|MadSci Network: Astronomy|
At one level the Hubble law is just a correlation between two observables, namely redshift and distance, so that's not in dispute. What you are asking, then, is whether the redshift has to be interpretted as a velocity of recession, which in turn implies an expanding universe. To see why most astronomers do interpret redshift as a velocity, let's first think about he history of the subject. Einstein proposed his theory of gravity (General Relativity) in 1915, and realised that it predicted an unstable universe. In 1922 Alexandre Friedmann solved the GR equations for a homogeneous universe, and found that the allowed solutions described universes which were either expanding or contracting. Now, at the time, the theoretical prejudice was for a static universe, so at first it looked like there must be a problem with Friedmann's results. By the late 1920s though, Hubble and others had discovered the Hubble law, so actually theory matched observations very well. Now in science when a theory makes a surprising prediction like that, which is then verified experimentally, it stands the theory in very good stead! People tend to think the theory is a good description of reality until there is compelling evidence to the contrary. Seventy years on, the Friedmann models still seem like a good approximation to reality (and have now passed many other tests, such as prediction of the cosmig microwave background radtion, and prediction of the abundances of light chemical elements in the universe), which is why the expanding universe is regarded as our standard model - our working hypothesis if you will. So we have a neat and pretty complete picture (not to say that there aren't many outstanding problems of course!), but you are right to ask if there aren't other plausible explanations for the redshift phenomenon. A number have been considered, I'll just mention a couple of illustrative ones: 1) gravitational redshifts. GR also predicts that high gravitational fields should produce a redshift - could this explain the Hubble law? The answer is no. The predicted redshifts are small except in the case of very dense objects (black holes etc), and the observed Hubble law and galaxy dynamics are not at all consistent with a gravitational explanation. 2) tired light models - could the photons just lose energy as they travel to us through space? This turns out to be an unattractive idea - firstly we know of no mechanism to make photons tired, secondly it would require a very finely tuned sort of "tiredness" to give the observed spectral changes, and thirdly, there is independent evidence (from supernova light curves) that the time dilation responsible for part of the standard redshift, really exists. In conclusion then, it's always possible that some other explanation for the redshift may be found, but the current interpretation fits remarkably well with other observations and theory, so it would be a big surprise if the standard explanation wasn't broadly correct.
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