Talk:Redshift

Previous talk at talk:Red Shift.

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I wonder if the redshift of light from distance galaxies is also a consequence of the fact that the space between the source and us has expanded as the light travelled through, "stretching" the waves out. Is that effect at work at all, and if so, how big is its effect compared to the Doppler effect? --AxelBoldt

I think (but don't quote me on this) that they are just two different ways of looking at the same thing. - AdamW

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I believe that Big Bang says that the whole universe started in a singular point, not just the observable universe. Even an infinite thing can start in a point: initially, the distances between all the infinitely many things were zero, then they took off. Isn't that true for the FRW models? --AxelBoldt

No, it's not. In the case of an infinite universe, the Big Bang starts from an infinite collection of points (infinite manifold is prefered by theorists, that I'm not).Remember that the universe has had a finite time to grow, if it was a point, now every point could only be at most at the distance it has had time to expand from that point. That is only true in the case of a closed, finite universe.

But it is true that even in an infinite universe, the distance between any two points is finite. Consider a one-dimensioinal universe: the real numbers R. We let this universe "expand" by defining the distance between the points x and y at time t to be d_t(x, y) = t |x - y|. As time t increases, the distances between any two points increases. The whole universe is of course infinite, but any two points, at any time t, have a finite distance. Now as you go back in time (t goes to zero), any two points come arbitrary close. At time t=0 you hit a singularity where all points have distance zero from each other, which one would typically call "a point". I'll have to look at Wald's book again. --AxelBoldt

Um, no. Try this: keep one point finite and take the lim as the other goes to infinity (perfectly valid in an infinite universe). Now take the limit as t goes to zero. In principle, the value could be anything (even zero), but because they are both linear terms order one, I'm inclined to say that the limit should be finite and non-zero. --BlackGriffen

plasma cosmology

It seems to be fringe stuff, but the plasma cosmology article claims other possible causes of redshift. Should we add these as well as others from other theories? - Omegatron 16:35, Sep 4, 2004 (UTC)

I just made some modifications and corrections to the article on redshift - I hope the original authors don't mind. In

answer to the question above: I don't think so. There are quite a few non-standard interpretations, such as tired light,

and this. I think it's fair to describe plasma cosmology as an extreme fringe position among the community, if indeed it

has any support, and my personal view is that the purpose of an encyclopedia should be to report the consensus view. Otherwise,

the reader might get the impression of a diversity of opinion which doesn't in fact exist. (Of course, this doesn't mean there

shouldn't be a dedicated article on plasma cosmology, or whatever else, provided the authors make it abundantly clear that

it is not the paradigm, and fairly report the level of support that the model entertains.) There might be a cause for mentioning

quintessence in the article on redshift though, if somebody working in this area is prepared to make a contribution.

serjeant Wed Sep 8 15:44:57 UTC 2004

Halton Arp I believe was Hubbles assistant at one point. More and more there are found anomalous redshift discordances. To believe that redshift could only be caused by the Doppler effect is pretty simplistic. Hannes Alfven won a Nobel prize for physics which is hardly fringe to name two plasma cosmologists. Gravity rules here on Earth because it's a dense place with dense Big Bang cosmologists. Space is the place for plasma. How come the Big Bang Cosmologists are continually surprised that they can't predict what space probes are finding? - an electric universe. Let's see - Venus - x-rays from comets - gamma ray bursts - quasars embeded in galaxies - need I go on Do I need to talk about Big Bangers believing in dark matter- WHiMps and Machos - it just goes on and on. - Gepay

I was just referring to their Forward Brillouin Scattering, not the entire article itself. I guess if it doesn't have a cosmological aspect it shouldn't be in this article, though. - Omegatron 18:20, Sep 8, 2004 (UTC)

Responding to Gepay's comments - Yes, these things are all just theories. Maybe the consensus is wrong. My personal feeling is that the wikipedia should tell people what most scientists believe, while still showing them some dissenting views (and portraying them as such). I'm hoping that the way we've structured the redshift article is achieving this. To me, it's important that people don't think of science as a didactic "this is what the truth is, the end" - when you're in it, it's more like a babble of competing views, and only when it's settled down to a more-or-less-consensus does it tend to filter out to the non-scientific world. But to me it's also important not to mis-represent a scientific consensus - where most scientists subscribe to a view, then I think wikipedia should say so very clearly. serjeant Tue Jun 21 16:52:19 UTC 2005

Redshift caused by Planck mass?

What are the reasons to belive that redshift is not caused by Planck mass? Planck mass is "everywhere" (sqrt mP=hc/G, the universal constants that are "everywhere") and thus may interact with photons along their way through the universe, making photons slowly losing their energy.

