The Beginning and Evolution of the Universe

It is the current opinion of many physicists that the Universe is well described by what Fred Hoyle termed a Big Bang Model, in which the Universe expanded from a denser hotter childhood to its current adolescence, with a present energy budget dominated by dark energy and less so by dark matter, neither of which have been detected in the laboratory, with the stuff biological systems, planets, stars, and all visible matter are made of (called baryonic matter by cosmologists) being a very small tracer on this dark sea, and with electromagnetic radiation being an even less significant contributor. Galaxies and groups and clusters of galaxies are locally distributed inhomogeneously in space, but on large enough scales and in a statistical sense the distribution approaches isotropy. This is supported by other electromagnetic distributions such as the X-ray and cosmic microwave backgrounds, which are close to isotropic. As one looks out further into space, as a consequence of the finite speed of light, one sees objects as they were at earlier times, and there is clear observational evidence for temporal evolution in the distribution of various objects such as galaxies.

At earlier times the Universe was hotter and denser, at some stage so hot that atoms could not exist. Nuclear physics reactions between protons, neutrons, etc., in the cooling expanding Universe resulted in the (nucleo)synthesis of the lighter elements (nuclei) such as D, 4 He, and 7 Li, with abundances in good accord with what is observed, and with the photons left over forming a residual cosmic microwave background (CMB) also in good agreement with what is observed.

Given initial inhomogeneities in the mass distribution at an earlier time, processing of these by the expansion of the Universe, gravitational instability, pressure gradients, and microphysical processes, gives rise to observed anisotropies in the CMB and the current large-scale distribution of nonrelativistic matter; the situation on smaller spatial scales, where galaxies form, is murkier. Observations indicate that the needed initial inhomogeneities are most likely of the special form known as scale invariant, and that the simplest best-fitting Big Bang Model has flat spatial geometry. These facts could be the consequence of a simple inflationary infancy of the Universe, a very early period of extremely rapid expansion, which stretched zero-point quantum-mechanical fluctuations to larger length scales and transmuted them into the needed classical inhomogeneities in the mass-energy distribution. At the end of the inflationary expansion all radiation and matter is generated as the Universe moves into the usual Big Bang Model epoch. Inflation has roots in models of very high-energy physics. Because of electromagnetic charge screening, gravity is the dominant large-scale force. General relativity is the best theory of gravity.

This review attempts to elaborate on this picture. Given the Tantalus principle of cosmology (and most of astrophysics), that one can see but not “touch” -which makes this a unique field of physics -there have been many false starts and even much confusion and many missed opportunities along what most now feel is the right track. Given space constraints we cannot do justice to what are now felt to be false starts, nor will we discuss more than one or two examples of confusion and missed opportunities. We attempt here to simply describe what is now thought to be a reasonable standard model of cosmology and trace the development of what are now felt to be the important threads in this tapestry; time will tell whether our use of “reasonable standard” is more than just youthful arrogance (or possibly middle-aged complacence).

In the following sections we review the current standard model of cosmology, with emphasis in parts on some historical roots, citing historically significant and more modern papers as well as review articles. We begin with discussion of the foundations of the Big Bang Model in Sec. 2, which summarizes research in the half century from Einstein’s foundational paper on modern cosmology until the late 1960’s discovery of the CMB radiation, as well as some loose ends. Section 3 discusses inflation, which provides an explanation of the Big Bang that is widely felt to be reasonable. Dark energy and dark matter, the two (as yet not directly detected) main components of the energy budget of the present Universe are reviewed in Sec. 4. Further topics include the growth of structure in the Universe (Sec. 5), observations of large-scale structure in the Universe (Sec. 6), and estimates of cosmological parameters (Sec. 7). We conclude in Sec. 8 with a discussion of what are now thought to be relevant open questions and directions in which the field appears to be moving.

We use hardly any mathematical equations in this review. In some cases this results in disguising the true technical complexity of the issues we discuss.

We exclude from this review a n

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