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Thread: What is the average temperature of the universe?

  1. #1 What is the average temperature of the universe? 
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    The question just poppedinto my head. I dont think I have heard of it ever been considered?

    Zero


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  3. #2  
    Forum Professor Zwirko's Avatar
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    A little under 3 K (-270 C).


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  4. #3  
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    Hi Zwirko, its cold outside then...

    this includes the temp in the suns, black holes, and is not just the temp of space?
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  6. #5  
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    thankis
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  7. #6  
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    My first thought was what is meant by average.

    I suspect a mass-weighted average would give a very different answer than one based on distance for example.
    Meteorologist/Naturalist & Retired Soldier
    “The Holy Land is everywhere” Black Elk
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  8. #7  
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    Quote Originally Posted by ZeroZero
    Hi Zwirko, its cold outside then...
    There's no kind of atmosphere... :wink:
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  9. #8  
    Moderator Moderator Dishmaster's Avatar
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    The fact that we measure a CMBR of 2.7 K does not mean that the universe has in fact this temperature. It is the net value of the measured radiation after subtracting contributions from other sources. Temperature is always an equilibrium between irradiation and cooling. There are many contributions, in particular from the stellar radiation field that raise the ambient temperature far above that range, while processes like infrared radiation and quantum mechanical recombinations and transitions lead to cooling. The canonical temperature of the ISM (interstellar medium) has something like 20 - 30 K, while it can be well below 10 K in just forming protostars, where the dense medium shields off the photon radiation. Only cosmic rays (particles, mostly protons) can penetrate the interiors and contribute to the heating. In other regions, it is more like 100 K. I don't remember that anybody tried to come up with a actual mean temperature of the universe, but 30 K is certainly a pretty good approximation.
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