| MadSci Network: Astronomy |
Hi Andrew,
Yes, the Doppler shift observed in radiation is very similar to the change in pitch that you hear, for example in the siren of a passing ambulance. Most introductory physics or astronomy texts will have good illustrations of what is going on.
When astronomers look at spectra from galaxies outside of our own, they find redshifts that increase with distance away from our own Galaxy. The simplest explanation for this, and the one that is generally accepted, is that the universe as a whole is expanding, carrying the galaxies along with it.
Other explanations have been proposed. Gravity can also lead to a Doppler shift in photons. If a photon has to "climb out" of a strong gravitational field, then an outside observer will see it redshifted. Something like this has been proposed as a possible explanation of the redshifts seen in the light from other galaxies.
There are a lot of problems with that explanation, though. Why, for example, would the redshift be the same for all galaxies at a given distance from us? There is no reason to expect them all to have gravitational fields of the same strength. Also, why would the gravitational redshift increase with distance?
So, any alternate picture would have to provide a natural way for all photons travelling the same distance to experience the same redshift, and by an amount that increases uniformly with the distance that they travel. Such theories can almost certainly be formulated, but they would not be as simple as the expanding universe hypothesis.
Another difficulty faced by any alternate theory would be to explain the cosmic microwave background radiation (CMB). This radiation originated early in the universe, when it had expanded just enough to become "transparent." After that point, the radiation could flow freely, while before that everything was opaque. When the radiation first began to freely expand, its temperature was several thousand degrees. Since that time, the universe has expanded so far that the radiation we see is redshifted into the microwave region, and has a temperature of about 3 degrees Kelvin, very close to the predictions of the first Big Bang model of the universe. In fact, the discovery of the CMB, combined with the redshift observations of galaxies, are generally taken to clinch the Big Bang theory.
(You'll notice that I've played fast and loose in switching between time and space above. This is because of the finite speed of light. We see objects that are further away from us as they were at an earlier time in the age of the universe. When we observe the CMB, we are seeing a signal from the universe at the earliest time possible--before that, everything was opaque and unobservable--and hence it is the most distant thing that we can see.)