MadSci Network: Astronomy

Re: Is expansion the only explanation for redshift/distance relationship?

Date: Mon Jan 29 07:38:25 2001
Posted By: Nial Tanvir, Faculty, Astrophysics, University of Hertfordshire
Area of science: Astronomy
ID: 979681694.As

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
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 
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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