Why is the universe considered as cold?

  1. Why do many claim the universe as a cold place? If it is the natural phenomena then how does formation of stars take place?
     
  2. jcsd
  3. mathman

    mathman 6,437
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    Although the average temperature is cold (~ 3 deg. K), stars get formed because the distribution of matter is far from uniform. If sufficient matter clumps together under gravity, the clump will heat up and, if large enough, become a star.

    http://en.wikipedia.org/wiki/Star_formation
     
  4. Chronos

    Chronos 9,881
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    The relative distance between stars is enormous. The high temperature of stars and low temperature of interstellar space is about as surprising as a bonfire in Maine and another in California failing to melt snow in the Midwest.
     
  5. Thanks for the reply. I would rather like to take the discussion a further ahead - whether light speed travelling through space gets affected by prevailing temperatures or is it always uniform. Also, what apart from black holes could obstruct the light travelling through space.
     
  6. The important aspect your missing is the sheer volume and overall energy density. If the volume of the universe increases, then the density of photons also increase. This will increase the pressure influence and temperature of the photons. In the case of stars which is far more dense than the surrounding space you have far higher temperatures. Google the gas laws as well as ideal gas laws (cosmology) for more details.
     
  7. phinds

    phinds 8,349
    Gold Member

    Uh ... anything that gets in its way? The Earth, for example. Other stars, other planets, interstellar gas. Now those things are so sparse that they don't stop much of it, but taken all together they stop way more than black holes do.
     
  8. Chronos

    Chronos 9,881
    Science Advisor
    Gold Member

    Objects, like black holes, planets and stars, are too uncommon to be a factor. The universe is more than 99.9% empty space.
     
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