  Einstein's theory of general relativity, which contains a universal value of
the Planck mass, has been so far successfully invoked to explain gravitational
dynamics from sub-millimeter scales to the scale of the cosmological horizon.
However, one may envisage that in alternative theories of gravity, the
effective value of the Planck mass (or Newton's constant), which quantifies the
coupling of matter to metric perturbations, can run on the cosmological horizon
scale. In this letter, we study the consequences of a glitch in the Planck mass
from sub-horizon to super-horizon scales. We first give three examples of
models that naturally exhibit this feature, and then show that current
cosmological observations severely constrain this glitch to less than 1.2%.
This is the strongest constraint to date, on natural (i.e. non-fine-tuned)
deviations from Einstein gravity on the cosmological horizon scale.

Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Theory