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

Ed T wrote:
> As a non-physicist, I am often left with the *impression* that dark
> matter and dark energy are concepts that have arisen as fudge-factors
> to preserve current theories. In other words, their existence is
> postulated to "make the math work" within the current cosmological
> paradigm. <snip>

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/detail/-/0674010795/qid=1128456013/sr=2-2/ref=pd_bbs_b_2_2/002-2810266-3838447?v=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/academics/courses/astro201/rotation_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/images/galaxy/mwrotcurv.gif

See also rotation-curve demonstration java applet -
http://burro.astr.cwru.edu/JavaLab/RotcurveWeb/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/nph-bib_query?bibcode=1974ApJ...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

.

Relevant Pages

  • Re: Nature of dark matter and dark energy
    ... > Two of the most mysterious concepts are those of dark matter and dark ... Dark matter is the extra mass that must exist in the galaxy to ... > of mass and is fundamentally made up of ordinary protons and electrons. ... > and the force of gravity. ...
    (sci.physics)
  • Re: Dark matter, antimatter, Pioneer slowing down, and so forth.
    ... > mist of dark matter... ... What if normal matter doesn't even really exist? ... > matter particles of the universe as mere residuals of the potent grand ... > the newly merged brane. ...
    (sci.physics)
  • Re: Dark fantasies?
    ... To date there is no evidence for and extra dimensions or other ... Dark Energy ... Dark Matter ... astronomy from 3 centuries of the stony silence of celestial sphere ...
    (sci.astro.amateur)
  • Re: Inflationary Theory ; Im confused
    ... homogeniety of elements in the universe, but my model makes the IP ... to congeal into matter, the matter would have acquired the property of ... was dark matter and dark energy reaching a point of repulsing visible ... >> AE once said that motion was meaningful only between two objects. ...
    (sci.physics)
  • Re: Inflationary Theory ; Im confused
    ... homogeniety of elements in the universe, but my model makes the IP ... to congeal into matter, the matter would have acquired the property of ... was dark matter and dark energy reaching a point of repulsing visible ... >> AE once said that motion was meaningful only between two objects. ...
    (sci.astro)