Friar Bellows

There is no need to postulate a new force. Gravity can repulse as well as attract. But repulsion requires that whatever is the phenomenon contributing to this accelerated expansion would need to have a negative pressure. Very exotic!

Oxymoron is spot on about the observed acceleration being only a tentative conclusion. I only know of two sets of observations which have claimed to detect this acceleration. More observations need to be done. The systematic uncertainties need to be decreased.

There are some very good papers on this subject at arXiv.org. Here are a couple:

<a href="http://arxiv.org/abs/astro-ph/0207297" target="_blank">Dark Matter and Dark Energy: The Critical Questions</a> by Michael Turner

<a href="http://arxiv.org/abs/astro-ph/0207347" target="_blank">The Cosmological Constant and Dark Energy</a> by Jim Peebles and Bharat Ratra

Liquidrage

Yes, it does need more research and I think I put it forth properly.
But if we do indeed live in an accelerating universe...
etc...
I did say generally accepted... because I believe it is. This topic is in the news again because the latest study confirmed what was found back in 1997 that suggested the same acceleration of the universe.

Regardless, if all the forces can be unified then it really doesn't matter if you call the new force "gravity acting differently" or not to me.
However, I don't know why you used "no need". As if what you put forth was somehow simpler or better proven.

Titanpoint

I am currently working on a PhD thesis that the Cosmological Constant is a chimera.

Friar Bellows

Again, it is not necessarily a "new force". No one (*) is talking about it at this stage as a "new force". The laws of general relativity (plus the usual cosmological assumptions) can easily accomodate an accelerated expansion of the universe. And it's not "gravity acting differently" at all; it's gravity acting the way we've known it to act ever since Einstein's general theory of relativity became a generally accepted law of physics. There is therefore no need to postulate a "new force". We have four fundamental forces. Why complicate that picture by adding another one? I'm sure you're aware of Ockham's Razor.

Of course, ultimately, you could be right and the accelerated expansion is due to a "new force". But these are early days and therefore there's no need at this stage to postulate a "new force".

(*) Well, there is the MOND theory, which is advocated by a small minority of scientists.

[ December 01, 2002: Message edited by: Friar Bellows ]</p>

Friar Bellows

Cool. Any references to your underlying idea or is it still secret at this stage? Maybe you could explain it here briefly?

Liquidrage

No, the laws of General Relativity with a Cosmological Constant, and only with the Cosmological Constant can account for the acceleration.

Yes, I'm aware of that the simplest solutoin is often the best. And it seems to me that a new force, considering that all the forces can be unified, is much simplier that what ever it is your suggesting.

What is the Cosmological Constant Fiar? You're suggesting it's the gravity on exotic material and that this exotic material is somehow simplier or even more to the point, not covered by what I've said here.

No one (*) is talking about it at this stage as a "new force".

I don't know why you think this. Practically every article I've read since this resurgance of the accelerating universe has called it a new or unknown force.
Lots of possiblities here.
And I still don't know why you're bolding what you are and acting like I'm posting in absolutes. I have stated nothing as absolute fact.

eh

It depends on what the cosmological constant is. If it's vacuum energy, then there is a problem. The calculated vacuum energy should cause an expansion so fast the galaxies would rip apart. No one really knows why such a constant would have a value so close to zero, which is needed to explain the current universe.

Quintessence is another constant energy source, but it's usually not refered to as a cosmological constant. It seems to be a competiing theory as to the source of the acceleration.

o

fando

Well, get the book and find out.

Answerer

Well guys, if the universe carries on in its expansion, I'm afraid we have a danger of watching ripping or tearing itself apart.

Friar Bellows

Firstly, you can't divorce the cosmological constant from general relativity. The most general form of the Einstein field equation has this constant present. What its real value is, well, that's up to astronomers to measure. But whatever its value (and that includes zero), it is an integral part of general relativity. Now many scientists don't like it for aesthetic reasons: a non-zero cosmological constant would mean that the left hand-side of the Einstein field equation would no longer be a measure of curvature.

Secondly, lets assume that the cosmological constant is zero. Even then, general relativity can still account for the accelerated expansion. All we need then is a form of "dark energy" with negative pressure, homogeneously distributed across the universe (quintessence? zero point energy? some combination of the two?). Perhaps it's related to the dark matter which astronomers have also postulated to account for various observations. Perhaps not.

Who says that all forces can be unified? We haven't been able to successfully unify the four known forces. So far we've done a pretty good job unifying three of them. But gravity remains the "problem" force for unification. And now you want to consider adding another long-range force to this unwieldy mix? Whoah Nelly!

Well, then I suggest you stop reading popular science articles and read professional science articles like the two I mentioned earlier in the thread.

Yes, but let's look for the simplest explanations first.