Noncommutative Manifolds the Instanton Algebra and Isospectral Deformations

Abstract

We give new examples of noncommutative manifolds that are less standard than the NC-torus or Moyal deformations of $\Rb^n$. They arise naturally from basic considerations of noncommutative differential topology and have non-trivial global features. The new examples include the instanton algebra and the NC-4-spheres $S^4_{\theta}$. The noncommutative algebras $\Ac=C^{\ify} (S^{4}_{\theta})$ of functions on NC-spheres are solutions to the vanishing, $ {\rm ch}_j (e) = 0, j < 2 $, of the Chern character in the cyclic homology of $\Ac$ of an idempotent $e \in M_4 (\Ac), e^2 = e, e = e^*$. The universal noncommutative space defined by this equation is a noncommutative Grassmanian defined by very non trivial cubic relations. This space ${\rm Gr}$ contains the suspension of a NC-3-sphere intimately related to quantum group deformations ${\rm SU}_q (2)$ of ${\rm SU} (2)$ but for unusual values (complex values of modulus one) of the parameter $q$ of $q$-analogues, $q=\exp (2\pi i \t)$. We then construct the noncommutative geometry of $S_{\t}^4$ as given by a spectral triple $(\Ac, \Hc, D)$ and check all axioms of noncommutative manifolds. The Dirac operator $D$ on the noncommutative 4-spheres $S_{\t}^4$ gives a solution to the basic quartic equation defining the `volume form' $ < (e - {1/2}) [D,e]^4 > = \g_5$, where $<$ is the projection on the commutant of $4 \ts 4$ matrices. Finally, we show that any compact Riemannian spin manifold whose isometry group has rank $r \geq 2$ admits isospectral deformations to noncommutative geometries.