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Dark energy

  1. Mar 19, 2008 #1
    I once read that dark energy(it may be dark matter,I am a bit confused), is ripping apart the universeand was awarded as the best theory of some year.

    What is this dark energy(/matter) and waht is it to do with the ripping of the universe??
  2. jcsd
  3. Mar 19, 2008 #2


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    Dark matter is matter in that it exerts a gravitational force. It is considered necessary to explain what holds galaxies and galaxy clusters together. No one knows what it is. It is unaffected by em or other forces, except gravity.

    Dark energy is even more mysterious - its effect is opposite to gravity. It was discovered about ten years ago and it is supposed to account for the fact that the universe expansion is accelerating rather than slowing down. Some versions of the theory lead to the idea of a big rip.

    Current theory has ordinary matter making up about 4% of the universe, dark matter around 20-25%, and dark energy the rest. It is important to keep in mind that both dark matter and dark energy are based on explanations of various astronomical observations. However, neither has been observed, and it is quite possible that astrophysicists will come up with completely different explanations in the future.
  4. Mar 20, 2008 #3
    Why rip?

    If its role is to hold together the galaxies, then why is it believed to rip apart the universe??
  5. Mar 20, 2008 #4
    As noted by mathman dark mattaer and dark energy are two different things.
    Dark matter - Contributes to the gravity holding galaxies together.
    Dark energy - Provides a repulsive force pushing the Universe apart.

    Dark matter, except for our inability to see it (apparently gravity only interaction), acts like extra regular matter. Dark energy is a repulsive force acting throughout the Universe rather than as local concentrations of attractive mass like in the case of dark matter.
  6. Mar 20, 2008 #5

    Ken G

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    All that has been said gibes with my understanding as well. I'll add that one possible way to understand dark energy is in terms of a "cosmological constant", which if it is there, can be thought of as saying that when we coordinatize the locations of things in general relativity, we automatically get an effect that acts as though "space itself had energy". These are rough terms because I am not a GR expert, but you can apparently reason that if space itself (whatever that means) has energy, then more space has more energy. Normally you can obtain energy when you let things with pressure expand, but if you let space expand, you have to provide energy. That would mean that space acts like it has "negative pressure". Here's the really weird part. Normally, positive pressure "wants to expand", so negative pressure "wants to contract", but not when gravity is the main player. Negative pressure implies an antigravity effect when you put it into general relativity, and it is that antigravity effect that makes "space expand" in the picture normally used. When you use the term "rip", however, it sounds like you may be talking about "quintessence" not a "cosmological constant", so that's even another way to get it and you'd be better off with Wiki on that one.
    Last edited: Mar 20, 2008
  7. Apr 8, 2008 #6
    Its a cosmological constant until its not a cosmological constant, just like newtons law of gravity (theory of relativity 1up)
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