Re: Expanding Universe

[Roger Freedman <airboy@PHYSICS.UCSB.EDU>]

Tim O'Donnell wrote:

> Roger,
> Let me clarify something after reading this article.
> There is limited Doppler Redshift for bodies that are close
> to ours, but most of the redshift we see is due to
> gravitational effects.  Is that correct or did I read
> something else into this article?
> Thanks,
> Tim O'Donnell


Tim,

There are actually three types of wavelength shift:

* Cosmological redshifts, due to the expansion of the universe (the
"Hubble flow"). As an example, a cluster of galaxies at a distance of
50 megaparsecs (160 million light-years) has a recessional velocity
of about 3500 km/s, corresponding to a redshift z = (wavelength
shift)/(rest wavelength) = 0.012.

* Doppler shifts due to motion within the universe. As an example,
the galaxies that make up a cluster move within the cluster at speeds
of a few hundred km/s. These motions can be in any direction, so by
themselves they could produce either a blueshift or a redshift. As an
example, the galaxy M31 in Andromeda (part of the same cluster as the
Milky Way, and a mere 750 kiloparsecs away) is approaching us at 300
km/s. For our example cluster of galaxies at 50 megaparsecs, the
superposition of the motion of galaxies within the cluster on the
cosmological expansion gives a range of recessional velocities from
about 3200 to 3800 km/s. Doppler shifts of galaxies within clusters
are less important for more distant clusters, which have larger
cosmological redshifts.

* Gravitational redshifts. These appear when light climbs out of a
gravitational well. They are quite small - for instance, the
gravitational redshift of light emitted from Sirius B, a white dwarf
star, is about 0.0003. Gravitational redshifts are unimportant in
cosmological problems.

Hope this helps.

Cheers,
Roger

PS - I'll throw in a shameless plug for my astronomy textbook,
UNIVERSE (http://www.whfreeman.com/universe/)
--
+++++++++++++++++++++++++++++++++++++++++++++++++++++++

Roger A. Freedman
Department of Physics and College of Creative Studies
University of California, Santa Barbara

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