Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Another stupid dark energy question

  1. Feb 2, 2009 #1
    rank amateur here - as i currently understand it, dark energy does not react with anything so we cannot detect it directly. DE is deduced from macro scale observations of the movements of distant galaxies, which arent behaving as they should given the amount of observed mass. (i hope all that is correct...)

    1. so, if energy is defined as the ability to do work, and DE does not interact with anything, how can we call it energy?

    2. DE is said to make up some 80% of the undetected mass/energy in the universe, and its main (only?) effect is the gravitational pull all the equivalent mass adds to a system - is that correct? if it is evenly distributed, how can it have any kind of local gravitational effect?

    3. is there some chance that DM/DE are artifacts of some slightly incorrect aspect of General Relativity, such as the grav constant, or cosmological constant, which might be tweaked to explain the observed galactic motions?

  2. jcsd
  3. Feb 2, 2009 #2


    User Avatar
    Science Advisor

    It's just a name for the effect we are observing. The term energy should not be construed to bear any resemblance to the physical concept. We might as well call it [tex]\phi[/tex]

    I believe you are confusing Dark Matter with Dark Energy. The two are very different. (Although perhaps equally mysterious)

    MOND is such an attempt at an alteration of newtonian dynamics in order to explain the observed galactic rotational phenomenon instead of appealing to Dark Matter. I haven't heard anything indicating Dark Energy might be explained through an alteration of any theory like that, though.
    Last edited: Feb 2, 2009
  4. Feb 2, 2009 #3
    There is one: our brane is slowly changing its signature from +++- to ++++, so time will end and our spacetime will become 4-dimensional euclidean space without time.

    Interestingly enough, the signature change is smooth: c just descreases. But inside the brane, for observers, c remain constant of course, but distant objects appear to be further away and tidal forces become stronger and stronger.

    Then there is a singularity, which appears to be a "Big rip" with infinite tidal forces, however it just what appears to be a singularity for an observer, outside the brane, again, everything is smooth and continious.

    I wish I did not lost a link to that theory...
  5. Feb 3, 2009 #4


    User Avatar
    Science Advisor

    I think you're mistaking energy with entropy.

    The local gravitational effect is absurdly minuscule, because it is so evenly-distributed, there's very little of it in small regions. You have to go to very large scales before the effect makes a difference.

    Furthermore, the total makeup of the universe changes with time. In the very early universe, radiation was the dominant type of energy density. Later, as the universe cooled, the radiation diluted faster than the normal matter and the normal matter came to dominate. Very recently, as the normal matter and radiation continued to dilute, the dark energy came to dominate as it doesn't diluted much as the universe expands (if at all).

    People are attempting to investigate this. It's basically been ruled out that dark matter can be an effect of an incorrect model of gravity. Currently cosmologists are rather agnostic as to whether dark energy is modified gravity or some unknown form of matter.
  6. Feb 3, 2009 #5


    User Avatar
    Gold Member

    Can you support this with some references?

    We have too-strong cluster-binding (attributed to DM) and too-flat galactic rotational curves (attributed to DM), and both require DM to be pretty obedient in its distribution and influence, yet undetectable. "Ruling out" an incorrect model of gravity is not possible, scientifically, with our current observations and models.
  7. Feb 3, 2009 #6


    User Avatar
    Science Advisor

    Some of the strongest support for this for some time has been the cosmic microwave background, but the most visual evidence stems from the Bullet Cluster. (Clowe et. al. referenced within is the paper to read if you want a scientific paper)

    Well, technically you can't rule out a model of gravity that also includes dark matter. But you can rule out one that is purported to explain the dark matter. The Bullet Cluster rules out most of those models. The ones that remain are vastly more complex than just adding dark matter anyway, so that they seem very unlikely.
  8. Feb 3, 2009 #7
    Also amateur here, but I think you are confusing dark energy with dark matter? My limited understanding tells me that the apparent movement of galaxies and the lack of mass etc is explained by dark matter.

    Also, Dark energy is the explanation for the apparent accelerating expansion of the universe.

    imho, 2 very different things?
  9. Feb 3, 2009 #8


    User Avatar
    Science Advisor

    Depends upon whether he meant the movements of far away galaxies with respect to us, or the rotations of galaxies, or the movements of galaxies within clusters.

    Right. Though there are attempts to develop exotic types of matter that would actually explain both sets of observations. Somehow I doubt they'll be fruitful, but we'll see.
  10. Feb 3, 2009 #9
    Yes, maybe. I think what prompted my response was the reference to 'observed mass' in the OP 's question. But maybe the OP can clarify? It just sounded like the 2 concepts were intemixed somehow.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook