Narrow Results

In models of universes that have constant negative spatial curvature, photon trajectories are exponentially unstable, which imply chaotic photonic dynamics. This has been put forward as an explanation of pre-inflationary homogeneity and to the fact that fluctuations in the cosmic microwave background are close to Gaussian. We show here that additional random fluctuations in the curvature can lead to stochastic stabilization of photon trajectories implying in several consequences for the statistic of temperature fluctuations of the Cosmic Microwave Background.

Let M be a complete, simply connected Riemannian manifold with negative curvature. We obtain the sharp constants of Hardy and Rellich inequalities related to the geodesic distance on M. Furthermore, if M is with strictly negative curvature, we show that the L^p Hardy inequalities can be globally refined by adding remainder terms like the Brezis–Vázquez improvement in case p ≥ 2, which is contrary to the case of Euclidean spaces.

The goal of this paper is to provide sharp spectral gap estimates for problems involving higher-order operators (including both the clamped and buckling plate problems) on Cartan–Hadamard manifolds. The proofs are symmetrization-free — thus no sharp isoperimetric inequality is needed — based on two general, yet elementary functional inequalities. The spectral gap estimate for clamped plates solves a sharp asymptotic problem from [Q.-M. Cheng and H. Yang, Universal inequalities for eigenvalues of a clamped plate problem on a hyperbolic space, Proc. Amer. Math. Soc. 139(2) (2011) 461–471] concerning the behavior of higher-order eigenvalues on hyperbolic spaces, and answers a question raised in [A. Kristály, Fundamental tones of clamped plates in nonpositively curved spaces, Adv. Math. 367(39) (2020) 107113] on the validity of such sharp estimates in high-dimensional Cartan–Hadamard manifolds. As a byproduct of the general functional inequalities, various Rellich inequalities are established in the same geometric setting.

Motivated by recent groundbreaking work of Ontaneda, we describe a sizable class of closed manifolds such that the product of each manifold in the class with ℝ admits a complete metric of bounded negative sectional curvature which is an exponentially warped near one end and has finite volume near the other end.

We describe the topological structure of cocompact singular Riemannian foliations on Riemannian manifolds without conjugate points. We prove that such foliations are regular and developable and have regular closures. We deduce that in some cases such foliations do not exist.