"Roland PJ" wrote in message oups.com...
On Jun 24, 10:39 am, "Martin Hogbin"
wrote:
"Roland PJ" wrote in ooglegroups.com...
Red-shiftof distant objects could be due to two different reasons:

1. Recession speed away from the earth, a special relativistic
effect.
2. Time dilation due to proximity to a concentration of mass, a
general relativistic effect.

It seems that most discussion ofred-shiftcentres around the
assumption that SR recession is the cause (and hence that the universe
is expanding).

Why has GR time dilation been eliminated as a cause?

It has not been eliminated it is a contributing factor.

Yes, I'm sure it _is_ at least a contributing factor, and thanks for
the links.

What I'm asking is why the history of cosmological distance
measurement (the 'distance ladder') seems to ignore it completely, and
focus only on recession speed.

I am not an expert but I believe that gravitational redshift is
a relatively minor effect for normal cosmological objects,
based on their expected masses. No doubt it has been taken
into account wher it is expected to be significant.


--
Martin Hogbin

On Jun 25, 8:22 pm, Sam Wormley wrote:
Roland PJ wrote:
On Jun 25, 7:53 pm, Sam Wormley wrote:
Roland PJ wrote:
On Jun 25, 7:41 pm, Sam Wormley wrote:
Roland PJ wrote:
And, generalising this, what proportion of stars in the Milky Way do
we find at various red-shifts?
The red/blue shifts for stars in the Milky way are not relativistic, but
simply Doppler effect due to relative motion between the star and the
earth. The sensitivity of measurement approaches 1 m/s.
Hi Sam - if there's a 'black hole' at the centre, then there _must_ be
some GR red-shifted objects nearby, surely?
Roland
Gas falling into the black hole can exhibit gravitational red shift.. But
this is local near the event horizon(s) of the black hole... not a galactic
phenomenon... do the calculations.

Sure. I'm busy gathering data... Wheeler reckons the 'black hole' at
the center of the milky way is estimated to be 3.8 x 10^9 m, which is
~ 3 x 10^-6 light years (is that right?)

Not very big, is it, on a galactic scale?

Measurements indicate that the mass of the big supermassive black hole
in our galaxy is about 3 x 10^6 Solar masses or 6 × 10^36 kilograms.
That makes the event horizon radius 0.05 AU or about one tenth the
radius of the orbit of Mercury.

Yes, that's a big bust for the absolute centre causing red-shift.

But what about the rest of the mass in the galaxy?

I'll do some thumb-sucks, which hopefully will be corrected, but maybe
they are enough to bust the theory on their own.

Firstly, what is the mass of the Milky Way 'core'. Perhaps 1/2 (say
3x10^9 suns?) of the total mass of the galaxy?

That gives an 'event horizon' radius for the core as a whole of ~ 3 x
10^9 x 3 x 10^3 m (the sun's 'event horizon' radius is about 3 km), or
about 10^13 m, or 10^-3 light-years (right?).

So, if a lot of the mass (stars) of the core was near the center (and
it's tiny - 10^-3 light years), then most of the stars in the core
will display exaggerated GR red-shifts to us.

This seems tremendously unlikely - the core itself is ~300 light-years
radius (is that correct?).

So, most of the stars in the centre of the Milky Way will show
exaggerated GR red-shift, only if most of them are in the innermost
3x10^6 th of the core. Again, doesn't seem plausible.

So, my last illustration. What if 1/2 of the mass of the core was
within 1/2 of the radius of the center and so on (which is a steep
density function).

1.5 x 10^9 suns in r 150 light-years.
7.5 x 10^9 suns in r 75 light-years.

We're not gaining on GR red-shift as we go in, so this scenario is
also a bust.

My conclusion: GR red-shift due to the mass of the galaxy itself can
only be a factor if the star density function is significantly steeper
than 1/2 the mass within 1/2 the radius at each 'step'.

So, I think I'm pretty thoroughly convinced. This is a really dumb
idea

Thanks for the kind feedback
Regards
Roland