arXiv subjects

Alain Blanchard

Publications and source records attributed to Alain Blanchard.

57 records · Page 4Linked to original sources

arXiv1996

On the inevitability of reionization: implications for cosmic microwave background fluctuations

Early photoionization of the intergalactic medium is discussed in a nearly model-independent way, in order to investigate whether early structures corresponding to rare Gaussian peaks in a CDM model can photoionize the intergalactic medium sufficiently early to appreciably smooth out the microwave background fluctuations. We conclude that this is indeed possible for a broad range of CDM normalizations and is almost inevitable for unbiased CDM, provided that the bulk of these early structures are quite small, no more massive than about 10^8 solar masses. Typical parameter values predict that reionization occurs around z=50, thereby suppressing fluctuations on degree scales while leaving the larger angular scales probed by COBE reasonably unaffected. However, for non-standard CDM, incorporating mixed dark matter, vacuum density or a tilted primordial power spectrum, early reionization plays no significant role.

astro-ph↗

arXiv1996

The Sunyaev-Zel'dovich Effect as a Probe of $Ω_{o}$

We examine how the Sunyaev-Zel'dovich (SZ) effect of clusters can probe $Ω_{o}$. Using self-consistent models of X-ray clusters in the context of hierarchical models of structure formation, we show that both the mean Compton {\it y} parameter and the number of clusters observed via the SZ effect strongly depend on $Ω_{o}$. These quantities are higher in open cosmological models due to the earlier epoch of structure formation. Specifically, we compare two models which are able to reproduce the present abundance of X-ray clusters, one for $Ω_{o}=0.2$ and the other $Ω= 1$. For $Ω_{o}=0.2$, ${\ymean}$ exceeds the current FIRAS limit of $1.5\times10^{-5}$. Also, the SZ redshift source distribution considerably differs in the two cosmological models. Thus, these results show that both an improvement in our knowledge of CMB spectral distortions and the realization of millimeter surveys to look for SZ sources over a large area of the sky should produce interesting constraints on $Ω_{o}$ and on the evolution of the baryonic fractionof virialized structures.

astro-ph↗

arXiv1995

The Significance of the Cosmic Virial Theorem

We discuss the development and physics of the cosmic virial theorem and its traditional application as an indicator of the cosmic mean density. The standard result that the density must be sub-critical relies on the assumption that the galaxy three-point correlation function accurately describes the mass distribution around a typical pair of galaxies. To challenge this assumption, we develop a simple model of this mass distribution based on the extension of galactic halos. In such a model, one sees clearly the importance of the local mass distribution around pairs to the prediction of the galaxy pair-wise velocity dispersion. The model provides insight into the mechanics of the theorem and demonstrates that a flat universe is {\it consistent} with the observations. More generally, we conclude that the relative pair-wise velocity dispersion probes the mass clustered around galaxies and that the observations, viewed in this light, indicate a lower bound of $\sim 300\; h^{-1}$kpc on the radius of galactic halos.

astro-ph↗