cosmic strings on the scale of 8 h,1 Mpc (,8), in both open cosmologies and flat models with a non-zero cosmological constant. We find a slightly lower value of ,8 than that obtained in the context of primordial Gaussian fluctuations generated during inflation. This lower normalization of ,8 results from the mild non-Gaussianity on cluster scales, where the one-point probability distribution function is well approximated by a ,2 distribution and thus has a longer tail than a Gaussian distribution. We also show that ,8 normalized using cluster abundance is consistent with the COBE normalization. [source]

The revival of cosmic strings

ANNALEN DER PHYSIK, Issue 4-5 2006
M. Sakellariadou
Abstract Cosmic strings are one-dimensional topological defects which could have been formed in the early stages of our Universe. They triggered a lot of interest, mainly for their cosmological implications: they could offer an alternative to inflation for the generation of density perturbations. It was shown however that cosmic strings lead to inconsistencies with the measurements of the cosmic microwave background temperature anisotropies. The picture has changed recently. It was shown that, on the one hand, cosmic strings can be generically formed in the framework of supersymmetric grand unified theories and that, on the other hand, cosmic superstrings could play the role of cosmic strings. There is also some possible observational support. All this led to a revival of cosmic strings research and this is the topic of my lecture. [source]