Motive (algebraic Geometry) - Tannakian Formalism and Motivic Galois Group

Tannakian Formalism and Motivic Galois Group

To motivate the (conjectural) motivic Galois group, fix a field k and consider the functor

finite separable extensions K of k → finite sets with a (continuous) action of the absolute Galois group of k

which maps K to the (finite) set of embeddings of K into an algebraic closure of k. In Galois theory this functor is shown to be an equivalence of categories. Notice that fields are 0-dimensional. Motives of this kind are called Artin motives. By Q-linearizing the above objects, another way of expressing the above is to say that Artin motives are equivalent to finite Q-vector spaces together with an action of the Galois group.

The objective of the motivic Galois group is to extend the above equivalence to higher-dimensional varieties. In order to do this, the technical machinery of Tannakian category theory (going back to Tannaka-Krein duality, but a purely algebraic theory) is used. Its purpose is to shed light on both the Hodge conjecture and the Tate conjecture, the outstanding questions in algebraic cycle theory. Fix a Weil cohomology theory H. It gives a functor from M_num (pure motives using numerical equivalence) to finite-dimensional Q-vector spaces. It can be shown that the former category is a Tannakian category. Assuming the equivalence of homological and numerical equivalence, i.e. the above standard conjecture D, the functor H is an exact faithful tensor-functor. Applying the Tannakian formalism, one concludes that M_num is equivalent to the category of representations of an algebraic group G, which is called motivic Galois group.

It is to the theory of motives what the Mumford-Tate group is to Hodge theory. Again speaking in rough terms, the Hodge and Tate conjectures are types of invariant theory (the spaces that are morally the algebraic cycles are picked out by invariance under a group, if one sets up the correct definitions). The motivic Galois group has the surrounding representation theory. (What it is not, is a Galois group; however in terms of the Tate conjecture and Galois representations on étale cohomology, it predicts the image of the Galois group, or, more accurately, its Lie algebra.)