Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does the CMBR have a detectable absorption spectrum?

  1. Jun 19, 2011 #1

    JDoolin

    User Avatar
    Gold Member

    I posted this http://answers.yahoo.com/question/i...GXejba9DH1G;_ylv=3?qid=20110618143816AAgRMyF"on Yahoo Answers yesterday, but it might be too specialized:

    If I understand correctly, the Cosmic Microwave Background Radiation is a perfect thermal (black-body) spectrum. But stars have an absorption spectrum that can be used to find out what kinds of elements are in them.

    In general can the spectral lines be used to calibrate, then compared to the thermal spectrum to find out how hot a star is? (I guess that's a second question.) Is that a technique in common use?

    Main question is still: Does the CMBR have a detectable absorption spectrum?
     
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Jun 19, 2011 #2

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Maxwell's equations are linear, so light waves pass through one another without interacting. Therefore a light wave cannot absorb another light wave. This can also be seen from conservation of energy and momentum.
     
  4. Jun 19, 2011 #3

    JDoolin

    User Avatar
    Gold Member

    Hmmmm, I guess more information may be in order...

    http://resources.metapress.com/pdf-preview.axd?code=702q017w7t7320g6&size=largest

    "The detection of hydrogen recombination radiation, in particular, radiation in subordinate lines, would be direct evidence for the validty of the model of a hot Universe."

    I was wondering whether they actually had FOUND the lines, or if they were just predicting how they would appear?

    In any case, if they can't find any evidence of any lines, wouldn't that call into question ALL forms of Big Bang models?

    (Or wouldn't it at least represent a failure to support the hot big bang, or several versions of the Big Bang theory? My own pet Big Bang Theory, admittedly, being one of them. But I think it equally hurts what is known as the Standard Model, if there are no Hydrogen absorption spectral lines.)
     
    Last edited: Jun 19, 2011
  5. Jun 19, 2011 #4

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Sorry, I see that I misinterpreted your original question. I thought you were asking whether CMB photons could absorb photons from other sources.
     
  6. Jun 19, 2011 #5

    JDoolin

    User Avatar
    Gold Member

    Ahh, that makes much more sense why you answered that way.
     
  7. Jun 20, 2011 #6

    Chalnoth

    User Avatar
    Science Advisor

    Not as far as I know. The issue is that the CMB was already redshifted by a factor of around fifty to a hundred before the universe became ionized again (which happened when the first stars turned on), so that basically all of the CMB photons were already at wavelengths far below the absorption lines of the newly-ionized atoms. Maybe the absorption spectrum would be detectable at the very highest wavelengths, but the problem there is that the dust in our galaxy starts to obscure things when you get to those frequencies (besides the fact that the CMB itself is quite dim at higher frequencies).
     
    Last edited by a moderator: Apr 26, 2017
  8. Jun 20, 2011 #7

    JDoolin

    User Avatar
    Gold Member

    So, yeah. Now the redshift of the CMBR is somewhere on the order of 1000. So the 13.6 eV Lyman line would look like a .0136 eV absorption line. Do we have gratings that could reliably detect and separate such a line?

    Or maybe it wouldn't even be a dark line at all. Just slightly dimmed from the underlying thermal blackbody radiation.
     
  9. Jun 20, 2011 #8

    Chalnoth

    User Avatar
    Science Advisor

    Well, no, you won't get absorption lines at the redshift of emission. The absorption lines would come later, when the universe became ionized again, at around a redshift of 10-20 or so. So we're talking about an absorption line at 13.6eV being redshifted to around 1.36eV to 6.7eV. The 6.7eV line would be at 1.62 Petahertz (10^15 Hz). By contrast, the CMB peaks at 160GHz. So this absorption line would be at a factor of 10,000 higher in frequency than the CMB itself.

    That said, there might be some interesting stuff about the first emission of CMB photons in the high frequency tail of the CMB spectrum, but that's going to be pretty hard to detect with the galaxy in the way.
     
  10. Jun 21, 2011 #9

    JDoolin

    User Avatar
    Gold Member

    Ah, so with a peak of the thermal spectrum around 160 GHz, we'd best be looking for spectral lines somewhere around the 1.875 mm range

    and if Planck's Constant is 6.58 times 10^-16 eV s, that peak would be around 105 micro-electron volts.

    .0136 eV would be way deep in the high frequency tail. A little more calculation is in order to see about the Balmer, Paschen series, etc.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does the CMBR have a detectable absorption spectrum?
  1. Does mass have latency (Replies: 3)

Loading...