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  1. #11
    tony_flanders@yahoo.com's Avatar
    tony_flanders@yahoo.com Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?



    Brian Tung wrote:


    It's probably more accurate to say that ordinary matter hangs around
    dark matter -- considering how much more of the latter there is than
    the former.

    the familiar universe that we all know and love is more or less
    of a side-effect. A galaxy is a large clump of dark matter that
    gathers up gas and dust, which in turn occasionally condenses into
    stars. And those second-generation byproducts are responsible for
    the lion's share of everything we can actually observe.

    - Tony Flanders


  2. #12
    Brian Tung's Avatar
    Brian Tung Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    Tony Flanders wrote:

    Fair enough.


    It may be that they are responsible for most of the phenomena, period.
    We just don't know right now.

    For example, the Earth weighs (or, I should say, has a mass of) about
    6 sextilllion tons. Forget about what that really means; it's a lot,
    that's all. Of that, the crust weighs (and this time I can really say
    "weighs") maybe 10 quintillion tons, a little more than a tenth of a
    percent of the whole Earth's mass. The Earth's biomass is, in turn,
    a tiny fraction of the crust's mass.

    Yet I wouldn't say it's a side effect, even though the vast majority
    of the mass is represented by core and mantle. It is where most of
    the phenomena happen. The phenomena that happen in the core and
    mantle are, I would guess, largely the same things happening over and
    over, at different size and time scales. Maybe dark matter is like
    that--a mantle within which visible matter is embedded.

    --
    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

  3. #13
    canopus56's Avatar
    canopus56 Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    Ed T wrote:

    Looking at the question of dark matter only, I found this book to be
    particularly helpful and easy to read:

    William H. Waller, Paul W. Hodge. 2003. Galaxies and the Cosmic
    Frontier. Harvard University Press
    http://www.amazon.com/exec/obidos/tg...glance&s=books

    For galactic dark matter, its existence is infered in part by measuring
    the speed at which various points in large spiral galaxies rotate. The
    speed of a galaxy's disk is measured by its redshift. Portions of the
    disk moving away from our position have more redshift; the disk
    portion moving toward our position has less redshift.
    http://zebu.uoregon.edu/~imamura/209/apr7/TF.html

    Now take the rotation curve of our solar system - an idealized system
    that does not have a large amount of unknown dark matter in it. The
    rotation curve of the planets follow Kepler's Third Law. In short, the
    further from the Sun, the slower a planet moves around the Sun. The
    velocity of a planet is inversely proportional to its distance from the
    Sun.

    For the solar system, Kepler's Third Law translates into a
    characteristic declining Keplerian rotation curve -
    http://astrosun.tn.cornell.edu/acade...ion_curves.htm

    (See lower right-hand corner of four-panel figure.)

    But bright parts of large spiral galaxies donot rotate with the
    characteristic declining Keplerian rotation curve -
    http://www.astr.ua.edu/white/ay101/i.../mwrotcurv.gif

    See also rotation-curve demonstration java applet -
    http://burro.astr.cwru.edu/JavaLab/R...eWeb/main.html
    (Launch the applet and use the slider marked "Dark mass to Light Ratio
    [Solar Units]")

    One means by which a bright galaxy can exhibit non-Keplerian rotation
    is if there is a large amount of unseen matter surrounding the bright
    inner disk of a galaxy. Think of a dark binary outside the orbit of
    Jupiter. Is gravitation could "pull" Jupiter along faster than
    Keplerian rotation.

    One source of this "dark matter" is neutral hydrogren. When you look at
    a spiral galaxy like M83, it is surrounded by a large invisible warped
    disk of neutral hydrogen that can be seen using radio astronomy -
    See Figures 7 and 8 in
    Rogstad, D. H.; Lockart, I. A.; Wright, M. C. H. Oct. 1974.
    Aperture-synthesis observations of H I in the galaxy M 83. Astrophys.
    J., 193:309-319.
    NASA/ADS Link:
    http://adsabs.harvard.edu/cgi-bin/np...pJ...193..309R

    Our Milky Way Galaxy is also surrounded by similar optically invisible
    disk of neutral hydrogren gas that extends out probably 60,000 kpc -
    almost to the Magellanic clouds.

    But even adding in the neutral hydrogren disk does not account for all
    dark matter needed to make spiral galaxies rotate with their
    characteristic "flat" non-Keplerian rotation curve.

