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Questions on dark matter and dark energy

  1. Dec 10, 2012 #1
    I have been researching online, but there are many things I am still unsure of because its hard to find the answers to specific questions.

    1.) I know how scientists observe dark matter. They can view its gravitational effects. I want to know exactly how it is distributed throughout the universe though. I've heard that it clumps together on the edges of galaxies, but is this true for all galaxies? Can clumps of dark matter be seen out in the vacuum with no proximity to normal matter at all, or are they always attached to "normal" matter?

    2.) Does dark matter move with "normal" matter or does it generally stay stationary at one point in the vacuum?

    3.) Does it appear to be the same kind of dark matter throughout the entire universe, or do some clumps of the same size appear to have larger gravitational effects than others?

    4.) How do scientists observe the effects of dark energy other than seeing that the expansion of the universe is accelerating?
  2. jcsd
  3. Dec 10, 2012 #2


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    Dark matter and dark energy are related by name only, they are distinctively different. We infer the existence of dark matter by its gravitational effects. We have no idea about the properties of DM particles. It does, however, appear to be 'cold' - i.e., generally not moving at relativistic speeds. The existence of DE is inferred strictly from its effect on expansion of the universe.
  4. Dec 11, 2012 #3


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    In the early universe, normal matter and dark matter tended to attract one another, so that an overabundance of dark matter generally also coincides with an overabundance of normal matter.

    The normal matter, however, experiences friction, and so collapses much more than the dark matter, which experiences no friction. So it's not so much that dark matter collects around the edges of galaxies, but that galaxies form in the centers of large blobs of dark matter.

    Now, it isn't quite as simple as this, because when stars start to form, things tend to get violent, and a low-mass overdensity will blow out most of the matter shortly after the first stars form, leaving a galaxy with almost nothing but dark matter.

    Yes, dark matter moves with normal matter. Most of the time. As above: normal matter experiences friction, dark matter does not. Gas clouds tend to collide, while dark matter will pass right through a collision. See this blog post for a neat description:

    There's no evidence of any difference in the makeup of dark matter, and there's no good reason why we would ever expect this.

    A smaller but still noticeable effect is on the rate of formation of galaxy clusters. Dark energy tends to make such clusters form more slowly. This effect can be seen in the CMB with what is known as the Integrated Sachs-Wolfe effect, and there are also other studies directly measuring the formation of structure in the universe (such as weak lensing surveys) that are investigating this.
  5. Dec 11, 2012 #4
    My big question is whether or not there is a significant (statistically averaged)derivative in redshift with respect to radial distance(δz/δr where z is the redshift). In particular if nearer intergalactic objects(ok, galaxies included :p) have greater or lesser redshift than very distant objects. And by very distant I mean a radial distance r≥ 1,000,000,000 light years.
  6. Dec 11, 2012 #5


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    Average redshift increases monotonically with distance. Precisely how it increases with distance depends upon the rate of expansion over time, which is not constant, and not terribly simple at longer distances.
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