Re: neophyte question about hubble's law
- From: Oh No <NotI@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 17 Sep 2009 22:31:01 EDT
Thus spake dfarr --at-- comcast --dot-- net <dfarr@xxxxxxxxxxx>
The 'Hubble's law' Wikipedia article states '...that the velocity at
which various galaxies ARE receding from the Earth IS proportional to
their distance from us.' (emphasis added)
Bear in mind that this applies only for small cosmological distances
My question is about the tense of the two verbs in all caps above.
Aside from assuming things are orderly,
do we have any way of inferring that a galaxy that was moving away
from us 12 billion years ago is still doing so? The light from the
galaxy which is reaching us now indicates it was moving away, but do
we have any way of inferring that it has not slowed down or started to
approach us, or disappeared off the 'edge'?
I'm not an astronomer or even a physicist, just an aging isolated
mathematics amateur, so go easy on me if this is something all
freshmen astronomy students know. Thanks.
Basically we have general relativity, plus a bit of common sense.
General relativity is itself based on the common sense principle that
the laws of physics are locally the same everywhere, and if we can't be
sure of that principle we cannot be sure of anything.
Under the assumption that matter is reasonably uniformly distributed we
can solve the equations of general relativity, and show that if the
universe is expanding now, then it has always been expanding (since the
big bang).
For much more (very clear and insightful) about what Hubble's law does
and doesn't say, see
Edward R. Harrison
"Cosmology: The Science of the Universe", 2nd Edition
Cambridge U.P., 2000,
hardcover ISBN 0-521-66148-X
As Phillip Helbig said later in the same sci.physics.research thread
from which I quoted above, "EVERYONE interested in cosmology should
read this book at least twice.".
Perhaps. It's just a pity Harrison's ideas about the expansion of space
time are somewhat inaccurate. I would recommend everyone should read
some real general relativity also.
Regards
--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces
and
braces)
http://www.rqgravity.net
.
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