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Dark energy

  1. Feb 20, 2016 #1
    In popular science around 70% of the total energy in the universe is dark energy. I’m a little bit confused regarding what this energy does. Does it influence all matter with a pushing force to make the expansion of the universe accelerate or does it create new space time fabric?

    I guess that it must be the latter because otherwise the energy will be restricted to pushing the galaxies apart from each other no faster than the speed of light.

    If new space time fabric is created by this energy how do you calculate how much energy is needed? Do we know how to create new space time fabric and how much energy is needed?
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  3. Feb 20, 2016 #2


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    It increases the expansion, so it contributes to "making more space between matter".
    Not more spacetime, because spacetime is the whole thing that contains all time-dependence in it already.
    There is no energy value in Joule you could assign to this, and I don't see how you could explain that without going into details of general relativity. Both energy density and pressure influence the expansion of the universe, and the right relation between both give a contribution that accelerates expansion.
  4. Feb 21, 2016 #3


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    Mentor note: removed quote of deleted post

    The simplest interpretation of the accelerated expansion is that there is a small, constant negative spacetime curvature (a.k.a. the cosmological constant, [itex]\Lambda[/itex]) that was present since the end of inflation. Normal matter and energy cause positive spacetime curvature, which has been overwhelming the cosmological constant for the first half of expansion history. Presently [itex]\Lambda[/itex]-curvature dominates, resulting in overall negative spacetime curvature. This causes accelerating expansion.

    Just like the positive spacetime curvature caused by matter and radiation can be converted into an energy density, [itex]\Lambda[/itex] can be converted into an equivalent energy density. That's why it is usually given as a fractional part of critical energy density, i.e. [itex]\Omega_\Lambda \approx 0.7\Omega[/itex]. Critical energy density is reduced by expansion while [itex]\Lambda[/itex]-density remains constant, hence the fraction [itex]\Omega_\Lambda[/itex] increases over time.

    PS: observation still favors a constant [itex]\Lambda[/itex], but it is possible that it is changing very slowly.
    Last edited by a moderator: Feb 22, 2016
  5. Feb 22, 2016 #4
    I don't follow this. If the expansion was due to the curvatures in space time the matter should behave like it does in gravitational fields? That is, never exceed the speed of light, experience time dilation and so forth.
    What I've heard in popular science is that the matter is almost stationary and that new space is created between the galaxies which makes them further and further apart from each other. Is that an incorrect picture?
  6. Feb 22, 2016 #5


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    That's correct. It doesn't
    That's the correct picture. There is no proper motion involved, things just get farther apart. The RECESSION velocity, which is not constrained, is about 3c for objects at the edge of our Observable Universe.
  7. Feb 22, 2016 #6


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    The large scale gravitational potential is approximately homogeneous, i.e. we do not consider the localized potential wells caused by mass concentrations. Large scale positive spacetime curvature result in decelerating expansion and negative spacetime curvature in accelerating expansion. It does not tell us what originally have caused cosmic expansion. There are various theories for that, e.g inflation, but it is generally not considered as part of the LCDM cosmological model.
  8. Feb 22, 2016 #7


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    And just to add one more clarification, NOTHING ever "experiences" time dilation. That's something that is seen by observers who are not at rest in the frame of an object or who are at a different gravitational potential.
  9. Feb 22, 2016 #8


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    Because of the terminology that is often used, its often interpreted that 'space' is some physical thing that expands with the universe, or that 'new' space is created with expansion. That is not strictly true. Things do move further apart, as in the distance grows between them, but no new space is created between them. That 'space' already existed.
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