My knowledge of physics and cosmology is limited to some general works on the subject, so I hope more knowledgable Dopers will forgive me if I ask a few naive questions concerning the use of red shift as a means of measuring the distance and relative velocity of other objects in the universe.
Is it generally assumed that Doppler shifting of the emitted light from distant objects is the only significant factor in determining the objects’ velocities and distances?
Is it possible that a significant component of the spectral shifting of a distant object is due to attentuation of the emitted light by intervening matter (I believe this is called the Compton effect)? If so, is there any way of knowing the relative importance of Compton vs. Doppler effect for a given object?
What other means exist to confirm that Doppler shifting is a reasonably accurate measure of distance and relative velocity?
Some form of these questions has been in the back of my mind ever since I first learned about the Doppler Effect in Junior High, but for some reason I’ve never seen them addressed in any of the texts I’ve read.
Redshift is the easiest and usually the only way to measure the radial (line of sight) speed of an object. It does not tell you the distance to the object, unless you invoke Hubble’s Law which says that the farther away the galaxy, the faster it’s travelling away from us. But we need other ways of measuring distance to confirm Hubble’s Law - or to have come up with it in the first place. There are many ways to do this. One is to use certain types of variable stars, whose true brightness can be calculated from the pulsation period. Compare the true brightness to the apparent brightness and you know the distance. (This is what Edwin Hubble did.) Another way is to use certain types of supernovae, whose true brightness can be calculated from the light curve (i.e. how long it takes to reach maximum brightness). These measurements all confirm that more distant galaxies have higher redshift, and hence are travelling away faster.
Redshift is very different from effects of dust. When we measure redshift, what we’re looking at are spectral lines - emission or absorption lines at specific wavelengths. These form a definite pattern, and when the whole pattern is shifted towards the red by a few hundred angstroms, there aren’t many other explanations besides redshift. If it was dust extinction, you would see the blue light absorbed more than the red, but it doesn’t shift spectral lines.
Compton effect is something else - it’s interaction between photons and charged particles. It’s usually negligible for visible light, and even if it weren’t, it would cause smearing of spectral lines, not a shift.
Parallax is useful only for relatively close objects. Remote objects do not move appreciably against the “fixed stars”, and thus parallax is impossible. (They are the “fixed stars”.)
For objects with a significant red shift (i.e. at a great distance), the methods that scr4 alluded to are needed.
Thanks to those who responded, thanks for your patience; obviously I’ve had a somewhat mistaken idea about the application of red shift.
Re Chronos’ explanation of distance measurements, I can see how this could be applied to stars or star-like objects, but it’s not clear to me how one could reliably establish the distance of large, remote structures (such as galaxies) using these methods.
If you’re talking about the “standard candle” method for determining the distance to galaxies, then it’s reliable in the sense that you have the distance to one of the stars of the galaxy. The size of the galaxy can be estimated based on type, brightness, etc. But really, a galaxy like ours (which is fairly good sized) is only 100,000 light years across, and an error of a onl a few hundred light years (or whatever it turns out to be by assuming the distance to the standard-candle supernova is representative for the whole galaxy) is negligable as compared to the overall distance of the galaxy from us (which is millions or billions of light years away).
So like scr4 said, redshift gives you a general idea of distances to other galaxies but you need more reliable methods like Chronos mentioned to get more accurate results.
Question:
When calculating redshift/hubble based distances for galaxies, super-massive stars, etc, how do we account for gravitational redshift? Is gravity ignored and all redshift attributed to speed & distance? I’m sure an entire galaxy has a significant redshift, enough at least to skew a distance calculation…
Sten Odenwald of “Ask the Space Scientist Fame” states the following:
(emphasis added)
So, it seems as if gravitational red shifts do not need to be taken into account as per your question.
Notwithstanding Odenwald, I know that some will claim that the gravitational redshift is significant eg. accounting for “erroneous” quasar distance calculations. This, in turn, has been countered (see here for an example)