George Dishman wrote:
Note: follow-ups to sci.astro only, this seems off-topic
for the other groups.

"Bjoern Feuerbacher" wrote in message
...

George Dishman wrote:

"Val Rogers" wrote in message
...


The idea of the universe collapsing and then exploding out again is old
(by your definintion of "old"). It's just a little too much deja'vu when
I hear essentially the same thing all over again.


Bjoern has covered most of your post but I think you may still
not be seeing the key difference in Steinhardt's proposal. I
also missed it from the abstract. In his case the universe
doesn't collapse, it continues to expand. Measurements a few
years ago showed the expansion is accelerating and if that
continues it will become exponential just as early inflation
proposed by Guth many years ago. Steinhardt's proposal is that
the brane collisions of string theory may recur and inject new
energy into the expanding universe. There is no cycle of
expansion/collapse as in the QSSC but continuous expansion at
a cyclically varying rate.

Sorry, but that is *not* a difference. Reading the page
http://wwwphy.princeton.edu/~steinh/npr/, I also thought, like you,
that there is no contraction in Steinhardt's model. But on another page,
http://wwwphy.princeton.edu/~steinh/cyclintro/
he clearly says that there *is* indeed a contraction in his model.


I may be wrong, I skimmed through the "Simplified" paper
http://wwwphy.princeton.edu/~steinh/dm2004.pdf
and though he talks of contraction I got the impression
it was significantly different from QSSC.

Yes, his idea is indeed significantly different from QSSC. I did
not dispute that above. I only disputed your assertion that in his
model, there is only expansion, but no contraction.


The first page also says that the ekpyrotic model is the precursor to
the cyclic model, so apparently they are not the same - but from these
popular-science articles, it is not clear to me if there is contraction
in the ekpyrotic model or not, and if, according to Steinhardt, a cyclic
universe could work without contraction or not.


From the abstract:

"In particular, we show that the contraction phase has equation
of state w 1 and that contraction with w 1 has a surprisingly
similar properties to inflation with w -1/3."

Half way down the second page:

".. we find that there are remarkable, unanticipated parallels
between inflationary expansion and the contracting and bounce
phases of the Cyclic Model."

And in the last paragraph of Section I:

"We focus on the two key ingredients needed to understand the
contracting phase: branes and the equation of state w 1."

Section II starts:

"The Cyclic Model was developed based on the three intuitive
notions:

* the big bang is not a beginning of time, but rather a transition
to an earlier phase of evolution;

* the evolution of the universe is cyclic;

* the key events that shaped the large scale structure of the
universe occurred during a phase of slow contraction before
the bang, rather than a period of rapid expansion (inflation)
after the bang."

So clearly he is describing a period of contraction, but
starting on the second line of page 4 he goes into more
detail and seems to describe something significantly
different from the usual understanding of contraction:

"The universe switches from expansion to contraction. The
branes themselves do not contract or stretch significantly.
Rather, the distance between them shrinks as the two branes
crash together. That is, the contraction only occurs in the
extra dimension between the branes. ...
During the contraction phase, the branes stop stretching
and quantum fluctuations naturally cause the branes to
wrinkle."

I understood that to mean that while the distance betwen
the branes is reducing the scale factor of our universe
(which is one of the branes) remains roughly constant
hence the hypervolume(?) product of separation and volume
would decrease, hence the term 'contraction'.

That seems borne out by his summary in the last sentence
of the third paragraph on the page regarding entropy:

"The simple reason is that the branes themselves do not
contract. Only the extra dimensions contract."

This stuff is really over my head but have I got the gist
or have I misunderstood his model?

Yes, I think you have indeed got the gist. I can't say for sure, since I
did not invest much time so far in studying his ideas, but what
you said above looks consistent with my understanding.


Bye,
Bjoern