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Dark stuff

  1. May 10, 2004 #1


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    Does the path of light from distant galaxies curve according to with or without dark matter and energy?
  2. jcsd
  3. May 10, 2004 #2


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    according to astronomers, it curves according to all the mass concentrated in the galaxy (including dark matter as well as ordinary matter)

    one way they have of estimating how much dark matter is clumped in and around a galaxy or in a group of galaxies is to measure
    how much the galaxy or group bends light
    that gives an estimate of the mass concentrated in it
    which they can then compare with the mass of visible matter

    the total mass (measured by bending) might turn out to be 4 or 5 times
    the visible matter mass----so the difference can be attributed to dark matter

    but that is not the only way to estimate the dark matter in or around some object----there are several ways and so far they seem to roughly agree, which is reassuring

    several other PFers have more detailed knowledge of how concentrations of dark matter are being estimated and mapped, maybe they will chime in here

    AFAIK dark energy does not enter here because it is, as far as they can tell, uniformly distributed. It doesnt contribute to bending light because it doesnt clump together by the pull of gravity
    It is evenly spread out thru space
    so it doesnt contribute to the lensing by galaxies and clusters

    there is as much dark energy per cubic mile in the space between clusters where there is almost nothing else
    as there is in clusters, on a per cubic mile basis

    why doesnt dark energy get clumped together by the pull of gravity?
    why should it stay evenly spread out? (thus not contributing to lensing)
    I havent heard any very satisfying conjectures about this and
    suppose it's an outstanding problem.

    but dark matter, by contrast, is believed to gather around galaxies and to contribute to lensing
  4. May 10, 2004 #3


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    http://www.astro.queensu.ca/~dursi/dm-tutorial/dm1.html [Broken]
    this is a fun site about DM, is it brown dwarfs, machos, other?
    the observational evidence for it is compelling.
    Last edited by a moderator: May 1, 2017
  5. May 10, 2004 #4


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    There is a distribution of dark matter predicted (actually estimated) from observations of the CMBR (such as WMAP) and surveys of large scale structure (such as 2dF and SDSS). There is a distribution of dark matter in clusters and around individual galaxies observed (actually inferred) by weak and strong gravitational lensing, galaxy velocities, and X-ray emission. There is an absence of 'lumpy' baryonic matter (black holes, stars, planets) in and around us in the Milky Way (from surveys such as OGLE and MACHO - not to be confused with MACHOs (massive compact halo objects), which are what the MACHO survey sought to find!). All estimates are in broad agreement; of course, there are some disagreements, and much work still to do.

    Dark energy is a mystery. Postulating its existence makes a good deal of sense of some observations (WMAP, distant supernovae, etc), and neatly fits many cosmological models. Some broad statements can be made about its properties (such as those pointed out by marcus). However, its nature is, IMHO, pretty much speculative.
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