This is an example of "tired light" models of cosmological redshift. Tired light was considered at one point, but for example tired light models don't reproduce the time dilation of high-redshift supernovae. Incidentally, Planck Mass is everywhere in the sense that my mortgage is everywhere - there's no escape from the fact of the debt but however dark it makes the world feel my mortgage isn't interfering with photons! "Planck Mass" has the same sort of meaning as "inch" or "year" - it's a concept. We have no idea what causes the cosmological constant, and (speaking as a non-specialist) perhaps the Planck mass scale (or particular multiples of it) may get involved in the physics of that somehow. serjeant Fri Mar 4 10:53 2005 (UTC)

Thanks. Did the considerations mentioned also specifically check the potential "interactions" of Planck units with the time dilatation of high redshift Supernovae? Even in case of Doppler effects (if really present they certainly make their contributions) this does not necessarily mean that interactions with Planck units do not contribute as well under "normal" circumstances, away from Supernovae. Not sure whether your mortgage really is everywhere. I rather think it's documented somewhere in writing. And without that documentation there will be no mortgage. What if sqrt of the Planck mass is not just a concept but a "particle" with specific physical properties? E.g. transmitting Planck impulses without transmission being detected because fully isotropic and place of transmission being of the size of Planck lengths? - RABecker

NB: One way of detection might be to make the isotropicity become anisotropic by e.g. putting a force to something, let's say a car. It usually does not accelerate from 0 to 100 in fractions of a second, despite of the force. The force (=impulses/time) has to overcome another, opposing force that certainly also is to be seen as impulses/time.

It might be helpful for you to try substituting the word "inch" for the words "Planck mass". Inches don't interact with real objects. Real objects interact with real objects. "Planck mass" is a concept like "Thursday" or "inch" or "overweight" - useful as a mental concept but not physical on its own. It may form part of a theory though. serjeant Tue Jun 21 16:52:19 UTC 2005

Landmarks

Can this article post some landmarks ie. comparisons of average red shifts of objects for the reader to gain a general idea of a standard? Ie. the red shift of a radio wave travelling from the sun to the Earth, or from London to New York (which would be very very small of course, like to the hundreds of negative powers, but still...) then increasingly for longer distances. I don't know this myself, that is why I ask. -- Natalinasmpf 21:10, 8 May 2005 (UTC)

Sure. Objects have been seen out to redshifts beyond 6 (writing as of June 2005). At redshift z=6, we see objects as they were when the Universe was only around 7% of its current age. Astronomers find objects right down to redshift zero, but when you get to very low redshifts (below z=0.02 or so) you tend to find local effects starting to dominate over the expansion of the Universe. For example, our galaxy is moving towards the Andromeda galaxy, so Andromeda has a negative redshift (a blueshift) caused by the Doppler effect. Also, the Solar System isn't expanding with the expansion of the Universe, because the Sun's gravity is much, much stronger from where we are. The Earth isn't expanding either, because the local binding forces of the Earth also overwhelm the expansion of the Universe. Of course, if everything expanded with the expansion of the Universe, then we wouldn't know it was expanding, because all our rulers would be expanding too! The rulers don't stretch because the forces that keep a ruler from falling apart are much bigger than the cosmic expansion. The cosmic expansion is very subtle on a local scale and only detectable by looking on cosmic scales. serjeant Tue Jun 21 16:52:19 UTC 2005

Redshift: Distance or Recession

could we tell the difference if Redshift is due to distance as well as recession speed?

Good point. The answer is no, unless we have additional information. Some of that additional information could be observational, or some could be whether the underlying theory appears simple or contrived. serjeant Tue Jun 21 16:52:19 UTC 2005

We do have that additional information. For instance, we can see that far-away supernovas appear to take longer to happen than close ones. Now, those durations are well-understood and regular, so if the rules of physics are the same over there we need an explanation. If those stars are receding, the light at the end of the event would get to us later than expected, because it comes from farther away. On the other hand, if the light is simply redshifted by the distance, the frequency would change but the timing would not. Current calculations show expansion/recession as the complete explanation. There are other such arguments that I don't remember right now.

So, I say the direct answer is "yes," we have enough data to tell the difference. Sergeant's description of the scientific philosophy is, however, absolutely correct, and a good point. Additional data has been required, and even more would be helpful.

Chris Mid 22:38, 22 Jun 2005 (UTC)

Chris Mid is quite right. We do have such additional information for the majority cases one meets in practice in extragalactic astronomy. Things get a bit uncertain when peculiar velocities are known to be significant, at the lowest redshifts, but once you're into the Hubble flow it's not a problem. serjeant Sat Jun 25 14:27 UTC 2005

Big Bang?

