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Dark Energy Interation With Gravity?

  1. Jul 9, 2007 #1
    So we don't really know what DE is quite yet, but because it is a form of energy density, does it bend space to produce gravity like other, more dense, concentrations of it (like mass)? On a universal scale, does dark energy curve spacetime? I'm thinking this is an untestable idea, since it would tend to rely on absolute space, but what does the PF community think?
     
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  3. Jul 10, 2007 #2
    Thinking of dark energy in terms of the cosmological constant, it clearly curves spacetime. Like all curvature in general relativity, it does not require absolute space to describe or measure.
     
  4. Jul 10, 2007 #3
    Dark Energy has never been directly detected, rather its existence has been inferred precisely because of its gravitational effects. It is not believed to have any form of interaction other than gravitational, so if it did not curve space-time, it would have no effect at all, and there would have been no reason to assert its existence.
     
  5. Jul 10, 2007 #4

    pervect

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    There are several sub-types of "dark energy". The most common is a cosmological constant. In this case, dark energy has a positive energy density and a negative pressure.

    If we imagine a box containing dark energy in a room containing dark energy, the box acts like it has a negative pressure on the inside and on the outside, so the box does experieicnes no forceson its walls. If we imagine a box containing dark energy in a "true vacuum", the box tends to want to implode because there is zero pressure on the outside and a negative pressure on the inside.

    The pressure is important because pressure contributes to gravity. If you look at a static system, you can define the Komar mass density to be the energy density plus three times the pressure. This Komar mass turns out to be negative for dark energy, which is why it makes the expansion of the universe accelerate rather than deaccelerate.
     
  6. Jul 10, 2007 #5
    DE is not my speciality but its existence is suggesting three questions that I believe it is correct to ask here:

    1) Are we sure that there is only one type of gravitationnal field in the universe (of course: the Newtonnian one)?

    2) If there were others, would we be able to detect them and how far would they have a contribution to this DE?

    3) Is there any relationship between the type of the gravitationnal field we know and the classifiacation of the particles?

    Thanks for the answers.
     
  7. Jul 11, 2007 #6
    Well, the one thing we DO know is that it isn't the Newtonian version - Einstein's General Theory of Relativity has been verified to be the more correct description of the classical gravitational field.
    That's like asking "if there's something that no one knows, what would it be?" There are currently no theories that I'm aware of that require additional interactions coupling only to mass/energy, which is what a "new" gravitational field would have to be. But really, even if you had one, it wouldn't be "another type" of gravity, it would simply alter the one we have now. Maybe you'd say, "gravity is transmitted by two particles, the Type I graviton and the Type II graviton," or something like that.
    There's not currently any accepted single theory that involves the classification of fundamental particles and gravity. Superstring Theory does incorporate both into a single framework, but it's not universally accepted.

    - Bruce
     
  8. Jul 11, 2007 #7

    pervect

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    I'm not aware of any proposals that have more than one type of gravitational field. GR only has one sort. (I suppose one could try to say that gravitomagnetism is not a part of gravity, but a "different field", but that would be very strange and non-standard).

    This is rather different than being "sure" - is science ever "sure" about anything?

    On a related note, there are some proposals such as Branse-dicke that have unknown sorts of fields that cause gravity, but these unknown fields aren't themselves gravity, for instance they have the wrong spin characteristics. They contribute to the stress-energy tensor of GR just like any other field does.
     
  9. Jul 13, 2007 #8
    Oh fine, it works again ! (Thanks). I was so glad to become an answer and could not defend my opinion... no more probleme; super.

    Within a relativistic approach (so far I understand it), gravitation is correlated to deformations of the geometric structure. Each type of field (gravitation field inclusively) is carrying a certain amount of energy (materialized via its associated stress energy tensor) which is contributing to a deformation of the geometric structure and we have to use the equations of Einstein to get an idea of the modification of the Ricci tensor due to the different energetic contributions.

    When we are saying: “There is a certain amount of dark energy in our universe” (I think it is about 60% - 70% of all energies…); do we say: measuring different trajectories of planets and galaxies and supposing the Newton’s laws are valid, we come to the conclusion that there must be a lot of undetected concentrations of invisible energies to explain what we see; and do we say: because it is influencing the positions of the visible matter relatively to the theoretical positions it was expected to be, then it is proving that this dark energy owns a gravitational effect?

    My ignorance of the answers to these above questions was motivating my question 1) a few days ago. If we are only equipped to detect gravitational fields obeying the Newton’s law and if we cannot measure the gravitational fields with a technology including directly the relativistic approach, then, perhaps, we miss a part of the gravitational effects. This was my idea (certainly wrong).

    Secondly, since every type of field can act on the local curvature and after that on the geometry and on the universal gravitation, how can we measure the part coming only from the attraction between the visible pieces of matter (that we could call the true gravitation or the classical one) and make the difference with the other sources? Do the measures automatically include all the sources? (I suppose that your answer will be yes)

    Question 2) is certainly not the best question I did formulate during my life, I agree too. That is why I thank you for the effort you have made to give it a consistency via the introduction of the hypothesis of a graviton type I, II, … No, what was always fascinating my mind as I was a young boy is the following. Look at the universe like if it was an enormous box. Inside of this box you observe (specially at night if the weather is good and if you are not living in a too polluted area) small corns of matter (asteroids, comets, planets, galaxies, clusters of …). Either if you look near you and scrutinize the structure of the matter (atoms, particles, sub-particles, …) or if you study the repartition of matter in the box, you will come to the unique conclusion: vacuum, vacuum, and only vacuum… More than 99% of all volumes in the box are “fulfilled” with vacuum! At this moment, looking for convenient similar natural phenomenon in the nature, I could not avoid the comparison with the birth of a crystal. Corns of matter are like the first stones of a crystal. The second image that was arising was: as soon as these corns are observable, they attract each other, they undergo the universal law of attraction and this law seemed to be a characteristic of this kind of matter (the observable one). With other words: attraction is the force in the box that explains the growth of the crystal and the crystal is the super-structure observed by the satellite Hubble.

    The fact that gravitation seems to be a natural property of the type of matter that we are able to observe is the motivation for the question 3). A veiled relationship seems to exist between the gravitation -which is only one type of acceleration field in the box- and the existence of observable matter.

    For sure: nothing is sure inside the box; everything only is more or less probable ! But, by chance some part of physics can make predictions with a extremely good accuracy.

    Once more time: thanks for the participation to the debate.
     
  10. Jul 13, 2007 #9

    pervect

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    As far as discrepancies in particle motion go, we do see them, but this is caused by "dark matter" rather than dark energy. For instance, galactic rotation curves show that galaxies must contain "invisible" gravitating mattter. This is not really a relativistic effect though - Newtonian gravity also needs dark matter to explain the rotation curves.

    There is now other evidence for dark matter as well, see for instance http://www.msnbc.msn.com/id/14453775/

    But your original question was about dark energy, not dark matter.

    Dark energy is needed to explain the fact that the "scale factor"of the universe is accelerating. The idea that the scale factor is accelerating is relatively new, and is due to using supernovae as "standard candles". FRW models would predict that the expansion should be deaccelerating, "dark energy" (such as a cosmological constant) is what's needed to explain this.

    Thus we don't really see any obvious particle deviations due to dark energy like we do for "dark matter", what we do see is a very weak but large scale effect on the geometry of the universe at very large distances.

    There are other arguments for the existence of dark enregy as well - see for instance http://prola.aps.org/abstract/PRL/v84/i16/p3523_1 (or at least the abstract).
     
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