INTRODUCTION

The variety and relative abundances of the 90 elements in the Solar System must have been produced somewhere else before its formation about 4.6 by ago. The earliest time some or all of them could have been produced somewhere is 13.7 billion of years ago when the universe is believed to have originated in a spectacular explosion—Big Bang—of what is known as a singularity in cosmological parlance. At least three sets of observations support the Big Bang theory.

1. Expanding universe Hubble discovered that galaxies are receding from each other at a speed proportional to the distance between them, based on red shifts of light coming from distant galaxies. This implies that at some time in the past all the matter in the universe must have been concentrated at one point.

2. Big Bang nucleosynthesis According to the Standard Cosmological model, the matter in the universe 30 minutes after the explosion must have consisted mostly of hydrogen (H) and helium (He) with only traces of deuterium (2H) and helium (3He), as confirmed by recent observations.

3. Cosmic microwave background Theory also predicts that the universe must now be filled with an isotropic background radiation in the microwave region of the electromagnetic spectrum as a relic of the initial radiation from the Big Bang. Penzias and Wilson (Penzias and Wilson, 1965) have indeed detected such a cosmic microwave background radiation corresponding to black body emission at 2.7 K.

The predicted sequence of events within the first 3 minutes of the Big Bang, according to the Standard Cosmological model, is given in Table 3.1.

STELLAR NUCLEOSYNTHESIS

For geologists used to events on a time scale of at least a few thousand years, incredibly short time intervals like 10–50, 10–43, and 10–35s make no sense. Even a comparatively much longer time interval like 10–10 s is so short that light with a velocity of as high as 3,00,000 kilometres per second will cover just about 3 centimetres in this interval. The important point for geologists to note is that Big Bang, or cosmological nucleosynthesis, was restricted to only H and He and to within the first 3 minutes.