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The first radiation?

  1. Nov 17, 2013 #1

    When did the really first radiation occur in the universe?
    I'm very confused from different sources:

    1." The oldest radiation in space comes from 380 000 years after the big bang.
    2. In the photon epoch three minutes after the bigbang was almost only gamma radiation.
    3. The WMAP shows the radiation of the first second."
  2. jcsd
  3. Nov 17, 2013 #2


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    Before year 380k space was made opaque because the gas filling space was so hot.

    It's a very interesting effect that we can see already exemplified by the sun and other stars. The outer layers of a star are gas that WOULD be transparent if it were cooler, so that we could see into the even-hotter insides of the star. But because the outer layer gas is dazzling glowing hot it scatters whatever light tries to get thru it and we CAN'T see.

    Around year 380k the gas filling space cooled down to below 3000 kelvin and by a combination of thinning out and cooling became effectively transparent!

    The sun's surface is about 5000 kelvin. 3000 kelvin is like the surface of a smaller more reddish star than the sun (less massive stars burn slower and cooler and are more orange color than the sun). The universe was full of that orange color light (and very sparse thinned out hot gas) and the light could finally survive and get somewhere rather than being constantly absorbed and re-emitted and re-absorbed and re-emitted and blocked and distracted and trapped in the gas. So that light from year 380,000 GETS to us.

    You may be wondering why when you heat gas hot enough it becomes more effective at scattering (i.e. blocking) light. If you can guess, fine. If you can't guess and ask, fine. Somebody will almost certainly see the question and respond
    Last edited: Nov 17, 2013
  4. Nov 17, 2013 #3
    Ok thank you,
    so there was radiation and later even optic light maybe before 3 min
    but everything was blocked by gasses until 380 000?
  5. Nov 17, 2013 #4


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    yes. gasses were in plasma state (hot).
  6. Nov 18, 2013 #5
    The CMb is the oldest radiation observed so far. I think most cosmologist believe that gravitational radiation is also there from a time way before 380,00 years after the big bang. Some even believe from before the big bang. There is also the possibility of detecting neutrinos from abut 1 second after the big bang.
    Neither primordial neutrinos or gravitational radiation has been detected and it is not likely they will be detected any time soon. But one can hope future science will get round to it. My guess is that’s decades away but Id love to be proved wrong. There is a chance one can detect gravity waves indirectly through CMb polarisation in what is known as the primordial B mode . Maybe Planck will do this with its polarisation (due in 2014) but people I have spoke to who work on the spacecraft don’t thinks its very likely, but again we can hope.

    So we are left with the Cmb which was emitted 380,000 years after the big bang. Before that the plasma was opaque as Marcus pointed out. But of course no one would suggest the photons didn’t exist before they decoupled and were set free so we can see them in the CMB.
  7. Nov 18, 2013 #6


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    Well, "before the big bang" in the sense that the big bang theory doesn't describe inflation, and gravity waves from inflation aren't much changed by any intervening matter. Perhaps interestingly, some of these gravity waves can produce a (possibly) measurable imprint on the CMB. I say possibly because we haven't yet detected them, and it's conceivable that the primordial gravitational waves were simply too small for us to ever be able to measure.
  8. Nov 20, 2013 #7
    Because Im new I cannot really discuss more in detail about complicated things yet..
    But I can try to.. :)
    If there was gravitational waves, wouldn't they leak out from any fog whatsoever? Also,
    would the waves even survive 13.7 b-years if we could detect them?
  9. Nov 20, 2013 #8


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    The imprint of ancient sound waves on CMB has already been seen and studied for over 10 years. Goes back to balloon born detectors in late 1990s, IIRC. Very low frequency density-pressure oscillations.
    Probably you can google "acoustic peaks CMB" and get some of the history.

    Waves in progress at the time the medium became transparent can leave their imprint in the temperature and polarization map which then can be seen long afterwards in the ancient light.

    Two things to remember: the matter whose light we see when we look at the CMB was only 41 or 42 million LY from here---i.e. from the matter that became us---when the light was emitted. that is not far by cosmic standards. It was a small world. The imprint of small oscillations in that world is now stretched way out and covers the microwave sky---a 360 degree surround picture.

    The other thing is that the speed of sound depends on temperature---essentially proportional. So since the gas was hot (3000 kelvin) the speed of pressure density oscillations, and the wavelengths, can be calculated.

    The detection of the acoustic peaks in the CMB is a fascinating chapter of science history, maybe you can find a good online source. I don't happen to know one off hand. If you get curious and find one, Toldox, please share it. Goes for anybody else who does too!

    About gravity wave imprint. I think that would require making a high resolution POLARIZATION map of the CMB, over and above the TEMPERATURE fluctuation map that we already have. I don't have anything definite to say about that---taking a waitandsee attitude.
  10. Nov 20, 2013 #9


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    I believe the first to confirm the anisotropies was COBE, launched in 1989, but it was extremely low-resolution. See here:

    While you could tease out the acoustic peak from the combination of COBE and balloon experiments in the mid-late 90's, it wasn't until WMAP that the acoustic peak was really clear and obvious (prior to WMAP, other structure formation scenarios than inflation were in contention, such as cosmic strings).
  11. Nov 24, 2013 #10
    Sorry, have been off for some days..

    ..Thanks, the acoustic peak was something new. I will absolutely report back if I find any more of information from the web.
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