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Theories about why the universe is expanding faster

  1. Dec 9, 2015 #1
    I like physics involving the cosmos. I was wondering what theories are out there that explain how the observable universe is expanding at an increased rate rather then slowing down due to gravity or other variables. I know the dark energy theory that basically states there's a energy that affects gravity allowing the universe to expand faster. Forgive me i don't even know the full details on the dark energy theory but i would appreciate it if you give me other theories and explanations on why this phenomenon is happening. thank you
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  3. Dec 9, 2015 #2
    Also i just saw a diagram of the history of our universe and it showed that at a time our universe was expanding slower but then all of a sudden dark energy appeared and increased it's expansion to today's rate. How does dark energy appear when all the energy and matter that would ever be in the universe was created in the big bang?
  4. Dec 9, 2015 #3


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    Dark energy does not suddenly appear, but at one point it becomes dominant because it does not become diluted as the universe expands like matter and radiation does.
  5. Dec 9, 2015 #4
    so basically dark energy was there before but 7.5 billion years ago "if this chart is correct" it started to become dominant and now experts estimate that the energy takes up about 68.3 % of the known universe. What other theories are out there besides the dark energy theory for our increased rate of expansion?

    410px-Dark_Energy.jpg 244px-DMPie_2013.svg.png
  6. Dec 9, 2015 #5


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    Doesn't this suggests dark energy is more a property of spacetime rather than a object or whatever on its own?
  7. Dec 9, 2015 #6
    'Dark energy' is not really a theory in the usual sense, it's just a place-holder name for 'something' which we know exists from observations, and it has the characteristic properties associated with forms of energy.
    What the 'something' is remains a mystery at present, (hence 'dark'), but there is no leading theory.
  8. Dec 9, 2015 #7


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    Yes, the simplest interpretation is a "left-over" intrinsic spacetime curvature after inflation ended (Einstein's cosmological constant, Lambda). This interpretation still fits all the verified data, but we cannot be sure it is the 'correct' interpretation. Hence a lot of study and observation are presently trying to reduce the error bars.
    Last edited: Dec 9, 2015
  9. Dec 9, 2015 #8


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    It's not that the rate is increasing. The rate of expansion is slowing gradually. But it seems to be approaching a constant value. In a universe with a constant rate of expansion, objects accelerate away from one another. To see this, consider that expansion can be written as speed per unit distance. So an object that is twice as far away will, on average, be moving twice as fast away from us. Thus as an object moves from distance A to a distance 2A, it speeds up.

    As for why it's approaching a constant expansion rate, the expansion rate itself is a manifestation of space-time curvature. The space-time curvature is proportional to the amount of stuff in the universe. As the amount of stuff per unit volume approaches a constant (because the dark energy has a constant or approximately constant energy density per unit volume), the expansion approaches a constant. And constant expansion = things move away from one another at accelerated speeds.
  10. Dec 11, 2015 #9
    Thanks everyone this helps immensely
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