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

Dark energy - Expansion of the universe

  1. Jul 7, 2011 #1
    Hi everyone!!

    Could you explain me how is the dark energy related to the expansion of the universe??

  2. jcsd
  3. Jul 7, 2011 #2
    It is the cosmological constant in the Einstein Field Equations.
  4. Jul 7, 2011 #3


    User Avatar
    Gold Member

    We don't really know what it is; what we do know is what we observe: that the universe is expanding and accelerating.

    Even without knowing anything more about what is causing this expansion, we know it will require energy to do so. Whatever form it takes, we call it Dark Energy.
  5. Jul 7, 2011 #4
    But why the problem of the accelerating universe is addressing to some missing mass? how can we state that? From the einstein field equation?
  6. Jul 7, 2011 #5


    User Avatar
    Gold Member

    Please rephrase.
  7. Jul 7, 2011 #6


    User Avatar
    Gold Member

    I'm going to respectfully ask if it isn't just a bit bold to claim that "we know...the universe is expanding and accelerating".

    Wouldn't it be wiser and safer to say:
    We observe the angular rotation of galaxies and currently prefer dark matter as an explanation.
    We observe the redshift of quasars and currently prefer an accelerating universe/dark energy as an explanation.

    It is very uncomfortable to put all one's eggs in the basket of invisible and potentially perpetually undetectable forms of matter and energy, and blithely say it adds up to 94% of the universe. That's too great a leap of faith for me to accept such explanations as chiseled into the stone of certainty.

    In my lifetime, a number off these sorts of explanations have been propounded as the state of the art. I think it's too soon to be anything other than skeptical.

    Last edited: Jul 7, 2011
  8. Jul 7, 2011 #7


    User Avatar
    Gold Member

    That's pretty much what I am saying.

    We observe the universe ot be expanding. We don't know why, but through some mathematical origami, it can be viewed as energy.

    Even if we have no idea of the source or manifestation of the energy.

    Think of it kind of like this: a ball is magically lifted from the ground to the table. We have no idea what lifted it there, yet we can still confidently figure out how much energy this magical movement required.
  9. Jul 7, 2011 #8
    Dark matter and dark energy is something that is completely unaccounted for. As some of you pointed out we can measure that the universe is expanding, and we're pretty sure it's correct, I think it was Edwin Hubble who discovered that galaxy's was moving away from us (our horizon in space is centered on Earth) and galaxy's further away is moving faster away from us while nearby galaxy's is moving with a lesser speed which is very similar to an explosion in this case the big bang. It requires energy for an accelerating expanding universe and we don't understand what it is = dark energy. When we talk about dark matter it is another "thing" where we are completely ignorant of what accounts for it. We don't know if its matter, we know its there because we feel its gravitation so dark gravity might be the right placeholder term until we figure out what it is. Until then dark energy and dark matter could be called Christian and Martin - we have no idea what they are.

    Last edited: Jul 8, 2011
  10. Jul 9, 2011 #9
    Dark gravity?
    The thing which separates any normal-baryonic matter to dark matter is the fact that 'dark matter' as the name suggests does not influence Electromagnetic radiation at all,gravitation however does influence it. Numerous categories of candidates have been theorized however WIMPS have come out as notable candidates in recent studies.
  11. Jul 9, 2011 #10
    Hello Ibysaiyan.

    It's correct that "dark matter" does not interact with electromagnetic radiation in any way. However, as you pointed out, gravitation does influence it. Weakly interacting massive particles could be the right hypothesis but until then I would suggest that "dark matter" is a name that could lead to a misconception but equally dark gravity might and probably will do the same.


  12. Jul 14, 2011 #11


    User Avatar
    Science Advisor
    Gold Member

    I think dark matter will eventually prove to be more than one particle species. Perhaps WIMP's, axions, neutralinos and other exotics are all out there hiding.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook