Re: Expanding Universe

["John S. Denker" <jsd@MONMOUTH.COM>]

Roger Freedman wrote:

> > A useful set of articles about Big Bang cosmology from Sky &
> > Telescope and Astronomy magazines can be found at
> >
> > http://www.itss.raytheon.com/cafe/cosm/cosmol.html
> >
> > The article "The Cosmological Redshift Explained" is
> > probably the most relevant to Tim's question.

At 07:16 AM 3/20/01 -0400, Tim O'Donnell wrote:

> Let me clarify something after reading this article.
> There is limited Doppler Redshift for bodies that are close
> to ours, but most of the redshift we see is due to
> gravitational effects.  Is that correct or did I read
> something else into this article?

I didn't benefit very much from reading that article.  As far as I could
tell they had one valid point, which was a negative point, namely:
    If I show you a redshift without revealing its provenance,
    you can't tell how much of the effect is a Doppler redshift
    and how much is a gravitational redshift.

Except for that, the article was, IMHO, mostly nitpicking, pettifoggery,
and hype.  The article conspicuously lacked positive statements about what
we *can* measure.

So let me say what we *can* do:  We can take a bunch of data for various
objects at various distances, after the manner of Hubble, and plot redshift
versus distance.  Let's restrict the data to familiar objects under
ordinary conditions within a few billion lightyears of here.  There will be
some scatter in the data, but the general trend can safely be interpreted
as a Doppler shift due to a velocity proportional to distance.

(The negative point remains:  given the redshift of a single unfamiliar
object under unknown conditions, it would not be possible to infer its
velocity.  But that's a different question.)

=============================

Philosophical remarks:

1) There are lots of things in this world that we cannot do, but that does
not detract from the things we *can* do.

2) General relativity does not invalidate special relativity, and special
relativity does not invalidate Newton's laws.  In fact GR demands that SR
be valid locally.  In fact SR demands that Newton's laws be valid in the
appropriate low-velocity limit.  These are examples of what Bohr called the
"correspondence principle".  The Odenwald & Fienberg article exhibited what
I consider a bad attitude toward this important principle.