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B How can anyone explain the universal equilibrium density problem

  1. Jun 22, 2017 #1
    With NASA defining this equilibrium density at 9.9 x 10<30 gm/cm3, how can an increasingly expanding universe maintain a steady density as it expands?
     
  2. jcsd
  3. Jun 22, 2017 #2

    phinds

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    "Dark Energy" is an unknown something that causes the expansion of the universe to accelerate. It makes up something like 70% of everything in the stress energy tensor, and for unknown reasons, it maintains a constant density as the universe expands.
     
  4. Jun 22, 2017 #3
    Doesn't that suggest that new dark energy is being generated within the universe for there to be an increasing rate of expansion? Then the density could not remain constant without new material to equilibrate the expanding volume. A fixed amount of "stuff" should drop in density when volume expands, yet this seems to contradict pretty basic scientific laws.
     
  5. Jun 22, 2017 #4

    phinds

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    Yes, the fact that dark energy maintains a constant density as the universe expands does seem weird. If you can find an explanation for it, I guarantee you a Nobel Prize in physics.
     
  6. Jun 22, 2017 #5
    If it is happening there must be an explanation. The increasing dark energy that expands the universe should be countered by the dark matter noted by findings such as galactic rotational curves, as described by Vera Rubin. I have heard discussions about this, but while we can identify increased dark matter by the expansion of space-time, as we try to identify the entities of dark matter we are having such difficulties in finding dark matter that the only way to measure dark matter is through functions such as evaluation of Einstein-Chowlson rings. Are these findings precise enough to recognize a similar increase for dark matter as we see with dark energy?
     
  7. Jun 22, 2017 #6

    phinds

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    Dark matter and dark energy have NOTHING to do with each other and the unfortunate use of the similar names has cause more wasted keystrokes here on PF that any other single topic. There is no increase in DM to correspond to the increasing amount of DE.
     
  8. Jun 22, 2017 #7

    phinds

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    I agree, and again, if you can find it, I guarantee you a Nobel Prize.
     
  9. Jun 22, 2017 #8
    If we cannot define dark matter and dark energy well, how could we assure ourselves that do not have a relationship? We cannot estimate exactly what dark matter is so whether it might interact with itself or with dark energy has not been confirmed to the positive or negative, to my knowledge. However, if you and I can figure it out, we can share the Nobel money! ;-)
     
  10. Jun 23, 2017 #9
    Perhaps an interesting basic lecture, just for the pedagogy: arXiv:astro-ph/0703739v2, 14 Dec 2007, "Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations"; 32 pages. It doesn't give an answer to the initial question (of course) but it presents some basic elements with which a discussion can eventually start.
     
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