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Proof of Red Shift that indicates Expansion

  1. Oct 8, 2011 #1
    What experimental evidence or optical measurements are used to prove that red shift is due to gravitational fields of suns, and is not simply an artefact of light passing through and being diffracted by the matter in the heliosphere of many suns before reaching earth?
     
    Last edited: Oct 8, 2011
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  3. Oct 8, 2011 #2
    First off, I believe you have a slightly eschewed definition of redshift.

    When a wave is produced by an object moving with respect to an observer, the observer will see the wavelength of the wave moving in front of the object be shortened ('blueshift'), and the wave being produced in the opposite direction of motion will have a longer wavelength ('redshift').

    This is true for any wave, including light.
    Now the reason this is evidence for the Expansion of the universe, is that in the 1930's, Edwin Hubble did an exhaustive survey of deep-sky objects, and found that the Red-shift of an object is proportional to the distance to the object, i.e., the farther away an object is, the faster away it is moving.

    I don't think Redshift can be caused by the gravity of an object, but I don't have a good enough understanding of General Relativity to be sure.
     
  4. Oct 8, 2011 #3

    marcus

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    Vorde that is a very helpful (if incomplete in spots) response. I'm impressed and encouraged that you, as HS student, can essentially handle buckeye's question.
    Keep learning cosmology and please keep on contributing here.

    There is something called "gravitational redshift" but it is not the main source of the overall expansion-caused redshift. It is just responsible for slight percentage variation in the pattern. The overall pattern is called *cosmological* redshift and gravitational redshift effects only cause slight percentage variation to it.

    I'll try to explain but first let me offer this link:
    http://en.wikipedia.org/wiki/Sachs–Wolfe_effect

    A photon passing near a star is at first BLUEshifted by the stars gravity as if it were "falling"into the gravitywell and gaining energy. And then as it passes out of the star's neighborhood it is REDshifted by the same amount, as if it were "climbing out" of the well and having to work at it and lose energy. This is just an intuitive way of thinking. Light does not speed up and then slow down as it passes by a star, the way a flyby spacecraft would, but it does experience a gravitational blueshift that is then canceled by an equal gravitational redshift---so there is no net effect on the wavelength.

    So buckeye offered an explanation for cosmological redshift that is wrong. Gravitational redshift does not account for the overall pattern of redshift, which is caused by expanding geometry.

    As HS student you are going to learn somewhat more about cosmological redshift---the kind that is caused by distances expanding and is the main kind we observe.
    It is not caused by ONE SINGLE Doppler effect but rather you could think of it as the cumulative result of a long series of small doppler effects due to the history of gradual expansion of distance that occurred while the light was traveling.

    Or you might find it simpler to understand if you relate it directly to the history of distance expansion---rather than to a Doppler effect of actual motion. In Gen Rel geometry is allowed (typically even required) to change and in relevant cases the theory actually predicts either overall increase or decrease of largescale distances---we happen to be in the case of increasing, which is one possible solution to the equation.
    A general pattern of increasing distances does not get anybody anywhere so it is NOT LIKE ORDINARY MOTION but it can have the effect of lengthening wavelengths.

    We tend to trust Gen Rel not only because it predicts the patterns of redshift we observe---those are only SOME of its predictions. The same equation predicts phenomena that we can test in close proximity to Earth, in the solar system, and in the Galaxy around us. In these cases there is no perceptible distance expansion (that effect only arises at much larger scale). But the same equation yields other predictions which have been verified to high precision---like 6 decimal places. So there is a lot of confirming evidence that GR is right. The observed cosmological redshift is only ONE out of many successes.
     
    Last edited: Oct 8, 2011
  5. Oct 8, 2011 #4
    If we think that the CMB is due to the Big Bang, then this makes sense, but what do we think if the CMB is actually due to microwave emission by the endless hydrogen clouds throughout the universe?
     
  6. Oct 8, 2011 #5

    marcus

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    I do not know of anybody who claims to have proven that the main redshift we observe is due to gravitational effects.

    I am not sure what you are asking about. There is a slight gravitational effect. Some rather nice experiments have been used to probe this. You could be asking about them.
    Just passing through a dilute gas or plasma surrounding the star would not produce the observed effect. I've seen this discussed and papers quoted and I will try to recall where. In the meantime someone else may be able to answer.
     
  7. Oct 8, 2011 #6
    I believe, though I could be entirely mistaken here, that we have mathematical simulations for the period where the universe cooled down enough to become semi-transparent, and that the energy our models predict photons with this period would have line up very well with the observed energy of the CMBR, though someone feel free to correct me.


    To Marcus: Thanks for the high praise, I'm certainly going to try my best to keep going! One question I have though: Is the reason we can't consider the long-term red-shifting of photons as one redshift but instead as a sequence of many redshifts because we believe the rate of expansion of the universe is changing? And if so couldn't we go back to the one-redshift model and add in a variable of (redshift?).
     
