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

Dark matter and dark energy

  1. Jul 15, 2010 #1
    How it's possible to discern dark matter from dark energy?
    Why do we need both things? How we can tell so accuratelly that the universe
    is made up 71.3% of dark energy and 27.4% of a combination of
    dark matter and baryonic matter.
    Thanks for answer.
     
  2. jcsd
  3. Jul 15, 2010 #2
    I find it in wikipedia:

    The total amount of matter in the universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density.

    How it's possible to determine amount of matter from these measurements.
     
  4. Jul 16, 2010 #3

    Chalnoth

    User Avatar
    Science Advisor

    The behavior of the two is completely different.

    Dark matter is a name for the missing matter we see in objects like galaxies and galaxy clusters. Basically, when we look at these objects, we find that most of the matter is in a non-visible component. Currently we've narrowed down the possibilities so that we're reasonably sure that dark matter is composed of some sort of massive particle that interacts only weakly with itself and with normal matter (somewhat like neutrinos, but neutrinos would have had too much kinetic energy when produced to explain our observations).

    Dark energy, by contrast, is used to explain a very different sort of observation: the expansion of our universe after a few billion years ago has begun to accelerate. Right now it is much more speculative and open as to the possible explanations, but some of them have been eliminated, at least. Currently the prevailing explanations are some sort of modification of gravity at extremely large distances or an energy field that tends to stay constant or nearly constant in density as the universe expands.
     
  5. Jul 16, 2010 #4
    The 71.3% is the negative pressure energy that's required for the accelerating expansion model of the universe to be complete.

    Although Einstein believed the universe to be static at the time, GR had flexibility to accommodate "Dark Energy" known as The Cosmological Constant.

    Or so I believe.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook