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Accelerating Universal Expansion

  1. Jan 3, 2013 #1
    I have a question that I have tried to answer by searching the internet, and by writing a few astronomy professors and professors of astrophysics. One replied however in it, he only confirmed what I knew and didn’t address the question.

    As I understand the accelerating Universal expansion, the empirical evidence is that more distance supernova have a greater net red-shift, after adjusting for the expected red-shift due to the expansion of space, than was expected if universal expansion was slowing down.

    To be clear I have no questions regarding the evidence, it is empirical and has been verified by others since it was discovered. I also understand that uniform expansion produces a red shift that grows with distance, and that this effect is subtracted away and they look at the residual.

    It is the interpretation of the positive residual red-shift that I find counter to how I would expect to interpret that empirical evidence. The clearest way I can think of to explain why is to create an example scenario.

    I will round the age of the Universe to 13.7 billion years. If I were to compare a supernova red-shift of one 12 billion years distant, to one 6 billion light years distant, I would expect the residual red-shift to be greater in the supernova of 12 billion years distant than the one 6 billion years distant. The reason is due to the different travel times and the age of the universe they represent. Photons emitted from a supernova 12 billion light years distant are showing me velocity information from the supernova when the Universe was 1.7 billion years old. At that age of the Universe, I would expect a supernova, and any galaxy, to be receding at a greater velocity, than a supernova 6 billion light years distant after the universe has aged to 7.7 billion years old when they should have been receding more slowly.

    The photons we see from the closer supernova are showing its velocity when the Universe was 7.7 billion years old. The receding velocity of that galaxy should be less than a galaxy that was receding away when the Universe and its expansion was only 1.7 billion years old?

    Summing it up, I need help understanding why a residual positive red-shift for more distant galaxies indicates an accelerating expansion, when I would expect to see that greater residual red-shift in the more distant galaxies, because they represent an earlier point the Universal expansion, when it should have been expanding faster than later points in time?

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  3. Jan 5, 2013 #2


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    This is a confusing topic, as "greater than expected" is not well-defined - expected based on which value? The current expansion rate or an earlier one?

    Accelerated expansion means a current expansion rate which is more than the previous value - but this acceleration is a recent phenomenon, in the early universe expansion slowed down. If you take that into account and see a deviation from the model, this is an expansion of the universe, where the sign depends on the time-evolution of those deviations.
  4. Jan 5, 2013 #3
    Perhaps I don’t understand all the mechanics well enough? However what I have read and thought I understood was they make a calculation of red-shift for the expected expansion of space from relativistic dynamics, than they observe the residual.

    If the residual red-shift is positive, it indicates the net relative receding velocity of the super-nova after considerations were calculated and subtracted for the expansion of space. They have confirmed the more distant the supernova, the more positive the residual. My point of questioning lies with the interpretation because for more distant supernova I would think the residual represents its velocity at an earlier age of the Universe and I would it expect it to be more positive (receding faster)?

    Do you know of anything that can help me understand why or how I may have erred in that conclusion?

  5. Jan 6, 2013 #4


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    Maxila, it sounds as if you have this backwards. What is observed is that SNe of a given redshift are DIMMER than would be expected if we didn't know about the recent acceleration, i.e. if the cosmological constant Lambda were zero.

    The distance is estimated from the luminosity because this type of SNe is a "standard candle".
    So what this means is that SNe of a given redshift are FARTHER than would be if Lambda were zero.

    Another way to say this is that SNe of a given distance have LESS REDSHIFT than would be expected, without the recent acceleration. But you have said the opposite!!!

    Here is a curve showing the expansion history---the heavy solid curve is the right one. http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg

    You can see slowing down for first 7 billion years or so, then only a little acceleration, then more acceleration.

    Nearby things have had the benefit of the recent acceleration and so they have bigger than expected redshift. Far away things which we see, say, when expansion was only 7 billion years old, have not experienced any acceleration at all! Or only a little. So their redshift is LESS than what we would have expected if we measure Hubble constant based on nearby stuff, as has traditionally been done.

    That figure is from http://arxiv.org/abs/astro-ph/0305179

    If you want, study the curve, and ask questions. For instance: Why was there slowing at first? because earlier the density was big enough to overwhelm the slight effect of the small positive Λ. the cosmological constant only became significant when the density thinned out. Then the natural built-in tendency to expand became important. We actually do not know that Λ corresponds to an energy. It behaves like a slight intrinsic constant curvature---people sometimes call it the "vacuum curvature".

    the curvature may or may NOT be due to an energy. It is wrong to jump the gun and talk as if we know when we do not know yet what this inherent curvature constant is due to. It's a common mistake that scientists fall into as well as others. "Dark energy" sounds exciting, but it is misleading to use that term for the cosmological constant Λ.

    A good discussion of that issue http://arxiv.org/abs/1002.3966
    Last edited: Jan 6, 2013
  6. Jan 6, 2013 #5
    You are right; I did have it backwards, thank you. I thought the more distant the SNe; the red shift was greater than was expected, not less. However if more distant SNe have a lower red shift than expected shouldn't that imply less expansion, not more expansion? In other words doesn't a greater red shift indicate a greater expansion of space and/or relative velocity than a lower value red shift (excluding gravitational red shift phenomena)?

    After I've read the papers you've linked, if I have any additional questions I'll post them here.

    Last edited: Jan 6, 2013
  7. Jan 6, 2013 #6


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    It implies less expansion between their emission of light and now. For fixed scale now ("1"), less redshift corresponds to a bigger universe earlier.
  8. Jan 9, 2013 #7
    I was able to find several credible sources that make it clear what they have observed to conclude the Universe is accelerating. Nothing I read mentioned greater or lesser redshift than expected, which is what I originally thought why they concluded the expansion was accelerating.

    You may be interested to know mfb, it is not "less redshift corresponds to a bigger universe earlier"; rather it is that for a given redshift, more distant SNe are dimmer than expected, leading them to conclude the expansion rate has accelerated, making them more distant than their given redshift indicates.

    Here area few links where this is discussed:



    Source: http://www.thehindu.com/sci-tech/sc...niverse-wins-physics-nobel/article2512038.ece

    Hope this helps clear up what evidence was used to determine the acceleration. I still couldn't find how they accounted for an expected greater redshift in more distant SNe due to that being an earlier time in the Universe when they should have been traveling at a greater relative velocity? If anyone has any references to that part of the resolution of this phenomenon I'd appreciate a link.

    Last edited: Jan 9, 2013
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