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B Cosmic background question

  1. Mar 12, 2016 #1
    I have embarked in trying to really do my homework and understand cosmology.
    From what I understand one of the ways that we can measure expansion is the the cosmic background radiation doesn't expand thereby providing something to compare observed expansion to.

    Have I got that wrong?

  2. jcsd
  3. Mar 12, 2016 #2


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    I think so. The CMB has already been redshifted enormously and is becoming more and more redshifted as we speak. It's just that the rate of expansion is low enough that we don't see a change in the redshift of anything, including the CMB, over the timescale of a century.

    When we measure redshift, we compare the emission/absorption lines in the spectrum of the object to a non-redshifted source.
  4. Mar 12, 2016 #3


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    Cosmic backround [CMB] photons are affected by expansion just like all other photons emitted by distant objects - it is diluted [redshifted] by expansion. The CMB is used as the 'rest' frame of the universe, as a matter of convenience. By measuring the redshift of photons emitted by various bodies scattered across the heavens and deducting the redshift of CMB photons from those same region of the sky, we can derive the apparent velocity of those bodies with respect to the CMB. Since the CMB has virtually the same redshift in every direction [except for a slight difference bow to stern in the direction the solar system travels through space] this method of comparison is very convenient.
  5. Mar 16, 2016 #4
    I must not be making myself clear I think...

    Using the raisin bread loaf analogy...The expansion rate is the rate at which the raisins are getting farther apart. There will be a certain rate per a certain distance, say 1/2 inch per hour between two raisins 2 inches apart at the start while baking in the oven. In other words 1st hour 1/2 inch, 2nd hour another 1/2 inch (at the same starting distance). An accelerating rate would mean 1/2 inch 1st hour then 3/4 inch the second hour and so on for raisins at the same beginning distance.

    In the real world, whatever constant is being used now to determine that a galaxy redshifted by 100 units represents a distance of 1000 distance units, will mean that in the future a redshift of 100 units will represent a different distance??

    Maybe redshift doesnt factor in. 100 units of redshift will always represent a certain distance due to the speed of light constant. Galaxy X will accelerate (apparently) from us due to even constant expansion because of the raisin effect...the farther the galaxy the faster it moves away. But in an accelerating expansion that acceleration is accelerating? In other words ACTUAL additional acceleration on top of the APPARENT acceleration.

    This year a galaxy at 13 billion light years distance will be moving away from us at 100 units of speed. Next year a galaxy at 13 billion light years distance will be moving away from us at 110 units of speed.
    The year after that a galaxy at 13 billion lys will be moving away from us at 125 units of speed.

    is that correct?

    Last edited: Mar 16, 2016
  6. Mar 16, 2016 #5


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    Not as far as I understand it. It should still represent a distance of 1000 units. The CMB is currently redshifted by a factor of 1,092, meaning that the distance between ourselves and the surface of last scattering has also increased by a factor or 1,092. Given the distance to the surface of last scattering is currently around 46 billion light-years, that puts the distance to this surface, at the time the photons were emitted, at about 42 million light-years.
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