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Is cosmic back ground radiation able to somehow not dissapate?

  1. Aug 20, 2010 #1
    It says in numerous books that radiation dissipates after about a distance of from the earth to the sun, and also when the scientists sent the radio signal of the Beatles's music into outer space a while ago, that , even though they did that through SETI, or whatever program they did that through, that the signal will dissipate after a certain distance and not reach any aliens. If radiation dissipates and behaves this way, then how is cosmic back ground radiation evidence of the big bang? Are they saying that cosmic back ground radiation is radiation that was somehow able to not dissipate (or be absorbed)?
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  3. Aug 20, 2010 #2
    Well, if by dissipate you mean scattered in its travel, of course it can be scattered, there are many processes for that, like inverse compton scattering which creates the famous SZ effect.

    Now, if you mean that cmb looses energy, well, that does happen too. The wavelength of radiation gets stretched by a factor of a(t) because of cosmic expansion.

    You might also be wondering what happens with energy conservation if photons go to larger (and hence less energetic) wavelengths as the universe expands. To make a long story short, energy conservation in its "naive" form (dE/dt=0) doesn't holds in general relativity.
  4. Aug 20, 2010 #3


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    I think what land_of_ice is describing is simpler than what lasm2000 is talking about: radiation gets less intense as it spreads out into a larger volume of space from a particular point. It's just being spread more thinly. The CMB didn't come from a particular point, because the Big Bang wasn't an explosion at a particular point in space. The Big Bang occurred in all parts of the universe at once. Therefore the CMB wasn't spreading out into previously empty space from one point.
  5. Aug 22, 2010 #4
    As crowell explains CMBR is everywhere so it doesn't disippate in a traditional sense...but does in others.

    In a nutshell, electromagnetic radiation (photons) remains constant but space does not; Each photon continues at lightspeed, c, but their concentration and frequency changes.

    Radiation from a point source, such as a radio antenna or a pulsar, goes thru ever larger spherical areas as the radius of transmission increases....so the "intensity" or strength per unit area dissipates...a fixed number and concentration of photons at the point of emission gets "thinner and thinner" as more area is encountered. Like a light, such as a star, at a great distance looks really dim.....few photons enter you eye....but such a star would burn you in an instant up close, such as local star, "the" sun....

    On the other hand the CMBR has weakened due to the expansion of the universe: At the time the first CMBR passed thru where earth would have been back then, about 380,000 years after the bang, it would have been about 454,000 K....pretty hot....today it's only about 2.7 K...."empty" space is pretty cold these days. Over eons, that same expansion effect will also weaken our radio signals a bit, but in the few years we have been around I doubt it would be measureable...

    Check this out: http://en.wikipedia.org/wiki/CMBR#Temperature
    Last edited: Aug 23, 2010
  6. Aug 23, 2010 #5


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    We missed the really hot epoch. Our solar system is merely 4.5 billion years old. The CMB temperature has only dropped around 20 degrees since then.
  7. Aug 24, 2010 #6
    And a damned good thing we did too, or we'd be proto-nuclear soup. :smile:
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