# 000 Why stacks

Topic: Derived Geometry and Stacks

# Motivation for Stacks

- What we would really like is a quasi-projective variety $${\mathcal{M}_g}$$ with a universal family $$U_{g} \rightarrow {\mathcal{M}_g}$$ such that the fiber of a point $$[C] \in {\mathcal{M}_g}$$ is precisely that curve. This is where the difficulty lies-automorphisms of curves obstruct the existence of such a family-and this is the main reason we want to expand our notion of a geometric space from schemes to algebraic stacks. Algebraic stacks provide a nice approach ensuring the existence of a universal family but it is by no means the only approach.