INTRODUCTION

From the point of view of a present-day observer in the Solar System, the time scale of the universe can be split into two major subdivisions—presolar and solar—as shown schematically in Figure 8.1. The presolar period begins with the big bang at time to (as measured from the present) and ends with the isolation from the Interstellar Medium (ISM) at time tt of a small portion of the dust and gas in the ISM to form the Solar System. The immediately following solar era begins at time ti with the formation of the solar nebula as a separate entity, and ends at the present time. Within these two broad time zones, one can distinguish discrete events and protracted processes even if some of the processes are yet unfamiliar. For example, the presolar era includes the cosmological or primordial synthesis of hydrogen (H) and helium (He) immediately after the Big Bang, formation of galaxies, synthesis of heavier elements from H and He in the interior of stars, and its termination in the material destined to form the Solar System. The solar era comprises the collapse of the isolated gas cloud into a central massive body (the potential sun) and a thin disc around the central object, condensation of the material in the disc into chemical compounds at tc, aggregation and accretion of the chemical species into the planets and other objects of the solar system at ta, and subsequent internal evolution of individual planets to the present time.

As geologists recognized the immensity of geologic and, hence, the solar time from observations of large scale geologic structures and processes, astronomers and astrophysicists relied on observations of large scale structures in the universe, like stars, star clusters, and galaxies to infer an even longer presolar time scale. The best known of such indirect dating methods is the mutual recession of galaxies, as evidenced by their spectral red shift. The most precise result, so far, for the Big Bang, as based on the rate of expansion of the universe, is 13.4 ±1.6 Ga (Lineweaver, 1999).