The situation in the early s

Let us conclude with an overview of this wonderfully tangled situation. Some were aware that the abundance of helium is larger than seemed reasonable in the otherwise promising theory of element formation in stars, and some even recognized that a hot big bang could supply the helium. Others recognized that in a hot big bang universe space now could be filled with a thermal sea of microwave radiation at a temperature a few degrees above absolute zero. The spin temperature of interstellar CN molecules was known by some to be large compared to what might be expected from excitation by particle collisions,

46 The related issues are the effect of the CMBR on the evolving distribution of the matter and the effect that has on the distribution of the CMBR. The basic principles were developed in the 1960s after the identification of the CMBR. We will discuss how gravity disturbs the radiation (Sachs and Wolfe 1967), the coupling of plasma and radiation (Peebles 1965), the dissipative decoupling (Silk 1967, 1968b), and the patterns that decoupling can leave in the distributions of matter and radiation (Peebles and Yu 1970; Sunyaev and Zel'dovich 1970c). Detection of these effects took some three more decades.

and some who knew that had considered the possibility that the molecules are excited by some source of microwave radiation. Others knew how to build a receiver capable of detecting microwave radiation at a temperature a few degrees above zero. And some of them had indications that communications receivers may have detected a near-isotropic component of radiation.

Why were all these pieces not put together until the mid-1960s? It was in part a matter of contingency: it can take time to notice relations among such a broad variety of considerations. But an important fact to bear in mind is that it was not obvious in the early 1960s that these are the relevant set of ideas. In a cold big bang universe, neutrino degeneracy could control helium production during the early stages of expansion. And what actually would happen during the early expansion of a hot big bang universe would depend on the laws of physics, which people suspected then, and still suspect, evolve with the expansion of the universe. The cosmological tests summarized in Section 5.4 show that this evolution could not have significantly affected the early production of helium, but that was not at all obvious in the early 1960s. The steady state cosmology also commanded attention. It was clear enough then that in a steady state universe one could postulate that helium is continually created along with the continual creation of neutrons, protons, and electrons. Perhaps better, helium could be produced in a hypothetical -but physically possible - class of massive exploding stars, and the energy released from the burning of hydrogen to the observed amount of helium could supply a significant sea of radiation.

The essays recall how the community found its way through this thicket of clues and conflicting ideas to the ones that led the way forward, in an iterative consultation of theory and practice. The process tends to be haphazard, and exciting, and on occasion exceedingly awarding. It has yielded deep advances in understanding of the world around us. The essays in the next chapter reveal the beginnings of such an advance, and Chapter 5 describes what grew out of it.

0 0

Post a comment