Re: [asa] big bang question ... and the start of matter...

Yes, cosmic expansion does appear to be accelerating, which points toward some sort of dark energy which has large negative pressure & thus produces repulsive gravitation. The simplest model of that is Einstein's cosmological term which he introduced in 1917 & later rejected. When you include that the number of possible uniform model universes increases. In particular, you can have a flat accelerating model. De Sitter introduced such a model shortly after Einstein's work. It is devoid of ordinary matter but there's another model that behaves like one filled with ordinary matter close to the beginning of expansion when ordinary matter dominates & then approaches the de Sitter form at large times when the cosmological term/dark energy dominates. That seems to be a decent approximation to our universe: At present the density of darke energy is somewhat more than twice that of ordinary matter (in which I include stuff whose composition we know - atoms, photons &c and dark matter which apparently behaves like known forms gravitationally but is composed of we know not what yet.)

I should also note that whether space is flat or not depends to a certain extant on how you slice space-time into space and time. The form of de Sitter's model that he first worked out had space sections that are curved & static. But the flat expanding form is often the most convenient.

Shalom
George
http://web.raex.com/~gmurphy/

----- Original Message -----
From: <mrb22667@kansas.net>
To: "George Murphy" <gmurphy@raex.com>
Cc: <asa@calvin.edu>
Sent: Tuesday, May 06, 2008 11:38 AM
Subject: Re: [asa] big bang question ... and the start of matter...

Quoting George Murphy <gmurphy@raex.com>:
..
> which will eventually collapse. k = 0 is like the case of a rocket sent away
> from the earth at exactly escape velocity - it would in the limit creep out
> to infinity at a speed approaching zero but would never fall back.
>
> Both inflationary cosmology and present observations indicates that our
> universe is, on the average, spatially flat. But it's possible that it's a
> closed (spherical) space with an extremely large radius of curvature.
>
> Shalom
> George

Responding to the part I could understand:
I thought observations made in the late 90s led cosmologists to the surprising
discovery that the expansion was accelerating --hence the need for a
mathematical factor (dark energy?) to counteract gravity. Have new conclusions
in subsequent years brought the flat model back in favor?

--Merv

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Received on Tue May 6 12:11:41 2008