Hubble’s Expanding Universe Model
The work of De Sitter created considerable discussion and agitation within the astronomical community throughout the world. Among those who were deeply attentive to these developments was Edwin Hubble. Hubble had been present in 1914 when Vesto Slipher first announced his observations concerning the motions of the galaxies before a meeting of the American Astronomical Society. Many years later, in 1928, Hubble undertook systematic research at the renowned Mount Wilson Observatory, endeavoring to bring together De Sitter’s theoretical conception of an expanding universe with Slipher’s empirical observations indicating that galaxies were receding.
Hubble’s reasoning proceeded in the following manner. If the universe were indeed expanding, then the galaxies within it should be moving away from one another. Furthermore, the greater the separation between galaxies, the more rapidly they should be receding. From this it would follow that an observer situated at any point— including the Earth—should perceive that all other galaxies are moving away, and that, on average, the more distant a galaxy is, the greater should be the velocity of its recession.
With this hypothesis in mind, Hubble undertook careful investigation to determine whether such a relationship could be detected. He found that there appeared to be a proportional connection between the distances of galaxies and the extent of their red shifts. The majority of galaxies, he observed, exhibited red shifts, and those judged to be farther away generally displayed larger red shifts.
This, however, gives rise to a most perplexing question: by what means did Hubble determine the distances of the galaxies? This problem presented great difficulty for Hubble, and it remains a formidable challenge even for astronomers of the present day. There are, after all, no physical measuring rods that extend to the stars. The fundamental approach is therefore indirect. One begins by employing various methods to estimate the distances of nearby stars. Then, step by step, one constructs what is known as a “cosmic distance ladder,” by which estimates may be made for the distances of certain galaxies. If one can further make an assessment of the intrinsic brightness of galaxies, then unknown distances may be inferred by comparing apparent brightness with known cases, in accordance with the inverse square law.
The detailed procedures involved in establishing this distance ladder are exceedingly complex and will not be elaborated upon here. It is sufficient to note that they depend upon numerous theoretical interpretations that are burdened with uncertainty and subject to revision, sometimes in unforeseen ways. As the discussion proceeds, the significance of these uncertainties will become evident.
By employing his methods of approximating galactic distances, Hubble established a proportional relationship—now known as Hubble’s law—between the degree of red shift and distance. He concluded that the galaxies most distant from the Earth exhibited the greatest red shifts and were therefore receding at the highest velocities. This conclusion he accepted as compelling evidence that the universe itself is expanding.
In time, this idea became so firmly established that astronomers began to apply it in reverse. If distance is proportional to red shift, then, it was reasoned, one could determine the distance of a galaxy simply by measuring its red shift.
Yet it must be remembered that Hubble’s distance figures were not the result of direct or precise measurements. Rather, they were derived indirectly from observations of apparent galactic brightness. Consequently, the expanding universe model is subject to two potential weaknesses. First, the brightness or dimness of celestial bodies may arise from causes other than distance alone, and thus the distances inferred from such observations may be inaccurate. Second, it is possible that red shift itself may not be directly connected with velocity.
Indeed, a number of astronomers are persuaded that certain red shifts are not produced by a Doppler effect. Some have gone even further, calling into question the very notion of an expanding universe.
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