In his great summary (The Problem of the Expanding Universe) of the development of the velocity-distance relation, the Big Bang and all that,  Edwin Hubble offers concise appraisal of the research to this point (in which Hubble and his assistant Milton Humason were prominent members of its founding studies). Hubble provided the first experimental data supporting the expanding universe theory of Georges Lemaitre’s cosmology (the “hypothesis of the primeval atom”), Einstein’s GRT and the mathematical equations of Alex. Friedmann, finding that the distance of galaxies were proportional to their redshifts (Hubble’s Law, 1929).  The whole thing enjoyed a bigger, more explosive life once it had been given its  more provocative and user-friendly name by Sir Fred Hoyle, who called the idea “the Big Bang” in 1949.  (Sir Fred was actually attacking the idea and must’ve thought his tag was to be demeaning, but as a proponent of the steady state theory, Hoyle’s plan backfired, a proof that irony exists.) The steady state model was more or less put to sleep when the cosmic microwave background radiation (first proposed by George Gamow, Ralph Alpher and Robert Herman in 1939) was “discovered ” in 1964 by Penzias and Wilson, this providing further experimental confirmation for the Big Bang hypothesis. 

“Meanwhile, on the basis of the evidence now available, apparent discrepancies between theory and observation must be recognized. A choice is presented, as once before in the days of Copernicus, between a strangely small, finite universe and a sensibly infinite universe plus a new principle of nature.”

It is an excellent approximation for a semi-general-public speech (to the Smithsonian Institution crowd during the war in 1943), and reminds me of one of his other, similarly spot-on statements, one of the greatest understatements in 20th century astrophysics The statement appears in a further report on Hubble/Humason studies in The Velocity-Distance Relation Among Extra-Galactic Nebulae  (Mount Wilson Observatory,  1931) ,  a regarding their understanding of the phenomena and the expanding universe, the authors take many steps back, and say in the concluding paragraph:

“…the writers are constrained to describe the apparent velocity-distance displacements without venturing on the interpretation and its cosmological significance”.

They basically lay out the functioning of the universe and the best understanding of how we got to where we are in the cosmos (Vesto Slipher notwithstanding), and how according to their understanding that the universe is expanding and is consistent with all known laws, that they decide to not say anything, um, philosophical, in the light of disrupting the idea of the creation of all things.

Hubble did have a way with words, his own way, though he was able to communicate a difficult series of ideas simply, as we can see in the opening of his Expanding Universe paper: 

“Mathematicians deal with possible worlds, with an infinite number of logically consistent systems. Observers explore the one particular world we inhabit. Between the two stands the theorist He studies possible worlds but only those which are compatible with the information furnished by observers. In other words, theory attempts to segregate the minimum number of possible worlds which must include the actual world we inhabit. Then the observer, with new factual information, attempts to reduce the list still further. And so it goes, observation and theory advancing together toward the common goal of science, knowledge of the structure and behavior of the physical universe.”

And this lovely summation:

“Current theory starts with two fundamental principles : general relativity and the cosmological principle. General relativity states that the geometry of space is determined by the contents of space, and formulates the nature of the relation. Crudely put, the principle states that space is curved in the vicinity of matter, and that the amount of curvature depends upon the amount of matter. Because of the irregular distribution of matter in our world, the small scale structure of space is highly complex. However, if the universe is sufficiently homogeneous on the large scale, we may adopt a general curvature for the universe, or for the observable region as a whole, just as we speak of the general curvature of the earth’s surface, disregarding the mountains and ocean basins. The nature of the spatial curvature, whether it is positive or negative, and the numerical value, is a subject for empirical investigation.”

“The second, or cosmological principle is a pure assumption – the very simple postulate that, on the grand scale, the universe will appear much the same from whatever position it may be explored. In other words, there is no favored position in the universe, no center, no boundaries. If we, on the earth, see the universe expanding in all directions, then any other observer, no matter where he is located, will also see the universe expanding in the same manner. The postulate, it may be added, implies that, on the grand scale, the universe is homogeneous and isotropic – very much the same everywhere and in all directions.”

“Modern cosmological theory attempts to describe the types of universes that are compatible with the two principles, general relativity and the cosmological principle. Profound analysis of the problem leads to the following conclusions. Such universes are unstable. They might be momentarily in equilibrium, but the slightest internal disturbance would destroy the balance, and disturbances must occur. Therefore, these possible worlds are not stationary. They are, in general, either contracting or expanding, although theory in its present form does not indicate either the direction of change or the rate of change. At this point, the theorist turned to the reports of the observers. The empirical law of red shifts was accepted as visible evidence that the universe is expanding in a particular manner and at a known rate. This arose the conception of homogeneous expanding universe of general relativity.”

I like Hubble.

Notes:

The paper begins:

“I PROPOSE to discuss the problem of the expanding universe from the observational point of view. The fact that such a venture is permissible is emphatic evidence that empirical research has definitely entered the field of cosmology. The exploration of space has swept outward in successive waves, first, through the system of the planets, then, through the stellar system, and, finally, into the realm of the nebulae. Today we study a region of space so vast and so homogeneous that it may well be a fair sample of the universe. At any rate, we are justified in adopting the assumption as a working hypothesis and attempting to infer the nature of the universe from the observed characteristics of the sample. One phase of this ambitious project is the observational test of the current theory of the expanding universes of general relativity. ”