Hi there, this is my first post and should start by saying that I'm not an astronomer, just an educated engineer, so please keep the flames down if I say something stupid.

Recently I have come about to some scientific "findings" that seem to go against the BB theory. I just thought I could ask here about these findings and let you guys explain those to me in not too complex terms.

My first question is: what is the matter with the redshift? I was taught that it was caused by the doppler effect due to galaxies moving apart from each other, thus kind of proving the observable universe is expanding. Now I read that some quasars emit different redshifts at different wavelengths, that there is evidence of quasars kind of interacting with galaxies (suposedly much closer objects), that some spectral lines in the sun's chromosphere do show a redshift too, and that redshift can be explained by alternative theories without need for a doppler causing effect.

I have no clue about the validity of these claims. Is it true that the red shift cannot be used to prove that galaxies are moving away from us?

José

In some logical sense of the word, red shift cannot be used to "prove" anything.

In the scientific sense of the word, the redshift of (the great majority of galaxies, and all distant once) provides very strong evidence for the general expansion of the universe.

You listed four specific questions about redshift, to which the answers are

a) no, the redshifts of light coming from one source within a single galaxy or quasar are not different; choosing different sources within the same galaxy or quasar can lead to very slightly different redshifts, for perfectly obvious reasons

b) there is no unambiguous example of two objects with very different redshifts which are interacting physically and therefore at the same distance from us

c) yes, some lines in the solar atmosphere appear to have very very slightly different velocities than other lines. This happens on Earth, too -- check out the Doppler radar of your local weather report

d) of course redshift can be explained by alternative theories! However, those theories do not explain as many very disparate observations in different regimes as the "mainstream" theories. That is why most scientists prefer the mainstream theories: they account for more observations than the alternatives.

Yes, you are right, I did ask several questions in one. Thanks for answering them all.

I understand that a single source only emits radiation with a single redshift, good, that there are no clear cut examples of interacting objects with very different redshifts, good, that the sun thingies display only slightly different velocities, good too, and that doppler effect (and by extension the expansion of the universe) is still the best explanation for the red shift.

I'm happy with that. I can keep telling my daughters what I learnt when I was younger.

No offense but, you are an astronomer, aren't you? Just want to make sure I got my answers from some one who knows what he is talking about.

José

Just one more redshift question. Doing some googling on redshift I found a reference to "gravitational redshift". According to the explanation here redshift can be caused as well as by the known doppler mechanism, by the gravitational slowing down of light (I just read the first two paragraphs of the site before it became too difficult to follow). Is this gravitational redshift an accepted fact or is it disputed in the scientific community?

José

Nice to see another engineering educated aboard. Welcome.

I trust my PM answered your last question.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Well, I guess you could judge for yourself.

Bibliography on ADS

Gravitational redshift is an accepted fact. It has been measured both on Earth, and from some objects in space. You can read about the first experiment to detect it on Earth (way back in 1959) at

http://prola.aps.org/abstract/PRL/v3/i9/p439_1

or

http://hyperphysics.phy-astr.gsu.edu...iv/gratim.html

It is a fully mainstream result of General Relativity.
Basically, the stronger the gravitational field, the slower time goes for the transmitter, thus redshift.

This slowing down is an effect that is clearly measurable to the point where the clocks in GPS satellites have to be compensated for being further out in use than they where in production, they also have to compensate for moving faster.

Actually if different wavelengths of light where produced at different heights, you would see different redshifts for the different wavelengths without invoking any modifications to mainstream theory.


I'm probably going to have my simplified explanation corrected soon, but that's ok.
I think I have a fairly good grasp of most of the things in GR, but I'm fully aware that there are parts I don't get.

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And the "driving on the freeway on a scooter" analogy still holds true because the pilots are sitting in 7 to 30 ton aircraft o' doom and you are running around them in your very own Meatbody, Mark I. Beep, beep.
Trying to make sense of computers, The Error Log.

HenrikOlsen. You are correct. Sometimes, when explaining gravitational redshift, I mix SR with newtonian mechanics....ten story building. Every floor has a laser night-lite shining at you. Let E=mc², and E=hv. Convert photon energy to mass. Drop from each floor. Larger 1/2 MV² from the higher floors. Reconvert to photons as it hits the eye. Funny looking rainbow if you have an extreme field.
From the top floor looking down, progressive redshifts, from the bottom looking up, progressive blueshifts.

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

As the people above have indicated, there are several causes of redshift. One thing that most of them know, but didn't really elaborate on is that the cosmological redshift is not normally taken to be Doppler redshift so much as a result of the space in the universe expanding. The difference may be subtle, but depending on what level of understanding you want to have it may be important.

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Forming opinions as we speak

I think the difference between cosmological redshift and relativistic doppler effect is actually very important for all levels of understanding the expansion of space.

The expansion of space is not about objects in space moving apart through inertia, where their relative motions are shown in their spectra by (relativistic) doppler effect. This is an apparent redshift, dependent purely on the relative inertial velocities of the object itself and the observer measuring that object's redshift.

The expansion of space is a process where the metric that describes distance is changing over time. The spaces in between objects are growing, irrespective of those object's inertial velocities. This is shown by cosmological redshift, where the spectrum of an object has been changed in an absolute way. The spectral lines are actually shifted as the light travels through expanding space. Simply put, the light is "stretched" as it moves through expanding space. The effect is cumulative, for the more expanding space the light travels through, the more its spectrum will be shifted.

Let's see if I get this last point right.

A expanding universe creates an absoulte red shift in the spectrum of far apart objects, a shift caused not because of the apparent velocities of the objects but because of the very nature of space expanding, and its effect on the light travelling trough it.

