Introduction to the Universe the baryonic matter

The major goal of Part I of the book is to present the observed abundances of the elements and isotopes and their variations in space and time, describe the relevant nucleosynthetic processes and outline the evolution of the elements in the Galaxy. In order to provide a context for the observations and models, this chapter is a brief excursion through the Universe, as a unique assembly of matter, and the galaxies within it. Only the small visible baryonic part of the assembly is considered here; it is ~ 0.04 of the total. Small but important, as we are made of it! To appreciate dark matter, dark energy etc., we refer the reader to the recent book by Peacock (1999), and also to the fascinating breakthrough in understanding the Universe following from recent measurements of the microwave background radiation and high-redshift supernovae (e.g. Spergel et al., 2003).

In Fig. 2.1 the major constituents of the evolving Universe are shown schematically together with the relevant interactions and time scales. The lightest elements, H and He, originated simultaneously with the Universe, in a single explosion-like event, the Big Bang, for which the age is now reliably known, 13.7 ± 0.2 Gyr (Section 4.3). From denser fragments of the early expanding Universe galaxies were formed, giant stellar systems comprising most of the baryonic mass of the Universe. The time scale of galaxy formation is ~ 1 Gyr or less, and some models envisage a relatively long infall of intergalactic matter up to several Gyr (Chiappini et al., 2001).

The baryonic matter in a galaxy is divided among the interstellar medium, stars (some with planetary systems) and stellar remnants. Gas (mainly H and He) is the major constituent of the interstellar medium. In some regions of the galaxy the gas is dense and cold compared with others and forms molecular clouds that also generally contain dust. Under certain circumstances a cloud can contract to form a star. At present the average gas-star mass ratio of our Galaxy is between ~ 0.1 and ~ 0.2, and in the early Galaxy the proportion of gas is thought to have been higher. From a mass-balance point of view the compositions of both interstellar clouds

Non-baryonic matter

The Evolution of Matter 111

Non-baryonic matter

Baryonic matter

Baryonic matter

1,2H

3,4He

INTERGALACTIC MEDIUM

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