Why not an expanding/contracting universe? Energy constantly changing from place to place. No beginning, no end - If there was a beginning, what was before that? -- billm

nkos
big bang is just a theory. by the opinions of great physicist cannot describe the birth of a univerce but maybe only a birth of a part of the univerce. by the way what we see if very little to say what happens everywhere. the theory of bing bang is more an idealistic theory that can accept the idea of God.

I think that Billm and nkos are effectively wrestling with the often-asked, subtle, and very difficult question: "What caused the big bang?" (Forgive me if I'm wrong, folks.) There are theoreticians who try to make sense of this question (M-theory is currently fashionable); perhaps someone more au fait with these models could post a reply. My understanding as a non-specialist is that in M-theory our Universe is only one of a number of "branes", which effectively allows the possibility of causative agents from before our Big Bang. serjeant Tue Jun 21 16:52:19 UTC 2005

Revision of 2. Expansion of Space

Deleted Text:

I am not very happy with the lengthy explanation under CAUSES, Item 2. Expansion of space. I found it quite a stumbling block.

You say there is a subtle difference between expansion and recession but I can’t see it. The two words seem to me to be virtually synonymous. You say the galaxies are not believed to be receding but that simply begs a definition of the word ‘receding‘ does it not?

Would you object to a shortened statement roughly as follows;-

2.Expansion of space. While this may be an effect rather than a cause, it expresses the continuous separation of entities not too tied up by gravity. This pressure to drift apart has naturally generated far more examples of red shift and far fewer examples of blue shift.

Arthur Hinks.

To Arthur Hinks: First, let me give you some help by explaining that your comment was deleted (by somone not me) because you really shouldn't put those in the main body. Try this discussion/talk page.

Now, your concern about the article is valid. Things in the universe are generally receding from each other. It's just that experts are a bit leary of words like that becase they want to avoid words implying normal motion and make a very important point. They want to convey the extreme coolness of the fact that space itself, the empty fabric of reality, is malleable, shapeable, stretchable. The Big Bang did not send everything flying apart like a bomb, rather it was the start of the growth of all reality itself. Sortof. Best I can do in this space.

Finally, while clarification is probably in order, your suggested text is not really accurate. Expansion is in fact the cause of this redshift, not an effect. It's a different thing, not Doppler at all, and explaining that is the purpose of the section.
Chris Mid 23:34, 20 Jun 2005 (UTC)

From Arthur Hinks. Thanks for putting me in my place; i.e. ‘Discussion category!’ Thanks also for pointing out that expansion is a cause of redshift. I fully concede it. I can understand your desire, and a real need for clarification of the difference between expansion and individual localised recessions but I feel it would best be left as a separate issue because for many it can be mind boggling; frustrating; controversial and off-putting! I am hoping the following simplification would now be acceptable: 2. Expansion of space. This is the general separation of all entities not too tied up by gravity; as opposed to local recessions of individual light sources. An appreciable drift away from us, for whatever reason, being the cause of the redshift symptom. Thanks for help.

Well, for what it's worth, my opinion is that the text should be improved, not removed. You suggest leaving it as a separate issue, but where should it be addressed if not in a section on the causes of redshift? Again I say explaining it is the purpose of this section. The basic gist of it was covered in the introduction.

Chris Mid 22:39, 23 Jun 2005 (UTC)

Personally, I do agree with Chris Mid that we really need to link redshifting to the stretching of space in this section; this is one of the important causes, and the section is about these causes. However Arthur Hinks makes the very good point that local effects can sometimes overwhelm the global expansion (which is my reading of his phrase "not too tied up by gravity" although such local effects aren't restricted to gravity). That's why the Earth isn't expanding with the expansion of the Universe. The current article doesn't really make this point at present, so I'd agree that there's room for improving the text (and possibly elsewhere too). I also sympathise that this is pretty counter-intuitive stuff. Physics does make big demands on your imagination - I find imagining 3D space hard enough, without trying 4-D curved spacetimes with electromagnetic waves moving through them. One of the great enduring surprises to me is that the Universe is understandable at all. serjeant Sat Jun 25 14:27 UTC 2005

PS - I just slightly reworded the causes section, so the Wolf effect is given a special status. It's a physically subtle effect and probably most relevant to laboratory physics, and is something for the cognoscenti I think rather than the general reader. I worry that quoting it in the same heading as the very famous Doppler effect risks mis-representing the generality of the Wolf effect's applicability, though it certainly rates a mention. Incidentally, the opening paragraph of the article strays into discussing causes, which are discussed more comprehensively elsewhere in the article - does anyone think this introduction should be trimmed back? Serjeant 28 June 2005 13:55 (UTC)

I tightened up the language in the intro. I didn't mean to preempt discussion; I was going to do it anyway. What do you think of it this way? Do you think maybe the equation should be moved down into the details? I'm not sure, and not sure where to move it to.

