Limitations on Standard Sirens tests of gravity from screening
  Modified gravity theories with an effective Newton constant that varies over
cosmological timescales generally predict a different gravitational wave
luminosity distance than General Relativity. While this holds for a uniform
variation, we show that if locally screened at the source and at the observer
as required to pass stringent astrophysical tests of gravity, the General
Relativistic distance is restored. In the absence of such a screening, the same
effect must modify electromagnetic luminosity distances inferred from
supernovae Type Ia, to the extent that the effects can cancel in the
comparison. Hence, either the modifications considered employ screening, which
leaves no signature in Standard Sirens of a cosmological modification of
gravity, or screening does not operate, in which case there can be a signal
that is however well below the forseeable sensitivity of the probe when
astrophysical bounds are employed. We recover these results both in the Jordan
and Einstein frames, paying acute attention to pecularities of each frame such
as the notion of redshift or geodesic motions. We emphasise that despite these
limitations, Standard Sirens provide valuable independent tests of gravity that
differ fundamentally from other probes, a circumstance that is generally
important for the wider scope of gravitational modifications and related
scenarios. Finally, we use our results to show that the gravitational wave
propagation is not affected by dark sector interactions, which restores a dark
degeneracy between conformal and disformal couplings that enables
observationally viable cosmic self-acceleration to emenate from those.