  8. Oct 8, 2011 #7
    Sorry, I was not clear. I'm imagining the Big Bang never happened. If so, then can the CMB be explained by microwave emission from the hydrogen clouds that make up 90% of the known (no DM) matter in the universe?
     
  9. Oct 8, 2011 #8
    Sorry Marcus. You are correct. I was recalling Heaviside's work. But I think you clarified my mis-understanding. If I understand correctly, then part of the claim for an expanding universe is that photons from the outer edge of the universe are red-shifted due to the expansion. Is that a correct statement?
     
  10. Oct 8, 2011 #9

    marcus

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    Hi Buckeye! I wasn't sure what you were asking about. Now I see that you are questioning the cause of the cosmological redshift. At least I think that's the message I'm getting.

    But I may still not be understanding you right.

    The basic thing we observe in this regard is the pattern called HUBBLE LAW which says that to good approximation on average v = Hd that is the current rate a distance is growing is proportional to the distance. The bigger the distance d, the faster it is growing.
    The letter v stands for the recession rate.

    This was first suspected in the 1920s and over the years evidence that it is right has continued to pile up. Of course in science nothing is known for certain and ideas can always change. Maybe Hubble Law pattern is not right everywhere! Maybe there is somewhere we haven't looked yet! But so far observations keep on confirming.

    And it is one of the things Gen Rel predicts under mild/reasonable assumptions. So that is reassuring. Our law of gravity says expansion of distances is a very likely case.

    Are you saying, then, that the Hubble Law pattern of expansion is wrong? That the pattern does not exist? I want to be clear about what you are asking about. Are you saying that there is some other explanation for the observed pattern of increasing redshift with distance?

    The thing is, we can measure distance fairly accurately out to, I guess now, z = 1.5
    and so we can check the pattern by correlating recession rate with distance.
    And there is a nice sharp straightline linear correlation of redshift with distance.

    Or more exactly a linear correlation of the inferred recession rate with distance.

    It is going to be hard to explain this by any other circumstances---like light passing near stars, or through gas clouds. Isn't this what you are trying to do?

    Let me know if I'm misunderstanding what you have been talking about.
     
  11. Oct 8, 2011 #10

    phinds

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    That's like asking "if pigs could fly, would they be able to go faster that the speed of sound?"

    The big bang is a model for what happened in our universe starting at about the Plank time. Hypothesizing it didn't happen is not meaningful.
     
  12. Oct 8, 2011 #11

    marcus

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    Phinds,
    normally your posts impress me as well-grounded clear responses much more concise than mine.
    Exactly what is needed at this forum. But I wouldn't say Buckeye's line of questioning is meaningless. I think he wants to see if there is an alternative model that explains or is compatible with all the observations WITHOUT a very hot dense initial state and probably WITHOUT the Hubble pattern of percentage-rate expanding distances.

    We have to approve of Buckeye's initiative. It shows gumption and is the kind of thing scientists always do. You know plenty of examples of very capable astronomers who have tried to come up with alternative model cosmologies fitting the data.

    And failed repeatedly. The trouble is there are so many interlocking pieces to the puzzle.
    It is not just the CMB (which Buckeye desires to replace with something originating closer to home) and it is not just the galaxies' redshifts correlated to their distances. There are a dozen other things. None can be explained away without a model which accounts for all.

    So it's a problem knowing what to say. Buckeye wants to know the reasons for things and if there are alternative explanations. He thinks he does not like some inherent features of an organic interlocking whole, and he wants to see if they can be removed.
    This is not meaningless, I think, it is merely tedious and hopeless. Tedious because we have been over this same ground often enough before. Hopeless because by now, with so much confirming evidence in recent years, we can be sure that when Gen Rel evolves to the next theory, the next cosmological model will have all or most of the same unpalatable features.

    I'm not saying that Gen Rel is absolute truth and will last forever. Scientific theories are always eventually found lacking and replaced by improved versions. GR is bound to be refined in some way and replaced by something better. But when that happens, features that dismay folks, and confound intuition, will still be there. Or so I think.
    ===========================

    Buckeye, your headline indicates you want it explained WHY REDSHIFT INDICATES EXPANSION OF DISTANCES

    Can I say this: people have tried for 80 years to think of other physical mechanisms that could cause what we see. And failed.
    One reason they fail is the clear pattern of correlation with distance. The redshift observations agree with the pattern that, outside our particular cluster of galaxies bound by their collective gravity, largescale distances increase by 1/140 of one percent per million years. It is just how it is. We have independent means to measure distance and the pattern is crisp.

    The frontier within which this pattern of correlation has been checked is constantly being pushed out.

    The pattern fits beautifully with GR our theory of gravity-as-dynamic-geometry which is increasingly well tested and has proven exquisitely accurate.

    Redshift is the lengthening of wavelengths of individual spectral lines, individual photons.
    It is not like the reddening of sunlight which occurs by filtering out the shorter wavelengths, selective scattering. One can still see the hydrogen lines or the sodium line, they are just moved over into longerwave territory. There are no alternative mechanisms able to do this except as a small variation on top of the basic expansion.

