Re: [Phys-l] Hubble Law

["Krishna Chowdary" <professor.head@gmail.com>]

On 1/18/07, frederth@saline.k12.mi.us <frederth@saline.k12.mi.us> wrote:
> OK.  I know that the Hubble Law states that recessional velocity of
> distant galaxies is greater as distance increases.  However, I am
> forgetting why this should be intuitively correct?


Try this simple demonstration of a "one-dimensional universe" (which I
learned from our intro astronomy course).  Take a slinky and stretch it out
a little bit; fix the ends down so that it remains slightly stetched (w've
attached blocks of wood to the ends of a plastic slinky).  Now, attach paper
clips or post-it notes at various positions, and label them 1, 2, 3, 4, 5,
6, etc (these represent galaxies).   Now, measure the distance between, say,
marker 3 (your "home" galaxy) and the other markers.  Now, stretch the
slinky some known distance.  Measure the distances again.  Stretch the
slinky again the same known distance as before, and measure the distances
again.  That should be enough, but you can do this as many times as you'd
like.

For a first approximation, assume that the time between measurements was the
same.  Now you can calculate (average) velocity.  And then you can connect
this velocity to the distance between markers.  From this, you can see that
the recessional velocity increases as distance between markers increases.

As far as I know, the Hubble Law supports the idea of an expanding
universe.  Maybe this demo will help you to develop that intuitive
connection you are looking for.  However, there is no a priori reason to
believe the Hubble Law; it is experimentally deduced (I guess if you had
already had a model of an expanding universe, then you would expect a Hubble
Law type of relationship to emerge, but I don't think that Hubble had this
model when he discovered the relationship observationally.  I'm not a
historian of science, though).

Hope this helps.

--
regards
-Krishna