Okay, I can't promise a follow-up until thursday, but I have to make some comments on this thread.
ubavontuba said:
What I think it comes down to is a paradox of contention between the two forces. Dark matter that is weakly held in a halo around galaxies (in higher orbits) would seem to be particularly susceptible to the vagaries of dark energy and it would tend to slip away from galaxies under the influence of dark energy... essentially causing the same problem that dark matter was ostensibly supposed to fix. Simple conservation of angular momentum would speed this process along until the whole galaxy evaporated (in an infinite time model). Doesn't this sound reasonable?
You can understand how this would not occur on galaxy scales by simply recognizing two facts:
1) The energy density of matter and dark energy are comparable (i.e. within the same order of magnitude) at the present epoch.
2) Galaxies represent extreme overdensities of matter.
You don't even need to crunch numbers. Just ask yourself what the condition is for the repulsive effects of dark energy to become comparable to the attractive effects of matter. The condition is that, within a given spherical volume, you would need a comparable amount of dark energy and matter. If galaxies are extreme overdensities of matter and dark energy is smoothly distributed in space (as we believe), then can you see how extreme overdensities of matter (galaxies) will have much more matter than dark energy (bullet 2, above)? Furthermore, can you see how the spherical volume at which this will no longer be true will be on cosmological scales (bullet 1, above)?
That's the simple answer. The more technical answer is that, in a [itex]\Lambda[/itex]CDM universe, the growth of structure halts at a certain scale which is given by the value of the cosmological constant (dark energy density). It turns out that, in our universe, this scale is only relevant for the growth of
galaxy clusters, not galaxies themselves. The dark matter halos occupied by the latter are much smaller, in general, and would not feel the effects of dark energy. The outer boundaries of galaxies are already fuzzy and are more likely to be determined by tidal forces from other nearby galaxies and/or the "temperature" of the dark matter.
I should emphasize that the cosmological constant only
halts growth, it does not cause the clusters to evaporate. The reason for this is that, in a CDM universe (i.e. one with only dark and luminous matter), structure is forever
growing, and it's doing so on larger and larger scales. This was the basic behavior of our universe prior to z~1, when the dark energy density became comparable to the matter density. When dark energy began to make itself known, this "infall" and growth was thought to have slowed and it should eventually come to a stop (if it hasn't already). The whole reason it's called the cosmological constant is that it has a constant energy density with time. Combine this with the reasoning in my "simple explanation" and you should be able to see why growth halts at a certain physical scale.
The cosmological constant is not the only possible source of the dark energy. There are some (admittedly exotic) theories in which the density of the dark energy forever
increases with time. These are the "Big Rip" theories that matt.o was referring to and, in those theories,
everything eventually gets torn apart by the dark energy, not just galaxies. Again, this can be understood by the arguments I give above.
Finally, it's worth noting that gravitationally bound groups of stars (like globular clusters and perhaps galaxies), can undergo a sort "evaporation" as a natural consequence of their evolution. This is not related to dark energy, however, and is not just a consequence of conservation of angular momentum, as you seemed to suggest at one point. Rather, this occurs because, when a galaxy/cluster tries to relax (i.e. reach equilibrium), it excites a few stars to speeds larger than the escape velocity of the cluster/galaxy. These few stars will then escape the cluster. In order for more stars to escape, the cluster must relax again, so the timescale for evaporation is many "relaxation times". We do observe this kind of behavior in globular clusters. However, this relaxation timescale turns out to be longer for larger objects, so we won't notice these effects in galaxies or galaxy clusters whose relaxation times are comparable to and longer than the age of the universe.