Dark matter, time-varying<i>G</i>, and a dilaton field

Abstract

We demonstrate the observational viability of having an exactly massless dilaton field couple with gravitational strength to most matter in the Universe. This is done by constructing a generalized Jordan-Brans-Dicke model in which the scalar couples with different strengths to visible and to conjectured ``dark'' matter. In this model, improved \ifmmode \dot{G}\else \.{G}\fi{} measurements may provide nontrivial bounds on the coupling constants of the dilaton to matter.