modified gravity

I conversed with Matt Kleban (NYU) for a while today about modifications to gravity that could remove the need for dark matter or for a cosmological constant. I mentioned my heresy about acceleration—that it is not demonstrated at high significance with SNe alone; it requires the combination of SNe and LSS or CMB constraints in the context of a physical cosmogonic model (ie, the many-sigma demonstration of acceleration is indirect). This encourages me to investigate a(t) in other models, but unfortunately almost all theoretically allowed modifications to gravity create strongly coupled, long-range gravitational interactions. There is a conflict, in other words, between models that can be computed (and therefore constrained) and models that are theoretically possible/allowed. This is not a good situation, because the non-zero dark energy can only be said to be experimentally demonstrated if it beats some plausible competitors.