Re: Cosmology

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

Cliff Parker wrote:

> I wonder though why you would so definitively state that our
> galaxy is not expanding.  If the Milky Way was expanding at a rate similar to
> that observed in the universe at large would we have techniques capable of
> measure this expansion on scales as small as our galaxy?

The Hubble law is:

v = H0*d

v = apparent recessional velocity
H0 = Hubble constant (about 75 km/s/Mpc, give or take 10% or so)
d = distance

If this law applied within our galaxy, what recessional velocities
might we see? The distance to the galactic center is about 8 kpc = 8
kiloparsecs = 0.008 megaparsec = 0.008 Mpc. Hence the Hubble law
would predict a recessional velocity v = (75 km/s/Mpc)(0.008 Mpc) =
0.6 km/s = 600 m/s. Spectroscopists can readily measure Doppler
shifts corresponding to much smaller recessional velocities. (An
example is the detection of extrasolar planets, which have periodic
Doppler shifts due to their orbital motion of a few tens of meters
per second. Another example is a police radar gun, which can measure
speeds of less than 10 m/s!) So if our Galaxy were expanding as a
whole as part of the Hubble flow, or even at just a percent or less
of the rate predicted by the Hubble law, it would have been detected
by now.

Many of the most popular (and most vexing) cosmological questions are
answered at Ned Wright's web site. You'll find it worthwhile to take
a look at

http://www.astro.ucla.edu/~wright/cosmology_faq.html

--
+++++++++++++++++++++++++++++++++++++++++++++++++++++++

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

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