Re: Our Expanding Universe

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

Date: Mon, 06 Dec 2004 12:49:08 +0100

Alf P. Steinbach wrote:
> * Bjoern Feuerbacher:
>
>>* Alf P. Steinbach:
>>
>>>No. There are a lot of issues, including the following. But note that
>>>that doesn't mean GR is necessarily entirely useless in this regard; it
>>>just means that GR'anians have not solved or explained the problem of
>>>the expansion limit; the argument you used is effectively hand-waiving.
>>
>>I told you how one can quantify it: by studying the embedding of
>>a Schwarzschild metric into the Robertson-Walker metric. This is
>>covered, AFAIK, in most introductory textbooks on GR. So GR has a lot
>>more than hand-waving to say about this problem.
>
>
> Then please do.

Pardon? Do you really expect me to present a really complex mathematical
argument in a newsgroup post??? Why don't you simply look it up in
a textbook?

> I've raised precisely that question in other forums before, e.g.
> [sci.astro.research], and possibly you have been involved in those
> discussions -- I don't remember.

Probably not. I only recently began posting there, and don't remember
writing anything about that so far.

> No consensus has ever emerged, no hard non-disputed numbers have
> emerged, no clear criteria have emerged, no correlations with
> observations have been put forward, in short: nothing concrete, just
> ever more frantic handwaiving and ever more subtle assertions, and
> disagreements about interpretations & validity of observations.

Well, then try looking into a textbook... E.g. the one by Stephani
discusses it, and MTW also, I think.

>>>(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.

Oh, yes, that was indeed the source of the misunderstanding. Sorry.

BTW, I'm also not a native speaker. Considering your name, you
are probably German? Falls ja, koennten wir ja auch per Mail auf
Deutsch weitermachen. ;-)

[snip]

>>>It is a conceptual tool only; for a finite universe with radius R(t)
>>>multiply (conceptually) every distance by 1/R(t).
>>
>>Coordinate transformations act on coordinates, not on distances.
>
>
> Do you understand that a transformation of coordinates can transform
> distances, and vice versa?

Yes. It was a nitpick, I admit.

>>Do you want to multiply all (spatial) coordinates with that factor?
>
>
> No, I do not "want" that. But you can do that as long as you remember
> that it's a conceptual tool only, and that that transformation affects
> other things (as any transformation). Is this viewpoint still unclear?

No, I understand now.

And I admit that I can't think at once about how one could distinguish
the two...

>>>What you're asking
>>>for is details that constitute some heavy-duty research and probably
>>>won't give any more clarity. IIRC some British researchers did however
>>>do the transformation (anything is worth researching) -- not long ago,
>>>they had an announcement that they later had to retract that the
>>>expansion could be explained non-relativistically simply by assuming a
>>>random set of speeds initially -- and possibly you can find that.
>>
>>The information you give here is a bit vague for a search, don't you think?
>
>
> Yes it is. If you're interested in that you'll have to put in a
> bit of work... I'm sure you have better resources for that than I. ;-)

I have better resources for searching for research articles - but
searches in such archives usually require quite precise information.
"some British researchers" is way too vague.

> [snippety-snip]
>
>
>>>Enlightenment, part II: chop the infinite Universe into an infinite
>>>number of regions of size R(t). Run time backwards so that at t=0 each
>>>region becomes a point as per R(t)=K*t for some constant K. Assume that
>>>these points are still distinct, i.e. an infinite collection of points
>>>that still constitute an infinite Universe: what does it look like?
>>
>>Like an infinite space. As it did look all the time.
>
>
> Does it contain any matter (and if it does, in what form -- the
> original particles having been squeezed down to zero size)?

We don't know. The physics we have today does not work at such
scales.

BTW, according to the physics we have today, the elementary particles
already *have* zero size.

> Can we now apply the same procedure to the resulting infinite space (and
> so on, ad infinitum)?

Probably yes.

[snip]

Bye,
Bjoern

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