Velocities as a probe of dark sector interactions
  Dark energy in General Relativity is typically non-interacting with other
matter. However, it is possible that the dark energy interacts with the dark
matter, and in this case, the dark matter can violate the universality of free
fall (the weak equivalence principle). We show that some forms of the dark
sector interaction do not violate weak equivalence. For those interactions that
do violate weak equivalence, there are no available laboratory experiments to
probe this violation for dark matter. But cosmology provides a test for
violations of the equivalence principle between dark matter and baryons -- via
a test for consistency of the observed galaxy velocities with the Euler
equation.
