VCA 7.A. Hubble's Expanding Universe Model

De Sitter's work caused a stir among astronomers around the world. One of them was Edwin Hubble. Hubble had been present when Slipher had announced his original findings about the motion of galaxies to a meeting of the American Astronomical Society in 1914. In 1928 Hubble set to work at the famous Mt. Wilson observatory in an effort to bring together De Sitter's theory of an expanding universe and Slipher's observations of receding galaxies.

Hubble reasoned like this: In an expanding universe you would expect the galaxies to be moving apart from each other. And the further apart from each other they were, the faster they should be moving apart. This would mean that from any point, including the earth, an observer should see that all other galaxies are moving away and that, on the average, the further away a galaxy is, the faster this motion should be.

Hubble set out to see if this were true and discovered that there seemed to be a proportional relationship between the distance of galaxies and the degree of their red shifts. Most galaxies, he observed, had red shifts, and the greater the distance, the greater the red shift.

This raises a vexing question: How did Hubble know how far away any given galaxy was? That was a very difficult problem for Hubble, and it remains so even for today's astronomers. After all, there are no measuring rods that can reach to the stars. But the basic idea is this: We can begin by using various methods to estimate the distances of nearby stars. Then, proceeding step by step, we can build a "cosmic distance ladder" that gives us estimates of the distances of a few galaxies. If we can find a way of guessing the intrinsic brightness of galaxies, we can then relate unknown galactic distances to known ones by making measurements of apparent galactic brightness. This is according to the inverse square law.

Here we will not go into the details of the complex procedures used to establish this distance ladder. Suffice it to say that they involve many theoretical interpretations that are fraught with uncertainty and subject to revision, often in unexpected ways. This will emerge as we go along.

Hubble, using his methods of approximating distance, established a proportional relationship, now known as Hubble's law, between degree of red shift and distance for galaxies. He believed he had clearly shown that the galaxies most distant from us had the biggest red shifts and were thus receding from us most rapidly. This he took as ample evidence that the universe is expanding.

Eventually this idea became so solidly established that astronomers began to apply it in reverse: If distance is proportional to red shift, then one can measure the distance of galaxies simply by measuring their red shifts.

But as we have noted, Hubble's distance figures are not direct, accurate measurements of how far away galaxies are. Rather, they are derived indirectly from the apparent brightness of the galaxies. Thus the expanding universe model has two potential defects: First, the brightness and dimness of celestial bodies could quite possibly be caused by something other than how far away they are, and thus the distance figures derived from them could be flawed. And second, it is possible that the red shift might not be connected to velocity.

In fact, a number of astronomers are convinced that some red shifts are not caused by a Doppler effect. And some even go so far as to question the very concept of an expanding universe.