Re: Our Expanding Universe

From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 12/06/04 

Date: Mon, 06 Dec 2004 12:51:15 +0100

jacob navia wrote:
> Alf P. Steinbach wrote:
>
>> * Bjoern Feuerbacher:
>>
>>>> (2) You cannot have a Universe where any small part is non-expanding
>>>> and
>>>> the total is.
>>>
>>>
>>> Huh? Why not?
>>
>>
>>
>> Perhaps you misunderstood 'any'.
>>
>> In that case, substitute 'every'; I'm not natively english-speaking.
>>
>> As a very simple model, consider a systematic sprinkling of matter
>> particles, no voids greater than some very small size S, where in any
>> sufficiently small part of space greater than S they're at relative
>> rest: a grid of matter particles (instead of particles you might
>> consider just points in space). Now zoom out and find a scale at which
>> they're flying apart at the same time they're locally at relative rest.
>> If you can then you have successfully summed a number of zeroes, zero
>> relative speed, and obtained something >0.
>>
>>
>
> That's a very good argument.
>
> I have never understood that concept of "expansion stops" either.
> Galaxies are coming apart but atoms not.

Then you also might try reading this up in a textbook. I already
told you what to look up: embedding of a local Schwarzschild metric
in a global Robertson-Walker metric.

> At what distance does the "space expansion" stop?

I don't know the exact calculation, but I would estimate at
about 5 Mpc. Does obviously also depend on the mean local density.

> Is the earth expanding?

No.

> The sun?

No.

> The milky way?

No.

> The local group?

No.

> The Virgo Cluster to which we belong?

In the outer parts, perhaps. But not in the central parts, where
the density is quite big.

Bye,
Bjoern