Static near-horizon geometries and rigidity of quasi-Einstein manifolds

Static vacuum near-horizon geometries are solutions (M, g, X) of a certain quasi-Einstein equation on a closed manifold M, where g is a Riemannian metric and X is a closed 1-form. It is known that when the cosmological constant vanishes, there is rigidity: X vanishes and consequently g is Ricci-flat. We study this form of rigidity for all signs of the cosmological constant. It has been asserted that this rigidity also holds when the cosmological constant is negative, but we exhibit a counter-example. We show that for negative cosmological constant if X does not vanish identically, it must be incompressible, have constant norm, and be nontrivial in cohomology, and (M, g) must have constant scalar curvature and zero Euler characteristic. If the cosmological constant is positive, X must be exact (and vanishing if dimM=2$$\dim M=2$$). Our results apply more generally to a broad class of quasi-Einstein equations on closed manifolds. We extend some known results for quasi-Einstein metrics with exact 1-form X to the closed X case. We consider near-horizon geometries for which the vacuum condition is relaxed somewhat to allow for the presence of a limited class of matter fields. An “Appendix” contains a generalization of a result of Lucietti on the Yamabe type of quasi-Einstein compact metrics (with arbitrary X).