Re: Quintessence and Curvature

From: Phillip Helbig---remove CLOTHES to reply (helbig_at_astro.multiCLOTHESvax.de)
Date: 03/23/05 

Date: Wed, 23 Mar 2005 10:33:32 +0000 (UTC)

In article <mrF2yeBhzoPCFwsZ@farmeroz.port995.com>, Oz
<Oz@farmeroz.port995.com> writes:

> >> >The age of the universe is inversely proportional to the Hubble
> >> >constant. The Hubble constant is essentially a distance measurement;
> >> >one has to measure distance and redshift, but the latter is quite easy.
> >>
> >> OK. What if (OK humour me) the redshift gives an incorrect figure for
> >> the velocity on galactic scales. What if the redshift is twice the
> >> velocity? Then galaxies will be going more slowly than you think, or
> >> more accurately the universe is expanding more slowly than we thought.
> >
> >What do you mean by "galactic scales"?
>
> Er, 'really long distances'?

Then say "cosmological" or at least "extragalactic". "Galactic" implies
the scale of a galaxy, which is much too small compared to the scale of
the Hubble flow.

> I am always being taken to task over my lax use of language.
> In this context surely we assign the redshift as a measure of the
> expansion of the universe between emission and detection.

Right.

> >Speaking a bit more generally, if you say "what if the relation between
> >redshift and distance is not what we assume", then this is tantamount to
> >saying "what if our theoretical basis of cosmology is not what we think
> >it is". That might be a valid line of argument, but without specifying
> >what this new theoretical basis for cosmology might be, there is little
> >point in asking such questions.
>
> I take your point, but I do not completely agree.
> MOND is of course precisely such a procedure.
> It has a use, it might even offer guidance.

MOND itself, in its original form, will not work, for a number of
reasons. One needs a more involved theory which has MOND as an
approximation. Bekenstein recently published such a beast, but it is
quite complicated, so the Occam's razor argument which favours MOND cuts
this new theory a bit too closely to the bone.

> I know. Very irritating of it for (some) theorists.
> I suspect quite a few spend the odd hour (when nobody is looking)
> playing about with it.

I know some who spend the odd hours playing with MOND, and the even
hours with more traditional stuff. :-)