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From: srp on 3 Aug 2010 09:52
On 2 août, 12:17, eric gisse <jowr.pi.nos...(a)gmail.com> wrote:
> srp wrote:
> > On 2 août, 15:40, Sam Wormley <sworml...(a)gmail.com> wrote:
> >> I gave a presentation yesterday to an audience that included
> >> one of my retired physics professors. I had responded to a
> >> question during the presentation saying that the universe could
> >> be infinite, but that since we cannot observe it, we cannot say
> >> for sure.
>
> > Carefull! If you go on giving presentations holding such
> > reasonable positions, you'll quickly end up being branded
> > a kook by every Cook in the book.
>
> >> After the presentation, Barney Cook, said I was wrong, that
> >> the the measured flatness of the universe means the universe
> >> is infinite.
>
> > Being established, he is right of course!
>
> >> I would appreciate comments from the physicists here. Thanks.
>
> > Wrong ng. Ask opinions on serious moderated ngs.
>
> >> -Sam
>
> > André Michaud
>
> Back again, Andre?

I never left.

Barely ever find anything worth commenting anymore.

Just wanted to give Sam a hint of what he can expect if
he starts airing constructive ideas, which are systematically
unwelcome by physicists more established than himself
and of course from the usual congregation of idiots howling
with the wolfpack.

What about you? Still nuts and bolting small local
networks?

André Michaud
From: Yevgen Barsukov on 3 Aug 2010 10:15
Sam Wormley <sworml...(a)gmail.com> wrote:
> I gave a presentation yesterday to an audience that included
> one of my retired physics professors. I had responded to a
> question during the presentation saying that the universe could
> be infinite, but that since we cannot observe it, we cannot say
> for sure.
>
> After the presentation, Barney Cook, said I was wrong, that
> the the measured flatness of the universe means the universe
> is infinite.
>
> I would appreciate comments from the physicists here. Thanks.
> -Sam

Friedmans' universe model consistant with general relativity
has only 3 possibilities - contracting, accelerated expansion, and
"balanced"
with expansion rate approaching zero at infinity.

The resulting space shape is "wrapped into itself" in model 1 (as a
sphere
in 4 dimmensional space-time), saddle in model 2 and "flat" in model
3.

Since both saddle and flat don't have a boundary, they are both
infinite
in the sense that you can start in one place and keep going
indefinitely without
coming back to where you stared.

Only wrapped into itself model 1 (contracting universe) is finite.
Popular explanation of this with some pictures can be found for
example in Hawking's
book "brief history of time".

On the side note, recent Shu model (not being relativistic in the
complete sense)
is also finite with space wrapped into itself, while allowing some
periods of
accelerated expansion consistent with observations.

Regards,
Yevgen
--
Tune in to "Strange Drawing of the Day" buzz:
http://www.google.com/profiles/100679771837661030957#buzz
From: PD on 3 Aug 2010 13:40
On Aug 2, 11:16 am, eric gisse <jowr.pi.nos...(a)gmail.com> wrote:
> Sam Wormley wrote:
> > I gave a presentation yesterday to an audience that included
> > one of my retired physics professors. I had responded to a
> > question during the presentation saying that the universe could
> > be infinite, but that since we cannot observe it, we cannot say
> > for sure.
>
> > After the presentation, Barney Cook, said I was wrong, that
> > the the measured flatness of the universe means the universe
> > is infinite.
>
> > I would appreciate comments from the physicists here. Thanks.
> > -Sam
>
> I'd ask him why measured spatial flatness implies an infinite universe,
> given that such a state can start from a finite amount of time ago.

That doesn't mean anything. You can have an infinite universe with a
finite lifetime.
Basic analogy: Imagine an infinite line. Take a point on the line and
observe that another point at distance x from it is receding with
velocity v, a third point at distance 2x is receding with velocity 2v,
and a point at distance nx is receding with velocity nv. This
*infinite* line will have have all had to diverge from a single point
at time x/v ago, provided that all the velocities do not change.
From: PD on 3 Aug 2010 13:41
On Aug 2, 2:40 pm, Sam Wormley <sworml...(a)gmail.com> wrote:
> I gave a presentation yesterday to an audience that included
> one of my retired physics professors. I had responded to a
> question during the presentation saying that the universe could
> be infinite, but that since we cannot observe it, we cannot say
> for sure.
>
> After the presentation, Barney Cook, said I was wrong, that
> the the measured flatness of the universe means the universe
> is infinite.
>
> I would appreciate comments from the physicists here. Thanks.
> -Sam

A flat universe can only be finite if it has a boundary.
A finite universe can only be boundariless if it is curved.

So it really hinges on whether there is a boundary.
From: Sam Wormley on 3 Aug 2010 19:08
On 8/3/10 12:41 PM, PD wrote:
> On Aug 2, 2:40 pm, Sam Wormley<sworml...(a)gmail.com> wrote:
>> I gave a presentation yesterday to an audience that included
>> one of my retired physics professors. I had responded to a
>> question during the presentation saying that the universe could
>> be infinite, but that since we cannot observe it, we cannot say
>> for sure.
>>
>> After the presentation, Barney Cook, said I was wrong, that
>> the the measured flatness of the universe means the universe
>> is infinite.
>>
>> I would appreciate comments from the physicists here. Thanks.
>> -Sam
>
> A flat universe can only be finite if it has a boundary.
> A finite universe can only be boundariless if it is curved.
>
> So it really hinges on whether there is a boundary.

Good Point!
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