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

What is Dark Energy?

  1. Oct 30, 2004 #1
    It's a very simple question i guess, what is dark energy? Is it like that proton and anti proton thing where the anti-proton contains dark energy or something like that? I read recently that they had found evidence of dark energy, what is it, what is the evidence, how did they find it and what does it all mean? Could someone clear all of that for me?
  2. jcsd
  3. Oct 30, 2004 #2


    User Avatar
    Science Advisor
    Gold Member

    The standard model of gravitation, GR, and its cosmological model, LCDM, require the existence of Dark Energy to fit cosmological observations.

    In the 1970's it was realised that there were three problems with GR, the horizon problem, the density problem and the smoothness problem. Guth discovered the theory of Inflation by studying the effect of the Higgs scalar field on the early universe. The universe is believed to have exponentially expanded at an enormous rate to 10^60 times its previous size at about 10^-33 second after the Big Bang. This sudden inflation solved the three problems but required the universe to have a density equal, pretty well, to the critical density at which the space-time of the universe is flat.

    However it seemed that not enough mass was around, only about 2-4% of that required. Dark Matter was invented, some sort of exotic particle that has not been discovered and this also helped to explain why galaxies and their clusters were revolving at too high a rate for the matter that could be seen. However even with DM not enough mass was around, only about 30% that required.

    Next distant Type Ia supernovae were discovered to be fainter than theory said they should be. To explain this it seems as though the universe has accelerated in its expansion against the normal expectation that it slows down because of the gravitational attraction of one galaxy on another.

    Finally the WMAP satellite looking at the fluctuations in the microwave background discovered the universe was flat after all. These were the major observations and there are several other minor ones. To make up the difference between matter, dark matter and the density needed to make the universe flat dark energy was invented, and perhaps it can explain why the universe is accelerating as well!

    If you don't know what it is, don't worry, nobody does! We don't know what dark matter is either and Inflation requires a 'Higgs boson' fundamental particle that hasn't been discovered. So make of that what you will. Some say there are other explanations for these observations but the cosmological community is very sure that they all do exist and they are known with "precision". (4% matter, 23% dark matter and 73% dark energy)

    So the LCDM model is robustly established, or so we think, and yet we have not the slightest idea what 96% of the universe is. That is the state of cosmology at the beginning of the 21st century!

  4. Oct 31, 2004 #3
    Hey, thx grath for clearing that up for me. You know, that's what i like about physics, like one eminent scientist once said (forgot the name) "we all agree your idea is crazy, but is it crazy enough to work?" ;)
  5. Oct 31, 2004 #4


    User Avatar

    Ha ha, that's great. Ehh... Are dark energy and the normal energy different?
  6. Oct 31, 2004 #5
    I believe the name is misleading, energy is energy, the ability to work is the same no matter what. A better name for it would be Dark Gravity.
  7. Oct 31, 2004 #6
    Its something physicists made up so that their theories would make sense. :)
  8. Nov 3, 2004 #7


    User Avatar

    Yep, that happens a lot, isn't gravity propartional to mass? Why's there so much more "dark gravity" than mass?
  9. Nov 4, 2004 #8
    Thats how the neutrino came to be 'invented'. As Beta decay seemed to break conservation laws, a massless, non-reacting particle was called in to solve the problem - the neutrino! It, of course, has since been detected.....
  10. Nov 4, 2004 #9
    see this link:
    http://universe.gsfc.nasa.gov/science/darkenergy.html [Broken]
    Last edited by a moderator: May 1, 2017
  11. Nov 4, 2004 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    the dark bit only comes into it because its difficult to "see"
  12. Nov 5, 2004 #11


    User Avatar

    NASA site's
    Is this ZPE?
  13. Nov 5, 2004 #12


    User Avatar
    Science Advisor
    Gold Member

    Indeed, 'grath' is rather familiar with these matters. FYI, the version of that quote I am familiar with"

    "Your theory is crazy, but it's not crazy enough to be true."
    - Niels Bohr (to a young physicist)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook