Theories that attempt to explain the observed cosmic acceleration by modifying gravity on large scales generally introduce a new long range force, typically mediated by a scalar. We will explain the fundamental reason behind the ubiquitous nature of this scalar force. Such a force has to be screened on solar system scales to satisfy existing experimental constraints, but can lead to O(1) violations of the equivalence principle in other environments. We will discuss what these environments might be where suitable astrophysical tests can be carried out.
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