A buyer’s guide to the Hubble constant

Since the expansion of the universe was first established by Edwin Hubble and Georges Lemaître about a century ago, the Hubble constant $$H_0$$ H 0 which measures its rate has been of great interest to astronomers. Besides being interesting in its own right, few properties of the universe can be deduced without it. In the last decade, a significant gap has emerged between different methods of measuring it, some anchored in the nearby universe, others at cosmological distances. The SH0ES team has found $$H_0 = 73.2 \pm 1.3 \; \;\,\hbox {kms}^{-1} \,\hbox {Mpc}^{-1}$$ H 0 = 73.2 ± 1.3 kms - 1 Mpc - 1 locally, whereas the value found for the early universe by the Planck Collaboration is $$H_0 = 67.4 \pm 0.5 \; \;\,\hbox {kms}^{-1} \,\hbox {Mpc}^{-1}$$ H 0 = 67.4 ± 0.5 kms - 1 Mpc - 1 from measurements of the cosmic microwave background. Is this gap a sign that the well-established $${\varLambda} {\text{CDM}}$$ Λ CDM cosmological model is somehow incomplete? Or are there unknown systematics? And more practically, how should humble astronomers pick between competing claims if they need to assume a value for a certain purpose? In this article, we review results and what changes to the cosmological model could be needed to accommodate them all. For astronomers in a hurry, we provide a buyer’s guide to the results, and make recommendations.

Homogeneity and Clustering

This chapter traces the history of the development of ideas on the large-scale structure of the universe. Modern discussions of the nature of the large-scale matter distribution can be traced back to three central ideas. In 1917, Albert Einstein argued that a closed homogeneous world model fits very well into general relativity theory and the requirements of Mach's principle. In 1926, Edwin Hubble showed that the large-scale distribution of galaxies is close to uniform with no indication of an edge or boundary. In 1927, Georges Lemaître showed that the uniform distribution of galaxies fits very well with the pattern of galaxy redshifts. The chapter then assesses several questions. The first is whether the universe really is homogeneous. Could the homogeneity of the universe have been deduced ahead of time from general principles? Or might it be a useful guide to new principles? It also asks how clustering evolves in an expanding universe, what its origin is, and what this reveals about the nature of the universe.

The Big Bang: discovery

A very small number of very smart individuals discovered the Big Bang (as it became known) and its aftermath. The fact that the stars are all moving away from us in a structured velocity-versus-distance relationship as first discovered by Edwin Hubble is simply fascinating, almost as fascinating as learning that just prior to this, most everyone—including Einstein himself—assumed that the stars were static, non-moving objects in the sky, just sitting there.

The Discovery of the Expansion of the Universe

Alexander Friedmann, Carl Wilhelm Wirtz, Vesto Slipher, Knut E. Lundmark, Willem de Sitter, Georges H. Lemaître, and Edwin Hubble all contributed to the discovery of the expansion of the universe. If only two persons are to be ranked as the most important ones for the general acceptance of the expansion of the universe, the historical evidence points at Lemaître and Hubble, and the proper answer to the question, “Who discovered the expansion of the universe?”, is Georges H. Lemaître.