What do we really know about cosmic acceleration?

Essentially all of our knowledge of the acceleration history of the universe (even the discovery of the acceleration itself) is predicated on the validity of general relativity through the Friedmann equation. Assuming only a flat spacetime that is homogenous and isotropic on large scales and described by a metric theory of gravity, we use SNe Ia to analyze the acceleration history. We infer robust conclusions about cosmic acceleration that are independent of a particular parameterization of the acceleration history. Specifically, we find (1) very strong (5 sigma) evidence for a period of acceleration, (2) strong evidence that the acceleration has not been constant, (3) evidence for an earlier period of deceleration, and (4) only weak evidence that the universe has been accelerating since z similar to 0.3.