The Redshift Trap

Shortly after stars were first seen in galaxies, confirming that they are outside our galaxy, Edwin Hubble and others in 1929 discovered that the redshift of light from nearby galaxies was proportional to the distance as calculated from apparent brightness of Cepheid variable stars within the galaxies^[1].  This is called Hubble’s Law and the proportionality constant is the Hubble Constant.  Because a redshift had been noted earlier in stars within our galaxy and had been attributed to movement of the source stars away from us, it was natural to assume, based on Hubble’s observations, that redshift of nearby galaxies was also caused by movement away from us.

This phenomenon is known as the Doppler Effect and is attributed to the fact that each wave of light is emitted just a little farther away as the source recedes, thus “stretching” the light to longer (redder) wavelengths.  Since farther is redder, farther must be faster by the Doppler Effect.

Since galaxies are light years distant we are seeing them as they appeared in the past.  Were the stars in the past moving faster than those in more recent times?  At first it appeared to be so.  Was the effect caused by the universe slowing down with time?  If the expansion is slowing down, could it eventually stop and then start to contract?  Instead, almost from the beginning, due to preconceived mathematically based theories postulating a beginning from a much smaller size, the redshift was seen as an expansion of the universe, not as contracting or slowing.  But what could explain the acceleration into the past?

After Einstein had defined space as being space-time, astronomers started to think of empty space as a thing the way the preceding generation talked about space filling aether.  Some theoretical astronomers, i.e. cosmologists, decided that the space between galaxies was expanding making more distant objects only appear to be moving faster.  (Like raisins on rising bread, all are moving at the same rate, but the expanding spaces between add up so that farther appears to be faster.) They never offered to explain the expansion of space; they just assumed it as a given.

After redshifts were found that indicated speeds near the speed of light, Hubble doubted that recessional speed was responsible for the redshift of galaxies.  In later years, he speculated about the intergalactic medium interacting with the light by gravitation or magnetism, etc. rather than expansion, as the cause of the redshift.  He is credited with discovering the expanding universe and thus the Big Bang, but after his earlier work, he spent the rest of his life working to refute it[2].

“[If the redshifts are a Doppler shift] … the observations as they stand lead to the anomaly of a closed universe, curiously small and dense, and, it may be added, suspiciously young. On the other hand, if redshifts are not Doppler effects, these anomalies disappear and the region observed appears as a small,homogeneous, but insignificant portion of a universe extended indefinitely both in space and time”

                             — E. Hubble, Roy. Astron. Soc. M. N., 17, 506, 1937

Link:  Hubble and red shift by Vincent Sauvé

[1] “A Relationship Between Distance and Radial Velocity Among Extra-Galactic Nebulae,” Edwin Hubble, Proceedings of the National Academy of Science, Vol. 15, 168, 1929.

[2] “The Problem of the Expanding Universe,” Edwin Hubble, American Scientist, Vol. 30, April 1942, No. 2