Dark-energy sources and respective implications

Firstly, I want this to be a very layman/simple level. According to Sean Carroll, we have certain possibilities for where dark-energy comes from:

Including

• A modification of general relativity
• Vacuum energy
• A modification of vacuum energy

And he goes on to say that if vacuum energy is the right answer the universe will continue to expand.

What would be the implications if the right answer is a modification of general relativity? Are there any other theories that explain dark energy that make predictions about the future like this?

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This has some part of the answer, or really just modifications to the question: blogs.discovermagazine.com/cosmicvariance/2011/10/04/… It seems that I might have been mis-characterizing vacuum energy. Dark energy is a given, provided the observation. Vacuum energy is the specific candidate. –  AlanSE Oct 10 '11 at 0:32
What's given is supernova experiments which indicate that the expansion of the Universe is accelerating, and the fact that the least painful way to insert this into General Relativity is through the constant $\Lambda$ in the Einstein-equation. –  mtrencseni Oct 10 '11 at 11:20