Re: general relativity website
- From: helbig@xxxxxxxxxxxxxxxxxxxxxxxx (Phillip Helbig---remove CLOTHES to reply)
- Date: Wed, 2 Apr 2008 07:35:40 +0000 (UTC)
In article
<6f80bed0-4f28-42de-bb76-133c26d66622@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
JimJast <jim_jastrzebski@xxxxxxxxx> writes:
"It was widely thought that the universe should endure from
everlasting to everlasting. In order to make this possible Einstein
made a modification to the field equation, by including a repulsive
force to balance gravitational attraction."
Another remark: Einstein, and most other people at the time, believed
that the OBSERVATIONAL evidence favoured a static universe. In some
sense, this implies an infinitely old universe which will last forever,
though of course a static universe could have arisen (don't ask me how)
at some time in the past and cease to exist at some time in the future,
or that could be the case for the objects (stars) in it. It was widely
believed that stars represented fair test particles for the universe.
Although some had been observed and even believed to be what they are,
extragalactic systems were left out of the picture. The introduction,
not just of the cosmological constant but of a specific value for it,
results in a static universe. While mathematically OK, this does seem a
bit ad hoc since the value is just right to cause it to be static. (Some
people have argued against the Einstein static universe by saying that
it is an unstable fixed point, i.e. perturb it in either direction
(expansion or collapse) and it will continue in that direction. Note
that the same argument applies to the Einstein-de Sitter universe (in
the sense that perturbing Omega away from 1 will cause it to continue in
that direction), though I have never seen the same argument used against
this model.)
And BTW, the remark that "the cosmological term was the biggest
blunder of his life" was made by Einstein not because of the math
(which is obviously flawless) but since after this discovery he was
botherd by every cosmologist and his brother until Enstein has
forbidden his secretary to let in anybody who wanted to talk with him
about the universe (soure: Roy Glauber, my teacher and Einstein's
coworker). Apparently George Gamow didn't have Einstein's sense of
humor and didn't get the joke (or didn't want to). How anyone could
think that the discovery of the cosmological constant could be a
blunder?
Had Einstein left it out, he would have concluded that a static solution
is not possible, and thus predicted the expansion (or contraction) of
the universe.
It was later shown that there are non-static models with a non-zero
cosmological constant. After the universe was shown to be expanding,
Einstein favoured setting the cosmological constant to 0 since, with the
data available at the time, such models were compatible with an
expanding universe. While it does simplify things mathematically, this
also led to the non-static models with a non-zero cosmological constant
being sort of forgotten and all sorts of "paradoxes" (universe younger
than the stars it contains!) showed up from time to time for which the
cosmological constant is an obvious explanation. The main motivation
for the Einstein-de Sitter model (with zero cosmological constant) and
Omega=1 was that the data were so poor at the time that one might as
well use the simplest model which was compatible with those data (even
simpler models, such as empty ones, being ruled out by the fact that the
universe is not empty).
In his later years, Einstein favoured a universe with a zero
cosmological constant (for the reasons mentioned above) and Omega>1,
since this implied a spatially closed universe.
.
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