Second, under Causes, I wonder if "3. Gravitational effects" wouldn't also be a less commonly cited cause? Although it's true, I've only seen it used in rare arguments against expansion.

Chris Mid 28 June 2005 22:24 (UTC)

P.S. In the intro, I also hate that "the universe is expanding" links to Dark energy. It's misleading. Several other articles link "expansion" directly to dark energy. But Expansion of space itself is a poor article. Fixing this strikes me as no small job.

Chris Mid 28 June 2005 23:51 (UTC)

It's misleading if it's suggested to be the only cause or explanation. I hadn't spotted that link, and I share your dislike, as it is misleading in the way you suggest. You're right that the Expansion of space article is poor. Your revised intro is fine, though it still touches on causes. I think the equation belongs there because it is purely descriptive and independent of the causes. I'm afraid that gravitational redshift is a commonly cited cause; it is one of the famous experimental tests of general relativity and has even been argued to have been observed via X-ray emission lines emitted near extragalactic supermassive black holes, etc. See e.g. http://astro.ic.ac.uk/Teaching/Postgrad/xrays.html Serjeant 30 June 2005 13:07 (UTC)

Non-standard cosmologies

Sigh. It would be nice to keep the nonstandard cosmology nonsense to the page that is about that. I have tried to tighten up the section as well as I can without deleting it altogether, but we should consider doing so. As most neutral observers will see, the arguments made by proponents of nonstandard cosmologies are piddling at best and wrong at worst. While Wikipedia is NPOV that does not mean we have to waste people's time with the machinations of a vanishingly small though vocal minority of naysayers. Joshuaschroeder 30 June 2005 11:47 (UTC)

Well, yes, but one of the disadvantages of wikipedia is that a vocal naysaying minority can cause a lot of confusion, and has led to other articles on scientific topics having their neutrality disputed. I feel that it's in the broad interests of the wikipedia to avoid such disputes. The solution I've preferred up to now is to give the naysaysers a medium for expression in this section, while still explaining that it is not supported by the overwhelming majority of the scientific community. Serjeant 30 June 2005 13:07 (UTC)

In the future, I think the best idea is to direct the naysayers to Non-standard cosmology. I often do this over on the Talk:Big Bang page. The problem is that a lot of naysayers want to broadcast their message in as many places as possible to try to lend their ideas a (false) sense of permanence and credibility. Wikipedia does have a place for these ideas, but it is not in the articles about science and using these articles as soapboxes is specifically forbidden by policy. Joshuaschroeder 30 June 2005 13:22 (UTC)

I notice you deleted the material on paradigms. This was intended to explain how science works to the non-specialist, and was inserted specifically in response to some vocal criticism by those naysayers. My idea was to short-circuit all the discussion of the form "we have this evidence, and that evidence" that has bedevilled other articles, and simply leave it as a statement of what the consensus is and is not. Before I edit this section any further, why do you feel we should not explain scientific paradigms? Serjeant 30 June 2005 13:07 (UTC)

The "paradigm" discussion occurs on the page of Non-standard cosmology where it belongs. This is a page about redshift, not a page about scientific paradigms relating to redshift. A discussion on scientific revolutions is an unexpected inclusion in any neutral encyclopedia's article on a scientific phenomenon, but if it is included here, it should be included on every page that discusses an observation or theory. Obviously that's an unreasonable expectation. Joshuaschroeder 30 June 2005 13:22 (UTC)

The Wolf Effect was also inserted originally by a naysayer. The key question is: can it cause a redshifting? The original article was not published in a crank's journal - do you feel there is an error in that article, or have later authors disputed it (as a lab effect)? I would like to avoid this wikipedia article having its neutrality disputed by the naysayers you mention, so perhaps the wisest course of action would be to mention the Wolf Effect in the context of non-standard cosmologies. However, I don't feel strongly enough to put it back in myself. Serjeant 30 June 2005 13:07 (UTC)

The problem with the Wolf effect being a "redshift" phenomenon is that the proportional implications of redshifts are not adequately addressed in the paper (despite what the naysayers say). Redshift is defined as a proportional frequency shift of all light from source to observer without associated scattering, interference, or change in photon count. In contrast, the "Wolf effect" is merely a combination of well-known scattering and interference effects that make shifts in frequency for certain wavelengths look possible (similar to how the Compton effect can change frequency) but this is not a mechanism for redshift because there is no proportional predictability associated with it outside of the lab. We should not be entertaining the nonscientific meanderings of naysayers in articles about science. Joshuaschroeder 30 June 2005 13:22 (UTC)