International Journal of Modern Physics D

The detected multiplicity of main-sequence and pre-main-sequence stars along with the emerging evidence for binary and multiple protostars, imply that stars may ultimately form by fragmentation of collapsing molecular cloud cores. These discoveries, coupled with recent observational knowledge of the structure of dense cloud cores and of the properties of young binary stars, provide serious constraints to the theory of star formation. Most theoretical progress in the field of star formation is largely based on numerical calculations of the early collapse and fragmentation of protostellar clouds. Although these models have been quite successful at predicting the formation of binary protostars, a direct comparison between theory and observations has not yet been established. The results of recent observations as well as of early and recent analytic and numerical models, on which the present theory of star formation is based, are reviewed here in a self-consistent manner.

Friedmann–Robertson–Walker universes with a presently large fraction of the energy density stored in an X-component with w_X<-1/3, are considered. We find all the critical points of the system for constant equations of state in that range. We consider further several background quantities that can distinguish the models with different w_X values. Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z~1. Therefore, it will require accurate measurements in the range 1<z<2 and independent accurate knowledge of Ω_m,0 (and/or Ω_X,0) in order to resolve a variable w_X from a constant w_X.

A lower limit for a neutral black hole size is obtained in the frames of the string gravity model with the second order curvature correction. It is shown that this effect remains when the third order curvature correction is also taken into account and argued that such restriction does exist in all perturbative orders of curvature expansions.

Quintessence is often invoked to explain the universe's acceleration suggested by type Ia supernovae observations. The aim of this letter is to study the validity of using a constant equation of state for quintessence models. We shall show that this hypothesis strongly constraint the form of the scalar potential.

The Burkert profile is a competing candidate for the analytic approximation of virialized dark halos especially when dark matter particles have a finite cross-section for elastic collisions. In this paper we reexamine its universality in massive systems, using an ensemble of 45 nearby X-ray clusters and 20 distant X-ray/lensing clusters. Despite the fact that this empirical profile turns out a great success on galactic scales and also reproduces approximately the X-ray observed surface brightness profiles of clusters, the dark matter cores of clusters predicted by the Burkert profile are too large to be reconciled with the strong gravitational lensing measurements. Specifically, the typical dark halo cores of clusters represented by the Burkert profile are about 0.2 Mpc, and only a small fraction (~1/4) of clusters can have compact cores smaller than 0.1 Mpc. This will constitute a critical challenge to the Burkert profile as a universal dark matter density law over entire mass ranges.

We propose a power filter G_p for linear reconstruction of the CMB signal from one-dimensional scans of observational maps. This G_p filter preserves the power spectrum of the CMB signal in contrast to the Wiener filter which diminishes the power spectrum of the reconstructed CMB signal. We demonstrate how peak statistics and a cluster analysis can be used to estimate the probability of the presence of a CMB signal in observational records. The efficiency of the G_p filter is demonstrated on a toy model of an observational record consisting of a CMB signal and noise in the form of foreground point sources.