    Last month, a photo of the Virgo cluster was published by Mihos et al,
    reaching down to mag 28. The photo shows illumination of these large
    clouds of normally "dark matter" gas surrounding galaxies.
    http://antwrp.gsfc.nasa.gov/apod/ap050926.html
    http://astroweb.case.edu/hos/Virgo/
    http://astroweb.case.edu/hos/Virgo/VirgoICL.mpg

    The larger the spiral galaxy, the faster it will rotate, because there
    is more mass. After surveying a large number of spiral galaxies, a
    formulae for the rotation speed of spiral galaxies was found. This is
    called the Tully-Fisher relationship.
    http://www.noao.edu/staff/shoko/tf.html

    While I have not done the measurements myself, finding the rotation
    curve of some bright galaxies with a spectrometer appears to be within
    the capabilities of amateurs with a 10" scope. In this sense dark
    matter is not a "fudge factor" but something that the tangible effects
    of which can be directly observed by amateurs armed with a good scope,
    a CCD camera, a spectrometer and a copy of Vizspec.

    Hope this helps - Canopus56


  4. #14
    canopus56's Avatar
    canopus56 Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    > For the solar system, Kepler's Third Law translates into a

    Should be - (See lower _left-hand_ corner of four-panel figure.)

    - C


  5. #15
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    On Tue, 4 Oct 2005 18:51:00 +0000 (UTC), brian@isi.edu (Brian Tung)
    wrote:


    Right, and I didn't mean to suggest that it is arbitrary, only that
    there are theories (such as branes) that don't require it in order to
    explain what we observe. There is a very real possibility that dark
    energy doesn't exist (even though it is currently the best explanation
    we have for the way the Universe is expanding). I'm not aware of any
    alternate physical theories that don't require dark matter. In general,
    it is pretty easy to fit dark matter in with our current understanding
    of physics; dark energy, on the other hand, is truly weird stuff.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  6. #16
    Bob's Avatar
    Bob Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    Sure it looks like random noise, but at least two things can be
    measured: the average amplitude deviations of this "noise" and also the
    various "grain" sizes. The grain looks to be a fairly consistant size.
    There's also a few larger lumps of above and below average too. Other
    grain of other sizes are not seen. One can then compare this to what
    various theories would predict it would be. Or different values
    plugged into the variables of a few theories. So many theories or
    variable values can be "falsified". That is, to say that that theory
    didn't predict anything even close to what we observed, so it has to be
    wrong, or more usually, needs tweaking. Which then needs new
    predictions and observations to test it further.


  7. #17
    Tim Killian's Avatar
    Tim Killian Guest

    Default How do astronomers know how much dark energy and dark matterin the universe from a radio map?

    Observation and measured data are crucial to the scientific method,
    except for the cases where observations don't fit predictions of popular
    theories, or when the necessary supporting data are entirely absent. In
    cosmology, it's now appropriate to ignore "erroneous" real-world
    observations and create computer models that use unobserved particles or
    energies. If enough assumptions are used, and enough variables are
    tweaked, then gosh-by-golly, the models often match (within the generous
    error bars) the favored observations!

    But science in the 21st Century does have its standards, and in the
    search for ultimate truth sometimes worldly things are sacrificed for
    the greater good.

    What makes all doctrines plain and clear?
    About two hundred pounds a year.
    And that which was proved true before,
    Prove false again? Two hundred more.
    --Samuel Butler, 1860


    John Deer wrote:

  8. #18
    John Deer's Avatar
    John Deer Guest

    Default How do astronomers know how much dark energy and dark matter in the universe from a radio map?

    Why can't dark matter be dust, gas or just unlit ordinary matter?
    Or is it exotic matter all around us but undetectable?

    JD



  9. #19
    Sam Wormley's Avatar
    Sam Wormley Guest

    Default How do astronomers know how much dark energy and dark matterin the universe from a radio map?

    John Deer wrote:

    It would absorb radiation--we would be able see absorption lines.


  10. #20
    Martin Brown's Avatar
    Martin Brown Guest

    Default How do astronomers know how much dark energy and dark matterin the universe from a radio map?

    Sam Wormley wrote:

    It isn't all that long ago that theorists could be taunted with the jibe
    that cold dark matter was probably broken chair legs and old biros.
    Observational constraints have become a lot better now.


    Or be clumped in such a way as to make it virtually unobservable. Modern
    instrumentation has rather put paid to that as a complete explanation
    but some ordinary dark matter could still be hidden in galaxies as
    Jupiter to brown dwarf mass objects or black holes.

    A reasonably accessible short explanation of interpretting the WMAP data
    is at:

    http://map.gsfc.nasa.gov/m_uni/uni_101matter.html

    I dislike dark energy and find it much more of a stretch to accept, but
    it does allow simulations to match the observed data very well.

    Ultimately our theories of the universe are tested against the
    observations - if a theory doesn't work it will get junked. And the
    quality of observations and instruments is improving all the time. The
    theorists do not have much wiggle room left to play with.

    Regards,
    Martin Brown

 

 
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