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I Density of matter in the expanding Universe

  1. Aug 27, 2016 #1
    As I understand it, the Universe is red-shifted (emission spectra) from any point of reference looking outwards. The Universe is expanding, but is matter being created at the same rate ? Does this mean that the density of matter in space is decreasing ? (density = mass / volume). What does this imply for the gravitational field balance between it's galactic components ?
  2. jcsd
  3. Aug 27, 2016 #2


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    Matter is not being created. Why would it be? The density of matter is decreasing outside of bound systems (galactic clusters and smaller) inside of which there is no expansion.
  4. Aug 27, 2016 #3

    Ken G

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    Some famous physicists, including Hoyle, felt that matter should be being created as you suggest, so that the mass density could be conserved. But this is because they felt the universe should not evolve with time-- it should be in a "steady state." I think that sprung from a mostly philosophical commitment to the idea that the universe could not begin or end. But with the discovery of all kinds of evidence (including the cosmic microwave background) that the universe does change with time and did have an early state that was dense and hot, the steady state model has been abandoned in favor of one where the universe does have an origin and does drop in density with time. There is no important impact on galactic potentials, they come from their own local matter distributions that are not affected by the expansion-- the expansion is seen only between clusters of galaxies, on scales unbound by the gravity of local mass distributions.

    By the way, modern views of the possibility of "universal inflation,", whereby baby universes are popping off all over the place in a vast universe that is otherwise in a steady state on the largest imaginable scales, are a kind of return to the steady-state model that Hoyle himself could not resist making comment on.
  5. Aug 27, 2016 #4


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    The conditions necessary to originate matter [iow, to convert energy to matter, to be specific] only existed briefly and in the very early universe, therefore no known physical process could account for the matter creation rate necessary to maintain the universe in a state of constant matter density. The alternative to matter creation is for matter from 'outside' the universe to 'enter' it from 'elsewhere' [e.g., via wormholes or as a consequence of expansion]. But, any such explanation has some severe and rather obvious problems.
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