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Questions and comments about cosmo FAQ

  1. Sep 8, 2013 #1

    marcus

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    In the interest of broader participation, feedback, transparency etc, maybe we can have (in regular cosmo forum) a thread for reaction to cosmo FAQ. Which articles have you found helpful? I've heard a number of positive comments in regular discussion threads, but ATM can't remember in which discussions they came up.

    Also have you found anything UNCLEAR? Do you think anything needs clarification (please specify which item, and quote the passage.)

    Jim Johnson recently had a question (in another thread) about this paragraph in the "total mass-energy" FAQ:
    "One can also estimate certain quantities such as the sum of the rest masses of all the hydrogen atoms in the observable universe, which is something like 10^54 kg. Such an estimate is not the same thing as the total mass-energy of the observable universe (which can't even be defined). It is not the mass-energy measured by any observer in any particular state of motion, and it is not conserved."

    https://www.physicsforums.com/showthread.php?t=506985
    I looked at it and it didn't seem clear to me why the total mass-energy in the currently observable region was UNDEFINABLE. So I was puzzled by the parenthetical clause, which I highlighted. Maybe whoever edits cosmo FAQ can put in some clarification or change it. Or maybe it doesn't need any attention!

    The main thing is it would seem to be definable, just as the observable region (from standpoint of comoving observer at solar system current location) is itself definable, and just as energy density is definable. Definable yes, but not a conserved quantity.
     
    Last edited: Sep 8, 2013
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