Dark Energy Epicycles
As is well known, Einstein thought the cosmological constant was a big mistake. The early solutions to his equations, without the constant, described expanding universes, but no one at that time had made any observations indicating that the universe was in fact expanding, and Einstein was horrified by a vision of the cosmos that went against all previous ideas. Eventually the constant was forgotten, except by a few die hard General Relativists who liked playing with the equations.
In 1998 an apparent (note this word) acceleration in the expansion was observed using Type IA supernovae, which act as standard candles due to the uniformity in their nature. In the Standard Cosmology, much other data appeared to confirm the assessment that the expansion was in fact accelerating. But in the last year or two it has become clear that the standard picture cannot fit the supernovae data. The word on the street has been that our understanding of the standard candles must be flawed. Funny then that they fit the data so well at the low z part of the curve.
Anyway, the Standard Cosmology is wrong. It is quite easy to explain away the apparent acceleration. In fact, some General Relativists have been doing just that using simple modifications of the standard picture. The true cosmology, however, is radically different, as is well known by the great cosmologist Louise Riofrio. It means thinking about a varying speed of light, at the very least, and I say that because it gets much, much more radical the more you look at it.
Amongst Quantum Gravitists it is well understood that the correct Background Independent unified theory will radically alter our conceptions of space and time. We are only in the early days of M theory, but we can see that a varying speed of light cosmology is probably inevitable. It also has the advantage of actually agreeing with the data.