In this expanding universe far apart objects do get farther apart with time, therefore creating an additional spectral redshift due to the apparent motion of these objects.

Question: Do the apparent and the absolute redshifts add to each other or are both the same thing but explained differently?

José

Jose,

THere were some straight lab physics experiments done in the last 40 years or so that were able to measure a shift in wavelength of light over a path of 3 stories. I can't think of the name of the experiment but it's quite indicative of a gravitational attraction of light here on earth and that light going up loses energy (red shifting) and light going down towards earth gains energy (blue shifting).

Why, yes, that's the very experiment I mentioned -- together with link to the original paper -- in my message above. Here's the link again:

Gravitational Red-Shift in Nuclear Resonance, by Pound and Rebka

jserrano asked:

I would not have chosen to draw the distinction between "types" of redshift that some other posters have done. We observe shifts in the spectra of distant galaxies and quasars. One can assign a velocity to each object based on this shift, if one wishes; and one can then break this velocity into pieces -- this much due to rotation of gas within the galaxy, that much due to the motion of the galaxy around the center of mass of its cluster, this other piece due to expansion of the universe. I don't see the point in drawing finer distinctions.

Sorry but I'm not currently at an institution nor am I a subscriber so access to that page is restricted for me - access to the pdf of the paper etc. At least I dont' think being a life member of sigma pi sigma grants me anything with AIP but decades of invitations to contribute. If it does, I have no clue anymore of any 'member number'.

However, there is some bit of information for anyone to access here

http://en.wikipedia.org/wiki/Pound-Rebka_experiment

key is the search for Pound and Rebka - the original researchers performing the experiment. There's quite a few references to this info that do not require subscriptions on something like Google.

The experiment was based on the Mossbauer effect and the date I believe was several years before the advent of such things as lasers and masers which permitted higher accuracies in measurement. The original experiment agreed to about 10% - and later ones got to maybe 1% accuracy with GR? Anyway, it's pretty certain that gravity sucks when it comes to photons (as well as with waste lines and fannies as one learns with maturity).

Thanks, I have a better picture now of the whole redshift thingy.

Redshift can have different causes, small shifts may be due to the local motion of the particular object (rotation, etc...), in some cases gravitational effects will be the cause of a larger part of the redshift as light tries to scape a big gravity well, and for the most part, specially when we take the average redshift of a whole galaxy, it can be assigned to the average speed of that particular galaxy as it moves further away from us. (if I got something wrong please correct)

So, if I got this right, the apparent discrepancy caused by low redshifted galaxies with what may seem close or interacting high redshift objects can be explained taking into account local motion, gravitational redshifts and coincidential aligning of distant objects.

Am I getting close to understand how redshift is understood in modern astronomy?

José

Sounds like you're getting closer to it.

Pretty much, objects far off are going to show where they're at by their redshift. Also, really far off objects can have multiple things between us and them with different redshifts that can affect what we see by imposing some lower redshift information on the light between here and there.

Also, there are some strange notions promoted by some fairly or formerly mainstream people like halton arp (arp peculiar galaxies catalog) suggesting that there are tie - ins between far redshift objects and apparently closer redshift objects supposedly ejected from galaxy cores. I don't think the ideas or the evidence have gained any support although they are out there.

It almost sounds like you may have encountered some of this. It makes for intriguing thought but I don't think there's enough evidence to even make a serious theory and it definitely isn't accepted at present time. Most of the original evidence seems to have not held up to scrutiny. THis should not be considered a part of an initial learning effort as it's very likely not of any substance.

In astronomy, we see the universe in 2 dimensions. The third dimension (distance from us) all must be inferred from other sorts of measurements and theories. Tremendous efforts have been done for many years to give us some reasonable clues about this but being an indirect measurement means it's subject to more error and sometimes potentially serious error. Only the closest nearby stars can actually be measured by direct methods - parallax. The rest vary in indirect techniques depending on apparent distance - the furtherest being the redshift amount indicating distance. In between, luminosities are used for particular stars. Our furtherest luminosity measurement is the supernova which does take us way out there as well as fairly close in.

I can't comment on the discrepancy, but I think you are getting closer to understanding how redshift is understood in modern astronomy.

Redshift cannot tell us all of the story, we need other kinds of measurement, other observations and interpretations of data to help us.

The way redshift works in practice is we have to decide ourselves what kind of redshift a certain object may be expected to show, and then apply the appropriate mathematical translation to our data to work out an objects apparent distance or relative speed.

The mathematical translation that is applied is different, depending on which kind of redshift you are working with. Over large distances, cosmological redshift (caused by expanding space stretching light) is the main component of an objects redshift. The redshifts of most objects over 5 billion light years away are almost totally caused by the expansion of space. Those object's relativistic redshift will be tiny in comparison to the redshift imparted by the expansion of space.

Whereas at the local scale, the close galaxies and the stars in our own galaxy have redshifts almost totally caused by relativistic doppler effect. And gravitational redshift plays a very small part in things in general with the exception of extreme objects like black holes.

As cosmological redshift measures the amount that space has expanded between an object and it's observer while the light was making it's journey, and is not caused by the relative inertial movements of the objects which are very small in comparison, at the scales which cosmological redshift is dominant the velocities we derive for objects are apparent velocities. The distant quasars haven't moved inertially from us faster than light, they could never have the energy to do that, the space between them and us has grown, and thus so has the distance.

So what is going to be the discernable difference between space expanding or time speeding up - or c slowing down when it comes to cosmological theory?