Please Register or Log in to view the hidden image! The above shows Einstein's field equation with the cosmological constant term added (viz. the second term on the left hand side). There's been lots of talk recently about the cosmological constant, but it seems to me that there is a lot of confusion about it and related ideas, not helped by the astronomer's typically lazy (ab)use of the term. The main problem is this: cosmological constant dark energy quintessence vacuum energy zero-point energy These terms are often used interchangably, or the difference between them isn't spelled out in detail. Here is my understanding of these terms; please correct me if I'm wrong: The cosmological constant is just a constant term added to the Einstein field equation. For reasons of simplicity and elegance (and wanting the Einstein tensor to vanish in flat spacetime), we might like it to be zero, but it is a constant that the astronomers should determine via observations (just as they determined the value of the constant on the right hand side of the field equation). And it may not have any physical significance at all. Dark energy and quintessence refer to same currently unknown thing, some weird and exotic "stuff" which does not tend to cluster on small scales like ordinary matter. It might mimic the behaviour of a cosmological constant, but I don't think it warrants that title, because it really is a contribution to the term on the right hand side, the so-called stress-energy tensor. Vacuum energy and zero-point energy refer to same thing, I think: the energy (density) of the vacuum as described by quantum field theory. Its magnitude is extremely large. I would like to end this post with some rampant speculation. Maybe the acceleration of the cosmological expansion is being caused by a combination of cosmological constant, dark energy, and vacuum energy. Actually, I would prefer the cosmological constant to be zero, for aesthetic reasons, so in that case, we have a combination of dark energy and vacuum energy. Perhaps the dark energy varies with time, and currently (almost) cancels out the very large value of the vacuum energy. Bah! All this baseless rambling leaves a bad taste in my mouth. Please Register or Log in to view the hidden image!
I think that an indirect way to describe it would be to describe the charge couple constants ( about 1/137) This dimensionless number depends on a few other constants ( correct me if i`m i forget some) velocity of light, gravity,electron mass and pi... there have been some studies of the spectra from quasars that show that certain absorbsion lines have been shifted(?) therefore there is an indication that the charge couple constant has changed...
There is the idea that dark energy could decay via the emission of (cold or hot) dark matter. There's also the XCDM parametrization and the scalar field theory. A lot of this is covered in the review by Peebles: The Cosmological Constant and Dark Energy
