Lefschetz extensions, tight closure, and big Cohen-Macaulay algebras

Abstract

We associate to every equicharacteristic zero Noetherian local ring $R$ a faithfully flat ring extension which is an ultraproduct of rings of various prime characteristics, in a weakly functorial way. Since such ultraproducts carry naturally a non-standard Frobenius, we can define a new tight closure operation on $R$ by mimicking the positive characteristic functional definition of tight closure. This approach avoids the use of generalized N\'eron Desingularization and only relies on Rotthaus' result on Artin Approximation in characteristic zero. If $R$ is moreover equidimensional and universally catenary, then we can also associate to it in a canonical, weakly functorial way a balanced big Cohen-Macaulay algebra.