<B>High Energy-Astrophysics</B><br><I>Equivalence Principle and Cosmic Acceleration</I>

Speaker: Lam Hui, Columbia University

Current ideas on modifying general relativity to explain the observed cosmic acceleration all require introducing new gravitational degrees of freedom, most commonly a light scalar field. Such a scalar field, while active on large scales to somehow drive acceleration, must be screened on small scales to match solar system and terrestrial experiments. These generalized scalar-tensor theories encompass both f(R) and extra-dimensional models such as DGP, where the graviton effectively has a resonance width. I will show how such generalized scalar-tensor theories generically lead to violations of the equivalence principle, and for certain screening mechanisms, the violations are order *unity* - such theories are nonetheless consistent with existing constraints, but lend themselves to interesting new astronomical tests. The most promising objects to study are galaxies in cosmic voids.