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I just noticed that when I quoted RM Mentock’s post I inadvertantly left in the line that indicated he was quoting me. Just wanted to let everyone know that I am not actually rebutting myself!
Oh, NOW I know what you’re talking about! :o It’s the cosmological constant that counteracts gravity and keeps the universe expanding. Try this other thread, entitled “Missing Mass”: http://boards.straightdope.com/sdmb/showthread.php?threadid=2881 The very first post, by Dvandom (who has made all of TWO posts since last October!) explains this. Since Dvandom is a physics education researcher at Ohio State, I’m sure he knows what he’s talking about.
If you want to read something that’ll REALLY make your head hurt, read my posts about the possibility there just may, indeed, be another universe and that its gravity is affecting ours. However, since light cannot travel from one universe to another, this other continuum will be forever invisible to us. This ain’t my theory, I’m just relaying it.
Thanks! Now I know that when my brain leaks out my ears it makes a really neat squishy sound!
No need to develop elaborate theories, Einstein did it for us. Now, as for the comment that it doesn’t mean much…well, I think it does. Look at how excited people get when you simply quote Einstein.
Perhaps we are talking past one another. What I am saying is that any point I decide to base a coordinate system on is arbitrarily designated to be fixed. I make no claim that this coordinate system is reflective of the actual nature of the universe. You seem to say that the fixed earth people can make such a claim and it is perfectly ok.
I’m sure they’re are other Universes. Our Universe is unique (well mabye not?), but I don’t think the Big Bang was an unique event. The Big Bang is like a seed sprouting, and I don’t think our Universe was the only sprout. Who knows mabye there are an infinite number of Universes out there. Sometimes you gotta wonder about the Great Design. Mabye an infinite particle arose from an absolute vacuum??
Just to tidy up the science a bit here, yes, there does appear to be a cosmological constant, but it’s possible to have a Universe that never re-collapses without it. The situation is, in fact, analagous to escape speed (not escape velocity, there’s no such thing). Ptahlis is correct that, no matter how great the distances, gravity never quite goes to zero, and that therefore, barring the influence of other forces, the expansion of the Universe is always slowing, but because it’s slowing at a decreasing rate, it’s possible that there’s some value below which it will never drop.
But if not the cosmological constant, what force will counteract the force of gravity? What exactly would cause there to be a threshold beyond which expansion will not slow?
Nothing need counteract gravity-- Like I said, in a non-lambda Universe, the expansion is always slowing slightly. If there is something counteracting gravity, then eventually, the expansion will start accelerating. If you’re having a hard time with the concept of an ever-slowing expansion which nonetheless never stops, you might consider, for example, the sequence 2, 1.5, 1.25, 1.125, etc. It’s always decreasing, but it never goes below 1 . The situation with the Universe isn’t exactly analogous, but it might help.
Oh, picky, picky, picky. If it looks like a duck, waddles like a duck, and quacks like a duck, it’s a duck. 
(Yeah, I know, and there’s no such thing as centrifugal force, either, it’s centripetal (sp?) force. Damned physicists…)
(Besides, it’s “analogous”, not “analagous”. Right back atcha.)
Actually, “centrifugal force” is perfectly fine, in the right context. It referrs to the apperent force experienced by an observer in a rotating coordinate system. Centripetal force is the force which keeps that system rotating. And thanks for the spelling correction-- There’s a reason I majored in physics instead of English :).
There is no such thing as centrifugal force in an inertial reference frame. But, yes, there are coordinate systems in which centrifugal force exists.
I’m forced to admit that the only reason I caught it was that I ran my original reply through a spell-checker to see if I’d spelled centrifugal and centripetal right (only to find that I’d gotten them both wrong) and the spell-checker caught your mistake as well.
OK, except for ulterior motives…but otherwise, the fixed earth people can make such a claim and no one can prove them wrong–unless they also prove Einstein wrong. Any appeals to common sense or reason are empty, and wrong.
Ok, I think we’re almost in sync now.
General relativity says that any coordinate system is valid within its own frame of reference. In other words, you may perform all observations and make any models you like from the perspective you have defined. The caveat in this is that there is no point in the universe that can be defined as fixed absolutely, as any reference frame is valid.
In turning to the theory of GR, the fixed earther’s defense is a flimsy one. They can certainly say that Einstein supports the idea that the Heliocentric view is no more real than a geocentric one. He does. Insofar as the true nature of things goes, the Heliocentric view is more predictive, more parsimonious, and more useful, but no more wedded to objective reality. But, in using the “all frames” argument they deny themselves, for their argument is that their frame is the only true one. It’s like saying “Everyone is equal except I’m better.” You don’t need to disprove Einstein to disprove the fixed earthers because Einstein himself disproves them.
Not perfectly ok, but they can’t be shown to be wrong, by our modern physics. Not since general relativity.