## Cosmology in the early s

The book Cosmology by Hermann Bondi (in two editions, Bondi 1952 and 1960a) gives a good picture of research in this subject at the time they were written. Bondi reported the vigorous debate on the relative merits of the steady state and relativistic big bang cosmologies, and he assessed the state of observational tests of these and the other ideas then under discussion about the large-scale nature of the universe. He also painted a vivid picture of the role of the philosophies that explicitly or implicitly inform our approaches to theory and observation.

Bondi surveyed a broad range of fundamental issues about the basis for physical cosmology. Is the universe really close to homogeneous in the large-scale average? Though astronomers were not talking much about it, they knew that the nearby galaxies are distributed in a decidedly clumpy fashion. Are the redshifts of the galaxies really due to the expansion of the universe, as opposed to a "tired light" effect? Perhaps, as Zwicky (1929) had remarked, light tends to shift toward longer wavelength as it moves across the immense distances between the galaxies. If the redshift is in fact an effect of expansion, how do we know the universe is evolving, as opposed to the idea that continual creation of matter is keeping it in a steady state? If the universe is evolving were the laws of microscopic physics really the same now and in the remote past, when the universe is supposed to have been so very different? In particular, is gravity now and in the past well described by general relativity theory? If the relativistic cosmological model were a good approximation, what would be reasonable values for its parameters? Does it make sense to allow a possible role for Einstein's cosmological constant, A (with the property illustrated in Figure 2.1)? Einstein and de Sitter (1932) noted that a realistic model for a homogeneous universe requires the mass density term in equation (G.1), but the observations then did not require nonzero values for either of the other two unknowns, the space curvature and cosmological constant terms. The Einstein-de Sitter model makes the simplifying assumption that we keep just the one term we know is required. Einstein's feeling about the A term appears in the second edition of The Meaning of Relativity (Einstein 1945, p. 111), where he added the comment that the cosmological constant is "a complication of the theory, which seriously reduces its logical simplicity." In the chapter added to the second edition of his book Bondi (1960a) took note of the "outstanding simplicity" of the Einstein-de Sitter case. But in this case, and other high mass density solutions, the universe expanded from a state of enormously large density, as in Lemaitre's primeval atom. What would the universe have been doing before that? Might we suppose the present expansion followed a bounce that terminated an earlier collapsing state? And if our universe is evolving how might it end, in a big freeze, as in the Einstein-de Sitter case, or a big crunch, as in higher density cases?34

This is a sobering list of issues,35 but it certainly does not mean that the cosmology Bondi described was an empty science. People were assembling observational evidence, in part out of simple curiosity, in part driven by the goal of testing theoretical ideas, and the observations were in turn driving theoretical developments. We consider first lines of research that were largely

34 Bondi (1952, 1960a) does not use the terms "big freeze" and "big crunch," and avoids also "primeval atom" and "big bang." In Gamow (1952) the phrase for the latter is the "big squeeze."

35 A review of the current and accepted answers to most of these questions might commence with the discussion in Chapter 5 of this book.

inspired by the steady state concept, and then the state of ideas about fossils from a big bang.

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