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I CMB radiation question

  1. Aug 21, 2015 #1
    Hello i have a question about cmb radiation.Cmb was release about 13.7 billion years ago and reach us from a point about 42 million light years.At the past were the age of the universe was smaller and the time that cmb release was smaller too,we were able to detect the cmb?I mean the cmb was always detectable?
  2. jcsd
  3. Aug 21, 2015 #2


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    It was always detectable from the time it was emitted, yes. The reason we didn't detect it sooner was that by the time we were around at all, the CMB was very, very faint, so it would take sensitive instruments to detect it. (If we had been around soon after it was emitted, it would have been easily visible everywhere--the sky would have been as bright as the Sun in all directions. But we weren't around then.)
    Last edited: Aug 21, 2015
  4. Aug 21, 2015 #3
    Thanks a lot PeterDonis!
  5. Aug 21, 2015 #4


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    Not quite that bright. The CMB started out at a lower temperature than our Sun, so it would have looked rather yellowish. The Sun's color temperature is around 5500K, which is white light (to us). The color temperature of the CMB at emission was about half that.. This also means it would have been roughly 1/16th as bright per unit area (energy output scales as temperature to the fourth power). But it would have been in all directions, so it would have been very bright indeed.

    Of course, there were no planets at all back then. Planets didn't start to appear until the CMB had cooled to around 30K or so, so it would have never been visible to any life form that can't see the sky in millimeter waves or longer (far infra-red).
  6. Aug 21, 2015 #5


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    An interesting question. The CMB photons we currently observe are at redshift 1090 and were emitted was about 380,000 years old. Substituting the corresponding values for the Hubble constant and lambda when the universe was a younger shows an equivalent redshift for the CMB of about 990 a billion years ago . Similarly 6 billion years ago, the CMB redshift would have been about 690. Around 11.5 billion years ago the CMB redshift was about 275. So you can travel backwards in time until the CMB photons were first emitted and still detect them, they just become less redshifted.
  7. Mar 21, 2016 #6
    What I don't understand is why there seems to be a steady supply of CMB photons arriving here from the great distances. Could the supply of photons incoming be expected to run out at some time? I never see this issue discussed in CMB descriptions.
  8. Mar 21, 2016 #7


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    CMB photons are essentially a gas of photons that permeates our entire universe. They represent around 99% of all of the photons ever emitted in the history of our universe.

    No, they won't run out any time soon.
  9. Mar 21, 2016 #8


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    I guess your question would make sense if CMB was a single source of radiation (like a supernova: happens at some point, and we observe it for some time; then it's gone)... However it is not; it is the whole observable universe becoming transparent to photons...that's also why it doesn't come from somewhere, it comes from everywhere [with small variations due to our asymmetric motion or other perturbations]
  10. Mar 22, 2016 #9
    That still does not seem to answer his question, if they are coming from everywhere they would also be coming from here, I cannot see how we can have the CMB radiation from recombination, being continuous if the source was transient. That event occurred for 20,000 years, so it has a leading and trailing edge, so once that event is over, they cannot be coming from everywhere, because they are coming from no where.

    I cannot see how you can have a continuous radiation (for all time) from an event that only occurred for a short period of time, even if that event was everywhere, it is not being created everywhere now. (if from recombination).

    I expect that the 3k radiation, is from the hydrogen in the Universe that has been independently measured to be at 3k, they are continuously radiating. (at 3k).
  11. Mar 22, 2016 #10


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    They are coming from a region that was far enough away at the time of emission that the CMBR from that region is only arriving here now. That region forms a spherical shell centered on us.

    Some the CMBR that we see today was emitted from the far side of that region (earlier and more energetically). Some was emitted from the near side of that region (later and less energetically). Due to cosmic red shift, the resulting distribution of energies we see is roughly the same regardless.

    The CMBR that we see tomorrow will have been emitted from within that same approximate region. On average it will have been emitted from a tiny bit farther away, but with the same range of emission energies and the same range of emission times. As a result, the CMBR that we see tomorrow will be a tiny bit more red-shifted than the CMBR that we see today.
  12. Mar 22, 2016 #11
    CMB is coming from a sphere of progressively more distant regions of Universe. There is infinite supply of more and more distant regions.
  13. Mar 22, 2016 #12


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    The source of the CMB is the primordial plasma that existed until our universe was some 300,000 years old or so. The CMB was emitted when that plasma turned into a gas and became transparent. Yes, sometimes those CMB photons strike something (like the Earth), and of course those photons stop traveling.

    But space is really, really big and really, really empty. Approximately 93% of the photons that were emitted have never run into anything before.
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