Modified Einstein versus Modified Euler for Dark Matter

Understand

Modifications of General Relativity generically contain additional degrees of freedom that can mediate forces between matter particles.

• One of the common manifestations of a fifth force in alternative gravity theories is a difference between the gravitational potentials felt by relativistic and non-relativistic particles, also known as "the gravitational slip".
• In contrast, a fifth force between dark matter particles, due to dark sector interaction, does not cause a gravitational slip, making the latter a possible smoking gun of modified gravity.
• In this article, we point out that a force acting on dark matter particles, as in models of coupled quintessence, would also manifest itself as a measurement of an effective gravitational slip by cosmological surveys of large-scale structure.