Speaker
Description
Scalar-tensor theories of gravity are known to generically admit an effective imperfect fluid description, which leads to a natural interpretation of GR as a state of thermal equilibrium. In particular, it is possible to assign a temperature to nonminimally coupled fields, which increases as the theory deviates from GR. However, this temperature is conformally dependent, and can be arbitrarily tuned, threatening its interpretation as an intrinsic property of a scalar-tensor theory (as opposed to a representation-dependent property). To remedy this, we employ the frame-invariant formulation of scalar-tensor gravity, in which all quantities are manifestly independent of the choice of conformal frame. We then find that all scalar-tensor theories, not just minimal ones, feature an identically vanishing temperature. As a result, the deviation from GR is a diffusive (rather than thermodynamical) process, controlled by the frame-invariant chemical potential, and so GR emerges as the state of diffusive equilibrium in the class of scalar-tensor theories (minimal or nonminimal).