| From: | Adam Getchell <acgetchell@*******.EDU> |
|---|---|
| Subject: | [OT] Cosmology (was Re: [OT] Alternity, physics, and scramjets) |
| Date: | Sat, 31 Oct 1998 21:46:16 -0800 |
>Astronomers announced that they might have discovered just that. It seems
>that the Universe is too large for it's age and that the rate of
>expansion in the Universe is increasing not decreasing or remaining
>stable. They may have come up with another explanation by now but I
>haven't heard about it (not suprising since I heard about it in the NY
>Times and stories like this almost never get followed up on).
I realize that this topic is almost instantly off-topic but I figure that
it won't generate too much discussion, and people who don't want to read
long physics tracts know to delete stuff by me anyways (at least stuff that
says Cosmology for the subject line ... ;-)
So I'll relay something I wrote to Robert regarding inflation of the
universe... Regarding Steve's comments, the Universe looks as if it is
accelerating its expansion due to Hubble's constant, which is something
like 60-75 kilometers per second per megaparsec (1 parsec = ~ 3.26 light
years). An object farther away has a greater velocity. Since the universe
is expanding, things are getting farther away faster all the time. ;-)
Hyperinflation is also a nifty solution to Olber's paradox, which states
that if there are so many stars, and each one has a subtended arc of
brilliance equivalent to the sun, then why isn't the night sky as bright as
day?
Hubble's constant is of extreme interest and astrophysicists try to get an
exact value as possible, because the inverse of Hubble's constant gives the
age of the universe.
>>Forwarded message
The topic that you are looking for is Cosmology.
I think Stephen Hawking's book "A Brief History of Time" is pretty good.
There can also be found some books at Amazon.com, including "Before the
Beginning" by Ellis, "The Accidental Universe" by Davies, and "After
the
First Three Minutes: The Story of Our Universe" by T. Padmanabhan (though I
haven't read any of these).
Briefly (and I may be rusty since my Astrophysics is dated before COBE,
which told us this information) a great deal of the present character of
the universe evolved during the hyperinflationary epoch. The universe is
much larger than it should be if the Big Bang "exploded" at the speed of
light. The mechanism to explain is "hyperinflation", which was a period of
time in which the universe expanded to something like ~90 % of the current
size in a few milli/nano/femtoseconds (I don't have my references, but I
remember it not being that long). Of course, having effectively faster than
light expansion in the universe means that anisotropy is possible, i.e.
physical laws could change because causality isn't communicated quickly
enough, so theories have to account for both inflation and isotropy.
At any rate, hyperinflation changes the geometry of the universe, which in
general relativity, is gravity. The problem becomes, what geometry does the
universe have?
There are 3 kinds:
Parabolic: If you make a large enough triangle (like light years) and
measure the angles, the sum will be less than 180 degrees. Such a universe
is closed, i.e. it will fall back on itself. Remember gravity is really
curvature, although we percieve it to be a force, it is really the way
things want to travel when unconstrained. (The classic example is a marble
rolling along on cheesecloth: it both indents the cheesecloth and is
affected by other indentations). As an aside, all forces are actually
curvatures, typically compactified dimensions, but that gets into string
theory and its successor, M and P-branes.
Flat: The sum of the angles of a triangle is 180 degrees. Globally, the
universe is nice Euclidean geometry (though not locally). The rate of
expansion will slow to a stop at some point, and the universe will remain
that size forever.
Hyperbolic: The sum of the angles of a triangle is greater than 180
degrees. Essentially the global curvature of the universe precludes the
disparate peices from ever getting back together again. The rate of
expansion will slow, but never stop.
This is where the mass of the universe comes in: enough marbles on the
cheesecloth will indent it such that its character will remain parabolic
even as the cheesecloth expands: not enough marbles, and it won't.
Well, good luck in your research, and I hope the above clarifies things or
at least gives you a decent springboard!
Cheers,
--Adam
>Adam,
>
>Can you give me a reference somewhere I can resolve a gravitational paradox
>that I've been trying to get my head around?
>
>Current astronomical theory states that the Universe is basically moving too
>fast to ever crunch up again (assuming the Big Bang theory, of course).
>
>How is this possible? Gravity is applying a brake on the expansion. That
>brake varies in strength, of course, but it is always a net result of
>pulling everything back together and slowing down the expansion. Unless
>there is something applying an acceleration at least equal to gravity, then
>sooner or later the expansion has to stop, right?
>
>Any ideas where I can find an explanation for this?
>
>--
>.sig deleted to conserve electrons. robert.watkins@******.com
>
>Steve
--Adam
acgetchell@*******.edu
"Invincibility is in oneself, vulnerability in the opponent." --Sun Tzu
