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Expansion-based cooling during Big Bang

  1. Apr 2, 2010 #1
    Simple question. Am I right in thinking that, according to current theory, the rapid cooling occurring after the Big Bang is a direct result of the universal spatial expansion (absurd as it may seem), for the following two reasons.

    1) as the (hot) particles of matter move into that new space, their relatively high linear speed is greatly reduced. They were moving very fast relative to other nearby particles, but now they're moving, with other particles, into newly created space, and so their relative velocity (relative to the other particles that also move into that now more sparsely populated space) is much less.

    2) the photons also are moving into newly created space, reducing their density and also shifting them to the red in that new space.
  2. jcsd
  3. Apr 2, 2010 #2


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    I'd say that is a good description, hkyriazi. Expanding geometry does have those effects.

    There's always going to be some awkwardness about a verbal description because our natural human languages (like English) evolved without the experience of expanding geometry.

    Personally I don't say "new space is created", but I don't see any reason to object. I focus on the idea that "distances increase" because I can say that without suggesting that space is a material substance. I focus on the geometric relations themselves.

    Expanding geometry has been very important in the formation of structure. For matter to fall together into the cobwebby filaments that eventually spawned clusters of galaxies, it needed some way to get rid of the kinetic energy acquired by falling!

    When they do computer sims of how the first structure formed, they USE the cooling effects that you mentioned. The expanding geometry does, in effect, reduce momentum, drain off kinetic energy, and help (*dark* and ordinary) matter to condense.

    To see animated sims of early u. structure formation, google "Smoot TED" and watch an 18 minute talk given at a TED meeting by George Smoot.

    The cooling mechanism for matter particles is especially important for dark matter. Other matter can collide and radiate away energy eg as infrared. Dark matter is supposed to be unable to radiate any kind of light. So whatever kinetic energy it picks up by falling it seems to be STUCK WITH. So how do concentrations ever condense? The sims show how.

    This is key to structure formation because there is more dark matter, like 10 times more, than ordinary. So the dark matter had to begin gathering into wisps and clouds first, and then those concentrations pulled in the more sparely distributed ordinary stuff.

    So expansion cooling is vital to the standard story.
  4. Apr 3, 2010 #3
    Thank you, Marcus! Hadn't known of, or thought about, any need for matter to lose energy in condensing into molecular/dust clouds in the expanding universe scenario. Had I been pressed on it, I'd have probably figured all that would be needed for the energy of the stars. I'll check out Smoot's talk.
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