Re: [Phys-l] Hubble Law

["Polvani, Donald G." <donald.polvani@ngc.com>]

Tom 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? Is it because they 
> were accelerating longer at the start of the big bang???  Or is it 
> because of pressure from "dark energy" causing them to speed up for a
longer time?
> Can someone help?

One explanation of the Hubble law which stands out in my memory is given
in "The First Three Minutes" by Steven Weinberg.  Weinberg explains that
the Hubble law results both from the empirical fact that the universe is
expanding and the principle that no one point in the universe is
"unique" (he had a name for this last principle which I've forgotten at
the moment).  He gave a simple one-dimensional figure which I found very
illuminating something like the following which assumes equally spaced
galaxies A, B, C, D, and E:

A ----- B ----- C ----- D ----- E

Suppose we are in galaxy C and observe empirically that galaxies B and D
are receding from us at speed v.  But by the principle that no one point
(or galaxy) has a unique position, an observer in galaxy B would see
galaxies A and C receding at speed v.  Therefore the galaxy B observer
would see galaxy D receding at speed 2v, and we in galaxy C would see
galaxy A receding at speed 2v, etc.

Don Polvani
Northrop Grumman Corp.
Undersea Systems
Annapolis, MD