    You mentioned something about selective scattering, as I recall, the sunset idea. It does not work. Nothing anyone has thought of works, except the expansion that GR says has to be there anyway. And that works beautifully.

    So here we are, monkeys on a smallrock planet, and we find this aspect of nature unintuitive. We just have to get over it. Distances between things can change in such a way that nobody gets anywhere. And this stretches the wavelengths of light that is traveling between them. :biggrin:

    Google "wright balloon model" and see if that helps.
     
    Last edited: Oct 8, 2011
  13. Oct 8, 2011 #12

    phinds

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    Marcus, I take your point. I was put off by the suggestion that the BB didn't happen. I was perhaps overly offended by what I took to be illogic. Saying that a theory never happened doesn't makes sence. Likely he doesn't think of the BB as a theory but as an EVENT and of course suggesting that looking at what might have happened if an event didn't occur is perfectly reasonable.

    I get a bit too literal-minded sometimes and I'm happy to have you chide me about that; keeps me more alert to my faults.

    Buckeye, sorry I got at bit snippy. I've found that it helps to realize that the term "Big Bang" really has two very different meanings. The first one is the "exposion" of the "singularity" and that is basically an undefined, not at all understood event (maybe) that took place before the Plank time. The "Big Bang" that is mostly what's talked about here is the theory of what the subsequent events looked like and why, and that part is well defined and reasonably well understood (with, admittedly, some holes in out knowledge but it really is amazing how much physicists HAVE figured out).
     
  14. Oct 8, 2011 #13

    marcus

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    YES! There's a German research institute that has a public outreach website that covers cosmology among other things, where one of their essays is about straightening out the confusion caused by having two meanings of BB.

    The essay is called "A Tale of Two Big Bangs"

    It is currently the first link in my signature at the end of this post:
    http://www.einstein-online.info/spotlights/big_bangs

    I'm always hoping newcomers will read that essay (which is just one page) and that they will find it helpful.
     
  15. Oct 9, 2011 #14
    Yes, I understand what I am asking.
    Let me ask one more question that is intricately related to the topic.
    Since the earth is not at the center of the expanding universe, should we not see two basically different types of CMB patterns or gradients of the CM redshift? One showing red shift and one not?
     
  16. Oct 9, 2011 #15

    marcus

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    Why? There must be some confusion. Could you explain what you you think "we should see"?

    You must have seen those oval blue and red mottled skymaps. They show a uniform redshift of right around z = 1100 over the whole sky.

    (After a 1/10 of a percent correction for solar system motion, the temperature in all directions is the same to within about 1/1000 of a percent.)

    Since the light started out to us from all directions at nearly the same temperature, around 3000 kelvin, and it is now about 3000/1100 kelvin in all directions, the redshift must be about 1100 in all directions of the sky.

    this is what theory predicts: distances in all directions have expanded equally, namely about 1100-fold.

    According to the model this would be the case at every location in the U. Our planet is not special. An observer in any galaxy would see that the distances all around him in every direction have expanded by the same factor. If he is contemporary with us then he sould see the same 1100-fold expansion.

    A couple of posts back I suggested you google "wright balloon model"
    The balloon surface is a 2D analogy of our 3D space.
    It takes only a few minutes to watch the animation which includes stationary galaxies and moving wigglers representing photons of light. It's a pretty good analogy and you can learn quite a lot in 5 minutes by watching carefully.

    For a 2D critter on one of those 2D galaxies, expansion looks pretty much the same in all directions he can point with his 2D finger. And there is no edge or boundary. For him, all expansion means is that distances between galaxies get longer, without any galaxy ever changing location.

    He is you, save for the fact that you live in 3D space, instead of 2.
     
  17. Oct 9, 2011 #16
    I thought that a universe that is ever expanding would have an outer surface. If the earth was unlucky enough to be on that outer surface, then the redshift would only be observed over one section of our sky. Is that reasonable?

    Moving the earth slightly away from the edge of the expanding universe, what would we see or be able to measure to decide what is happening?


     
  18. Oct 9, 2011 #17
    There is no outer surface to the universe. Think of it this way:

    You inflate a balloon half-way. You draw dots each a 1/2 inch away from each other in a pattern around the whole balloon. Now you inflate the balloon to its full size; now the dots are more than an 1/2 inch away from each other, but there is no 'outer dot'. They are all expanding away from each other.
     
  19. Oct 9, 2011 #18
    According to topology, everything has a surface. The only question is its density. Yes?

     
  20. Oct 9, 2011 #19
    I have never heard that, but I don't have a very deep knowledge of Topology. However I am quite certain what I said is correct, think about the surface of our planet (Imagine it as a curved 2D space, so no outer space/inside of planet), you can go in whatever direction for as long as you want and you will never reach an 'outer surface'.
     
  21. Oct 9, 2011 #20

    phinds

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    No, the universe isn't expanding INTO anything and it does not HAVE a "surface" so there is no answer to the rest of your question because it is based on a false hypothesis.
     
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