‘So, what happened after the Big Bang? How can the stars, planet and the human who manufactured me form? What happened to the radiation left after annihilation?’

‘To understand the formation of astronomical objects, we must have a clear concept about the legacy of the Big Bang.’

EVOLUTION OF THE UNIVERSE>AFTER THE BIG BANG>INTRODUCTION

A few minutes after the big bang, the nuclear fireworks ceased. The expansion of the universe continued rather uneventfully afterward for better than 300,000 years. We call this period the radiation era, and it marks the emergence of the primeval fireball.

How do we know that this hot, dense soup of primordial matter and radiation evolved in a quiescent and regular manner? As we saw above, the uniformity and isotropy of the cosmic expansion is theoretically constrained by our direct observations of the ubiquitous and more or less invariant helium abundance throughout the present universe and by observations of a much smaller hut still significant amount of deuterium. Thus, the expansion could not have been too dissimilar from the standard model

But we would like to have direct evidence from this early era of the universe. Of course, we cannot directly observe the primeval fireball and in fact direct observation would have been impossible even by a hypothetical human observer, for the universe did not become transparent until after 300,000 years. Direct observation of the early universe could not be feasible until the density and temperature had fallen to the point at which matter could form and radiation could propagate freely. Before 300,000 years had elapsed, observing the early universe would have been like trying to peer into a dense fog. We do, however, have a means of partially penetrating this fog, but before we describe it, we first turn to an important and pervasive constituent of the early universe that has never been directly detected.