From: Amara Graps (amara@amara.com)
Date: Thu Aug 15 2002 - 23:14:41 MDT
Anders Sandberg:
>I honestly don't know how astronomers measure it for galaxies.
The velocity is given by the redshift of the spectral lines, which
is straightforward to measure for an individual galaxy. However,
for measuring the velocity one must also consider the 'peculiar motions'
of the galaxies moving with respect to each other ('peculiar velocity').
The peculiar velocities are randomly oriented, and for a given
galaxy, one cannot split its measured velocity into the Hubble
expansion (Big Bang) and the peculiar velocity. However,the
cosmological principle says that the typical value of the peculiar
velocity should not depend on where in the Universe the galaxy
is. Therefore, it is independent of distance, whereas the Hubble
velocity is proportional to distance. If we look far enough away
(tens of megaparsecs) then the Hubble velocity dominates and the
unknown peculiar velocity can be ignored.
http://www.astro.ucla.edu/~wright/distance.htm
Given that the expansion velocity can only be accurately
distinguished from the peculiar velocity at large distances, we need
to be able to estimate these large distances in order to measure the
expansion rate (H_0, the Hubble 'constant').
Galaxies are too far away to be measured by parallax. So then
'standard candles' are used: Cepheid variables or supernova or the
brightest galaxies in galaxy clusters. All of these have good
success at determining the relative distances between two galaxies,
and relative distances are all that is needed to confirm the Hubble
law (expansion of the Big Bang). Edward Wright's site lists in
more detail the ways to measure distances:
http://www.astro.ucla.edu/~wright/distance.htm
The standard candles give the relative distances, but to measure the
absolute distance, we also need a calibration against an object of
known distance, which is much harder.
And this 'absolute calibration' is where the problem somewhat breaks
down, although the cosmologists have come a long ways in In the last
few years. If you type in 'cosmological constant' into the Los
Alamos Preprint archive, you'll se that it's a major effort. Edward
Wright's Web site lists the age of the Universe and the state of
that research:
http://www.astro.ucla.edu/~wright/cosmolog.htm
He has other things to say about 'variable constants' too.
Amara
-- ******************************************************************** Amara Graps, PhD email: amara@amara.com Computational Physics vita: ftp://ftp.amara.com/pub/resume.txt Multiplex Answers URL: http://www.amara.com/ ******************************************************************** "Dare to be naive." -- Buckminster Fuller
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