Chandra casts cloud on alternative to dark matter

Discussion in 'Astronomy, Exobiology, & Cosmology' started by thed, Oct 24, 2002.

  1. thed IT Gopher Registered Senior Member

    Messages:
    1,105
    <a href="http://chandra.harvard.edu/photo/2002/0021/index.html"><img src="http://chandra.harvard.edu/photo/2002/0021/0021_xray_dssopt_comp.jpg" width="425" height="189" border="0" alt="Click on image for more info"></a>

    "An alternative theory of gravity called MOND, for Modified Newtonian Dynamics, does away with the need for dark matter. However, MOND cannot explain the Chandra observation of NGC 720, which shows that the dark matter halo has a different shape from that of the stars and gas in the galaxy. This implies that dark matter is not just an illusion due to a shortcoming of the standard theory of gravity - it is real. "
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

    Messages:
    7,415
    They're leaping to conclusions. People came up with an idea called dark matter, and the moment they see something unexplained, they say "See? That proves dark matter!" Yet they don't really know much about what they're seeing yet.
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. thed IT Gopher Registered Senior Member

    Messages:
    1,105
    Adam

    It's not that simple, honest.

    It was assumed that Spiral Galaxies rotated as non-rigid bodies. The inner parts would rotate faster than the outer. When they measured the rotation it was constant across the disk. It rotated as a rigid body with constant angular velocity.

    This is only possible if either the galaxy was embedded in a much larger mass, a mass that is non-light emitting or dark matter, or basic theores of Gravity are wildly wrong. Astrophysicists chose the former.

    So if Spirals have large amounts of dark matter around them then so should Ellipticals. It was found that the velocities of stars in ellipticals are larger than the escape velocities of the Galaxy. Again, this can only happen if they are embedded in large amounts of non-luminous matter, dark matter, or basic theories are wrong. Again, the dark matter approach was took.

    You get the same effect at larger scales. Galaxies in clusters typically have velocities around or higher than the escape velocity of the cluster (as measured by light emtting matter).

    IMSC, more recent measurements of mass from gravitational lenses requires more mass than is luminous. All the evidence points to there being some large amount of mass out there that does not emit any radiation at all, or at least very weakly.

    Could the idea be wrong though, of course it can. So observations like this are important. An alternative theory to explain Dark Matter (by modifying basic physics) was proposed and the evidence is against it. Each observation I've seen recently supports the idea of there being some new form of mass that we don't know about.
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. Adam §Þ@ç€ MØnk€¥ Registered Senior Member

    Messages:
    7,415
    I'm well aware of the galaxy-spinning stuff, but I dislike the idea of scientists leaping to a conclusion as they seem to have. A working hypothesis may be useful for learning new things, but simply saying "We like our gravity theory so we'll assume this is happening instead" is not good enough.
     

Share This Page