The purpose of this post, the third in a series on deformation theory and DGLAs, is to describe the obstruction theory for a formal moduli problem associated to a DGLA.

1. Tangent-obstruction theories

Standard problems in classical deformation theory usually have a “tangent-obstruction theory” parametrized by certain successive cohomology groups. For example, let’s consider the problem of deformations of a smooth variety {X} over an algebraically closed field {k}, over finite-dimensional local {k}-algebras. Then:

  • The “infinitesimal automorphisms” of {X}—that is, automorphisms of the trivial deformation over {k[\epsilon]/\epsilon^2}—are given by {H^0( X, T_X)} where {T_X} is the tangent bundle (i.e., vector fields).
  • The isomorphism classes of deformations of {X} over the dual numbers {k[\epsilon]/\epsilon^2} are given by {H^1(X, T_X)}.
  • There is an obstruction theory with {H^1, H^2}. Specifically, given a square-zero extension of finite-dimensional local {k}-algebras

    \displaystyle 0 \rightarrow I \rightarrow A' \rightarrow A \rightarrow 0,

    and given a deformation {\xi} of {X} over {\mathrm{Spec} A}, there is a functorial obstruction in {H^2(X, T_X) \otimes_k I} to extending the deformation over the inclusion {\mathrm{Spec} A \hookrightarrow \mathrm{Spec} A'}.

  • In the previous item, if the obstruction vanishes, then the isomorphism classes of extensions of {\xi} over {\mathrm{Spec} A'} are a torsor for {H^1(X, T_X) \otimes_k I}.

One has a similar picture for other deformation problems, for example deformations of vector bundles or closed subschemes. The “derived” approach to deformation theory provides (at least in characteristic zero) a general explanation for this phenomenon. (more…)

Let {k} be a field of characteristic zero. In the previous post, we introduced the category (i.e., {\infty}-category) {\mathrm{Moduli}_k} of formal moduli problems over {k}. A formal moduli problem over {k} is a moduli problem, taking values in spaces, that can be evaluated on the class of “derived” artinian {k}-algebras with residue field {k}: this was the category {\mathrm{CAlg}_{sm}} introduced in the previous post.

In other words, a formal moduli problem was a functor

\displaystyle F: \mathrm{CAlg}_{sm} \rightarrow \mathcal{S} \ (= \text{spaces}),

which was required to send {k} itself to a point, and satisfy a certain cohesiveness condition: {F} respects certain pullbacks in {\mathrm{CAlg}_{sm}} (which corresponded geometrically to pushouts of schemes).

The main goal of the series of posts was to sketch a proof of (and define everything in) the following result:

Theorem 7 (Lurie; Pridham) There is an equivalence of categories between {\mathrm{Moduli}_k} and the {\infty}-category {\mathrm{dgLie}} of DGLAs over {k}.


4. Overview

Here’s a rough sketch of the idea. Given a formal moduli problem {F}, we should think of {F} as something like a small space, concentrated at a point but with lots of “infinitesimal” thickening. (Something like a {\mathrm{Spf}}.) Moreover, {F} has a canonical basepoint corresponding to the “trivial deformation.” That is, we can think of {F} as taking values in pointed spaces rather than spaces.

It follows that we can form the loop space {\Omega F = \ast \times_F \ast} of {F}, which is a new formal moduli problem. However, {\Omega F} has more structure: it’s a group object in the category of formal moduli problems — that is, it’s some sort of derived formal Lie group. Moreover, knowledge of the original {F} is equivalent to knowledge of {\Omega F} together with its group structure: we can recover {F} as {B \Omega F} (modulo connectivity issues that end up not being a problem). This relation between ordinary objects and group objects (via {B, \Omega}) is something very specific to the derived or homotopy world, and it’s what leads to phenomena such as Koszul duality. (more…)

There’s a “philosophy” in deformation theory that deformation problems in characteristic zero come from dg-Lie algebras. I’ve been trying to learn a little about this. Precise statements have been given by Lurie and Pridham which consider categories of “derived” deformation problems (i.e., deformation problems that can be evaluated on derived rings) and establish equivalences between them and suitable (higher) categories of dg-Lie algebras. I’ve been reading in particular Lurie’s very enjoyable survey of his approach to the problem, which sketches the equivalence in an abstract categorical context with the essential input arising from Koszul duality between Lie algebras and commutative algebras. In this post, I’d just like to say what a “deformation problem” is in the derived world.

1. Introduction

Let {\mathcal{M}} be a classical moduli problem. Abstractly, we will think of {\mathcal{M}} as a functor

\displaystyle \mathcal{M}:\mathrm{Ring} \rightarrow \mathrm{Sets},

such that, for a (commutative) ring {R}, the set {\mathcal{M}(R)} will be realized as maps from {\mathrm{Spec} R} into a geometric object—a scheme or maybe an algebraic space.

Example 1{\mathcal{M}} could be the functor that sends {R} to the set of closed subschemes of {\mathbb{P}^n_R} which are flat over {R}. In this case, {\mathcal{M}} comes from a scheme: the Hilbert scheme.

We want to think of {\mathcal{M}} as some kind of geometric object and, given a point {x: \mathrm{Spec} k \rightarrow \mathcal{M}} for {k} a field (that is, an element of {\mathcal{M}(k)}), we’d like to study the local structure of {\mathcal{M}} near {x}. (more…)