Gravity with Auxiliary Fields

Understand

Modifications of General Relativity usually include extra dynamical degrees of freedom, which to date remain undetected.

• Here we explore the possibility of modifying Einstein's theory by adding solely nondynamical fields.
• With the minimal requirement that the theory satisfies the weak equivalence principle and admits a covariant Lagrangian formulation, we show that the field equations generically have to include higher-order derivatives of the matter fields.
• This has profound consequences for the viability of these theories.