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Dark matter - dark energy

  1. Jul 31, 2004 #1
    dark matter --- dark energy

    Can anyone explain to me the difference between dark matter and dark energy ???

    What are their proper relations to the expanding universe???

    Dark matter should be taken into account because we know not enough matter that can be held responsible for keeping galaxies together right ???

    tanti auguri
  2. jcsd
  3. Jul 31, 2004 #2
    Dark matter is that matter which accounts for the gravitational mass in galazies which makes the rotation curve (rotational speed plotted against distance from center of galaxy) approximately flat. Dark energy is that matter in the universe which is responsible for the accelerating expansion of the universe.

  4. Jul 31, 2004 #3
    Dark matter is considered to be made from exotic particles like WIMPS (WEAKLY INTERACTING MASSIVE PARTICLES) or MACHOS or it could be made from other undiscovered exotic particles.Dark matter might account for the extra gravity that allows stars to have higher velocities than normal in spiral galaxies and that allows galaxies to orbit one another at speeds far higher than Newton's laws allow (Modified Newtonian Dynamics, MOND, a theory by Mordechai Milgrom,changes Newton's force law and accounts for experimental data very closely indeed but no-one knows why MOND works!).Dark energy is considered exist everywhere in the universe and to be more energy like than mass like - hence the name dark energy .Many theorists believe dark energy is a scalar field called quintessence (there is also a version called spintessence).Dark energy has a constant energy density throughout the universe
    and the total amount of dark energy increases as space expands (some people think the extra dark energy is made from gravitational potential energy).Dark energy appears as the cosmological constant in Friedmanns's equations and has negative pressure - it is gravitationally repulsive and it is causing the expansion of the universe to accelerate .Observations of the brightness of the same supernovae at different times have confirmed that the expansion is accelerating-the supernovae are dimmer than expected after a fixed time has elapsed, so they are further away than can be explained by a uniform velocity.It is not known for sure that dark matter exists but the
    evidence for dark energy is convincing.Dark energy may be dark matter in some theories.Most of the mass of the universe is considered to be in the form of dark energy - at least 70 per cent.
    Last edited: Jul 31, 2004
  5. Jul 31, 2004 #4

    Thanks, Pete for replying.

    I have one question though. Do we have any idea on much percent of the "total mass" is dark energy, dark matter and just the kown matter?

    for example : 75% dark energy, 14% dark matter, 1% kown matter???

    How can we distinguish between dark energy and dark matter ?? How were they discovered ???
  6. Jul 31, 2004 #5
    I heard that the universe is something like 70%dark matter. And to answer your last question, dark matter was first proposed because stars around gallaxies accelerate at the smae rate, no matter if they are close or near the black hole in the centre, this is why there is a lot of dark matter found at the edge of gallaxies.
  7. Jul 31, 2004 #6
    70 % dark Energy,25 % dark Matter, and 5 % baryonic matter.
    We are made from baryonic matter.
  8. Jul 31, 2004 #7
    But how is this socalled quintessence been discovered and why is the dark energy represented by a scalar field ???

    I know that scalar field do not interact through elektromagnetic interactions because of the spin 0 quanta that they carry. Does this have anything to do with the discovery of dark energy.

    Nenad, thanks for explaining the dark matter-discovery, i got the point...

    Thanks also to my dear Yang-Mills-friend Kurious

    marlon (not brando, unfortunately)
  9. Jul 31, 2004 #8
    Dark energy was discovered because of the supernovae being dimmer than expected - in 1998, I think.
  10. Jul 31, 2004 #9
    marlon: quintessence has not been discovered yet, it's only hypothetical

    there are other proposed candidates to act like dark energy: acceleressence, phantom energy, Chaplygin gas, kinessence,...
  11. Jul 31, 2004 #10

    ok, but how is this related exactly to dark energy and what about that scalar field quintessence ???

  12. Aug 1, 2004 #11
    ok, but how is this related exactly to dark energy and what about that scalar field quintessence ???


    The brightness of light from a supernova at one point in time can be compared to the brightness of light from the same supernova at a later time.The light is much less bright than expected when it is measured at the later time because something is causing the supernova to move away from the Earth faster than expected.
    Dark energy is believed to be causing this faster than expected movement by speeding up the rate of expansion of space.Einstein had already guessed that such a negative pressure - which is what dark energy is - could exist, and put it into one of his equations as the so-called cosmological constant.Nobody knows what dark energy is made from- is it waves particles etc.Some theorists are saying that dark energy is
    the energy associated with a scalar field called quintessence - a field new to science -
    that fills all of space.
  13. Aug 1, 2004 #12
    thanks to you all for explaining

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