What's the dope on the Red Shift?

Factual Questions
1

I recently heard about Halton Arp’s discoveries and I was interested to see what the members of this board thought about them. From what I understood, recessional velocity was the only thing that could produce a red shift, and a high red shift value meant an object was far away. What Halton Arp photographed, however, shows objects with a high red shift value in the same area and interacting with objects that have a low red shift value. This would mean that something else is causing the Red Shift, and that it’s not a reliable means of determining an objects distance.

What’s the dope on this? Are all these scientists wrong?

Is this picture genuine? I’m pretty sure it is.

2

I presume you mean this guy: http://www.haltonarp.com/

(And BTW I find his tagline “Communicating information on galaxies, quasars and cosmolology which ordinarily would not be available in professional journals or the public media” to be somewhat telling right off…)

Could you link to something to do with these “discoveries”? All that is is a black and white photo on a photobucket account - it doesn’t really mean much.

3

I can’t make any sense of the picture, but as far as i know, red shift means the object is moving away, not that it is far away. An object that is closer, but moving away faster, would have more redshift than an object much further away, but moving slower.
So, an object with high red shift could be interacting with an object with low red shift. The higher red shift object is just passing by.

4

There are at least three known mechanisms which can produce redshifts. First, the source and the observer can be moving away from each other. Second, the source can be deeper in a gravitational well than the observer. Or third, the source and observer can be located a great distance apart in an expanding universe (note that this is not actually the same thing as the first). All of the data seem to support the last of these three as the explanation for the redshifts of most galaxies (for some nearby galaxies, the redshift due to relative motion is greater, but only for a few of them).

There are some pictures which some… Alternative scientists, shall we say… claim show high-redshift objects interacting with low-redshift objects, but the claims for interaction are tenuous at best. Some of them are based on vague interpretations of shapes, and some are even based on known instrumental artifacts. The conventional explanation for such images, and the one best supported by the data, is that they show a foreground object which just happens to be in very close to the same direction as a distant object, and that any apparent interactions are just a coincidence.

5

I get the impression that Dr. Arp is a maverick and does not submit papers for peer review and publication in professional journals.
He seeks approval of his work/theories on his own forum/terms.

I saw this in the 40’s when a Prof, in Thermodynamics came up with a ‘new’ way of looking at cycles.
Published it in “Power Engineering”, McGraw Hill Mag.
A peer review would have shot it down as merely viewing cycles fron a different perspective,

6

… and my poor little high school physics gets shot out of the water…
Thank you, Chronos, for the new information.

7

I do not see how the first and third differ. Two bodies are moving apart, for whatever cause, light from one is red-shifted when seen from the other. How do you know it is red-shifted? Because you see some spectral llines, say from hydrogen, at a lower frequency (that is, redder) than they are here.

Now let us assume that you are on a star in a galaxy, in a cluster of galaxies. Then you may some motion with respect to the center of the galaxy or cluster and then you will be red shifted with respect to the nearby objects you are moving away from and blue shifted with respect to those you are moving towards. As long as you stick to nearby objects these motions will probably cancel. If, on the other hand, you find that all really distant objects are red-shifted, then there would appear to be only one readonable conclusion: the universe is expanding and all those distant objects are rushing away at increasing speeds. Not the only possible conclusion (see below) but the most reasonable.

It is worth asking how we know that there is a simple relation between red-shift and distance. For nearby stars, there is a class of variable stars called RR Lyrae variables (IIRC) whose distance can be measured by parallax and whose absolute brightness therefore can be measured and turns out to be proportional to their frequency. For relatively nearby galaxies, RR Lyrae variables can be found and the brightness/frequency relation can be turned around to yield distance and it turns out that distance is proportional to red-shift. Then for really strongly red-shifted galaxies it is assumed that the distance/red-shift relation continues to hold and it becomes a measure of distance.

I have heard at least one alternate explanation. While it is an alternate explanation, it has, as far as I can tell, no experimental or observational difference from the expanding universe hypothesis. The late Fred Hoyle gave a series of talks in which he theorized that the universe was of fixed size, but it had begun with no gravity and the force of gravity was increasing with time. As it increased, the light emitted was being shifted by the increasing gravity. This explanation would unite the red-shift with the gravitational red-shift, the seond kind above. This would leave only one kind of red-shift, not two, but has no consequences.

8

I think the supposed difference between #1 and #3 in Chronos’ post is that #1 is due to motion of the objects, and #3 is due to expansion of the intervening space between the objects. Theoretically, the rate of change in the expansion of the intervening space could be independent of the velocity of the two objects.

I also heard an alternative theory that red shift wasn’t due to an expanding universe, but due to photons decaying as a function of the distance they travel. I heard there is a proposed experiment to see if they can measure red shift in photons bounced millions of times between two precisely arranged mirrors.

9

It’s called the “tired light theory” and it seems to not be main line thinking. Wikipedia’s article is flagged as being of disputed neutrality and accuracy.

10

To illustrate the difference between a Doppler shift and a cosmological shift:

Suppose we have a hypothetical universe containing two objects, a source and an observer, plus some weird dark energy-like fields which govern the expansion. The universe starts off static, neither expanding nor contracting, and the two objects are at rest relative to each other. The source then emits a pulse of light. While the light is travelling, the dark energy fields kick in for whatever mysterious reason, and cause the universe to expand. When the universe has doubled in size, the expansion ceases, with the light pulse still en route. Then, the pulse reaches the observer. In this situation, there is no Doppler shift at all, since the source and observer are at rest relative to each other. But the observer will nonetheless measure the light pulse as being half the frequency it was when it was emitted. This redshift is due entirely to the cosmological expansion: The wavelength of the light is stretched by the expansion of the space through which it is travelling. Of course, the Universe we live in is not quite so accomodating, so the distinction is rather more subtle, but it’s still real, and for most cosmological models, distinguishable.

There are other hypotheses to explain the redshift, such as tired light, but they’re not generally accepted in the physics community (though there’s nothing at all wrong with conducting experiments to test them, anyway, if you can get the funding). Personally, I can’t see any problem with that Wikipedia entry (at least, as it was at the moment I read it), since it correctly notes in the body of the article that the tired light hypothesis is not currently generally accepted. It’s possible that that caveat was not present when the “disputed” label was added, or that the “disputed” label was added by proponents of the tired light hypothesis, to dispute the statement that it’s not mainstream.

11

Actually, I did an ADS search and found quite a few peer-reviewed papers on this topic with his name on it. This isn’t exactly my field and I haven’t taken the time to read any of the papers in full, but IMHO it’s always useful to look for and investigate observations that seem to contradict the “standard” theories.

12

To expand on the difference between Chronos’ #1 and #3 and apply it to what the good Dr. Arp is talking about:

If the redshift were due to the various non-Local-Group galaxies moving away from the Milky Way, which would make it the center of the Universe, then if we were able to tgransport ourselves to one of these other galaxies, the Milky Way would appear redshifted, but some other galaxies in that direction that happened to have a net velocity toward us would appear blue-shifted.

This does not seem to be what is happening. The only galaxies that are blue-shifted relative to us (moving toward us are fairly local, and the shift is explainable by gravitational effects. Never mind the low possibility that we just happen to be at the center of the Universe.

What we believe is happening is the expansion of the space of the universe as described in previous posts. In this scenario, if were in that distant galaxy, all galaxies not local to it would appear redshifted as well.

Now as long as we are there, let’s imagine viewing the Milky Way galaxy in one direction, and another galaxy with an equivalent redshift in the opposite direction. If we hop back to the Milky Way, that further galaxy would appear to have a greater redshift than the one we just visited. Because we believe that the Universe is expanding uniformly in all directions, we can guess the distance of a far-off galaxy by its redshift.

If we know the distance, we can use the speed of light to figure out how old the light from that object reaching us is, and when we figure out which are the farthest objects we can see, we can guess at the age of the Universe. A good-if-somewhat-dense-for-the-layman-but-nowhere-near-as-dense-as-it-could-be treatment of the current state of our understanding of all of this as of a couple of years ago can be found in Ken Croswell’s book, The Universe At Midnight.

What Arp is suggesting, therefore, is that he has evidence that two objects whose redshifts suggest they are thousands or even millions of light years away from each other are displaying some sort of interaction that their apparent relative disntances would suggest is possible. This would suggest that our belief that redshifts are caused by cosmological expansion are incorrect, and that everything we think we know about the Universe as a result of those beliefs is wrong.