Limited Modified Gravity
  We systematically assess several limiting cases of modified gravity, where
particular theoretical or observational conditions hold. This framework
includes the well known scalar-tensor gravity and No Slip Gravity and No Run
Gravity, and we extend it to three new limits: Only Run, Only Light, and Only
Growth Gravities. These limits give simplifications that allow deeper
understanding of modified gravity, including demonstration that gravitational
effects on light and matter can have opposite signs in their deviation from
general relativity. We also show observational predictions for the different
cosmic structure growth rates $f\sigma_8$ and the ratio of gravitational wave
standard siren luminosity to photon standard candle luminosity distance
relations, defining a new statistic $D_G$ that emphasizes their complementarity
and ability to distinguish models.
