Structure Formation Limits on f(R) Gravity*

Modifications to gravity may explain cosmic acceleration without envoking Dark Energy. Two features emerge in the power spectrum of structure formation in modified gravities that are not seen in standard GR: One, a strong suppression of power manifests at a potentially observable length scale. And two, the growth factor is scale dependent. Both these features make modified gravities difficult to reconscile with LSST observations and limit strongly the allowable types of f(R) modifications.