Astrophysical Tests of Modified Gravity: Constraints from Distance
  Indicators in the Nearby Universe
  We use distance measurements in the nearby universe to carry out new tests of
gravity, surpassing other astrophysical tests by over two orders of magnitude
for chameleon theories. The three nearby distance indicators -- cepheids, tip
of the red giant branch (TRGB) stars, and water masers -- operate in
gravitational fields of widely different strengths. This enables tests of
scalar-tensor gravity theories because they are screened from enhanced forces
to different extents. Inferred distances from cepheids and TRGB stars are
altered (in opposite directions) over a range of chameleon gravity theory
parameters well below the sensitivity of cosmological probes. Using published
data we have compared cepheid and TRGB distances in a sample of unscreened
dwarf galaxies within 10 Mpc. As a control sample we use a comparable set of
screened galaxies. We find no evidence for the order unity force enhancements
expected in these theories. Using a two-parameter description of the models
(the coupling strength and background field value) we obtain constraints on
chameleon and symmetron screening scenarios. In particular we show that f(R)
models with background field values fR0 above 5e-7 are ruled out at the 95%
confidence level. We also compare TRGB and maser distances to the galaxy NGC
4258 as a second test for larger field values. While there are several
approximations and caveats in our study, our analysis demonstrates the power of
gravity tests in the local universe. We discuss the prospects for additional,
improved tests with future observations.
