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Dark energy

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  1. Jun 15, 2014 #1
    Ok this may be another stupid Question that I always seem to ask! Does Dark Energy follow the same rule as energy? E=MC^2 or E=MC2?
     
  2. jcsd
  3. Jun 15, 2014 #2
    we don't know what dark energy is exactly, so we have no idea what particle to use in that formula.
     
  4. Jun 15, 2014 #3
    Thanks for your answer
     
  5. Jun 16, 2014 #4

    Matterwave

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    The name "dark energy" is perhaps a poorly chosen one. We are as yet completely clueless to the underlying mechanism behind it. The simplest model which explains dark energy is the regular cosmological constant. In this case, the dark energy is a manifestation of a constant of integration in Einstein's Field Equations. As such, it does not behave like any particles that we know of.
     
  6. Jun 16, 2014 #5
    Thanks so much for your answer! Dark energy is a mysterious thing. From what I've read it seems to be growing stronger too.
     
  7. Jun 16, 2014 #6
    I just wanted to clarify I meant to say seems to to growing stronger but not growing in the total percentage of it to everything else in the universe
     
  8. Jun 16, 2014 #7

    phinds

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    No, it is not. As far as is known, dark energy has been around since the beginning or the universe (or shortly thereafter) and has always had the same energy density per unit volumn. What you are probably getting confused by is the fact that the EFFECT is getting larger because the space between galaxies is getting larger. It's a snowball effect.
     
  9. Jun 16, 2014 #8
    Does dark energy actually exist or is it a theory? Any proof/evidence of it?
     
  10. Jun 16, 2014 #9

    phinds

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    I think you have a fundamental misunderstanding. Experimental evidence is unequivocal that SOMETHING is causing the universe's expansion to accelerate. We have no idea WHAT is causing it so someone made up the name "dark energy" as a stand-in phrase so that we don't have to go around all the time saying "whatever it is that is causing the universe's expansion to accelerate"
     
  11. Jun 15, 2016 #10
    Am I to understand the effect of dark energy is more prominent in areas of less baryonic matter?
     
  12. Jun 15, 2016 #11

    phinds

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    In bound systems such as galaxies and even galactic clusters, there IS no effect of dark energy. It's like an ant pushing on a house. It's not that the ant isn't producing a force, it's that the force is so insignificant that it has no effect on what it is being applied to because there are other forces involved that swamp it.
     
  13. Jun 15, 2016 #12
    So if the effect of dark energy is constant, but not as prominent within large amounts of matter (mass), then could a assumption be made that the effect of dark energy is countered (minimized) by the very existence of large clusters of matter (mass)?
     
  14. Jun 15, 2016 #13
    Apologies of the reiteration, but is dark energy only presumed to be a force in the presence of mass or is it based on some kind of test result?
     
  15. Jun 15, 2016 #14

    phinds

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    I don't know if there is any empirical evidence or not but it is assumed that dark energy exists everywhere with the same density. It just has no EFFECT in bound systems, as I said.
     
  16. Jun 15, 2016 #15

    Chronos

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    Expansion of the universe has been known to cosmologists for nearly a century. In the 1990's two research groups lead by Perlmutter and Riess observed distant supernova in an attempt to quantify the rate of expansion. By focusing on a specific type of supernova with a well known peak luminosity, they were able to deduce the rate of expansion of the universe has been speeding up over the past ~5 billion years. This is because supernova within 5 billion light years of earth were systematically fainter than they should have been, suggesting they were receeding more quickly than expected. Borrowing a concept from classical physics this phenomenon was dubbed 'dark energy'. The simple reason being it requires energy to increase the distance between massive bodies in classical physics. The nature of this alleged 'energy' is, however, completely unknown [hence the prefix 'dark']. The simplest explanation is called the cosmological constant, which is basically a small intrinsic curvature of spacetime in addition to that imposed by gravity. Einstein accomodated this tiny additional curvature by simply adding an integration constant to his field equations. He was among the majority of scientists of his time who believed the universe was static and unchanging and this was the easiest [and mathematically valid] way to avert the gravitational collapse of the universe which otherwise appeared inevitable. He subsequently rejected his cosmological constant idea [deeming it unnecessary] after Hubble discovered the universe was expanding. The tiny extra curvature imposed by lambda [the modern term for the cosmological constant] is too feeble to have any measurable affects on gravitationally bound systems.
     
  17. Jun 15, 2016 #16
    No, dark energy should exist independent of mass, it seems to be a property of spacetime itself.
     
  18. Jun 15, 2016 #17
    Exactly my point. The only observable counter to the effect of Dark energy is mass. Otherwise it would be quite likely that the there would be a run away expansion event. This begs the question of why this expansion effect is increasing if the mass of the universe hasn't changed (reduced)....This maybe a subject for another thread.

    Never the less, galactic formations seem to be overwhelmingly resistant to these effects. So could it be a rational conclusion that the mass within the universe is reducing its density by way of thermal and light exhaustion radiating away from its previously dense cluster?
     
  19. Jun 15, 2016 #18
    Note: density is used as the basis of gravitational cohesion...
     
  20. Jun 16, 2016 #19
    Sort of: gravity is the counter to dark energy, but it doesn't work the same way.

    The expansion effect is increasing exactly because the total mass in the universe hasn't changed. As the universe expands, the mass gets further and further apart, making their gravitational effect on each other lower and lower. Dark energy comes from space itself, so as everything gets further apart, the effect gets higher and higher. We're at the only time in the universe where gravity and dark energy are essentially balanced (within the same order of magnitude as each other,) in the next couple of billion years, dark energy will come to dominate.
     
  21. Jun 16, 2016 #20
    Could you explain this? I thought lambda appears in the EFE as an ad hoc term to describe what we see. I know lambda is derivable as a constant of integration in unimodular gravity for example, but is true for GR? Thanks,
     
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