CONSISTENT SPIN-2 COUPLING AND QUADRATIC GRAVITATION

A discussion of the field content of quadratic higher-derivative gravi tation is presented, together with a new example of a massless spin-tw o field consistently coupled to gravity. The full quadratic gravity th eory is shown to be equivalent to a canonical second-order theory of a massive scalar field, a massive spin-two symmetric tensor field, and gravity. The conditions showing that the tensor field describes only s pin-two degrees of freedom are derived. A limit of the second-order th eory provides a new example of a massless spin-two field consistently coupled to gravity. A restricted set of vacua of the second-order theo ry is also discussed. It is shown that flat space is the only stable v acuum of this type, and that the spin-two field around flat space is u nfortunately always ghostlike.