Projective space and Grassmannians

The real projective space Pⁿ is a moduli space that parametrizes the space of lines in Rⁿ⁺¹ which pass through the origin. Similarly, complex projective space is the space of all complex lines in Cⁿ⁺¹ passing through the origin.

More generally, the Grassmannian G(k, V) of a vector space V over a field F is the moduli space of all k-dimensional linear subspaces of V.

Chow variety

The Chow variety Chow(d,P³) is a projective algebraic variety which parametrizes degree d curves in P³. It is constructed as follows. Let C be a curve of degree d in P³, then consider all the lines in P³ that intersect the curve C. This is a degree d divisor D_C in G(2, 4), the Grassmannian of lines in P³. When C varies, by associating C to D_C, we obtain a parameter space of degree d curves as a subset of the space of degree d divisors of the Grassmannian: Chow(d,P³).

Hilbert scheme

The Hilbert scheme Hilb(X) is a moduli scheme. Every closed point of Hilb(X) corresponds to a closed subscheme of a fixed scheme X, and every closed subscheme is represented by such a point. A simple example of a Hilbert scheme is the Hilbert scheme parameterizing degree ${\displaystyle d}$ hypersurfaces of projective space ${\displaystyle \mathbb {P} ^{n}}$. This is given by the projective bundle

${\displaystyle {\mathcal {Hilb}}_{d}(\mathbb {P} ^{n})=\mathbb {P} (\Gamma ({\mathcal {O}}(d)))}$

with universal family given by

${\displaystyle {\mathcal {U}}=\{(V(f),f):f\in \Gamma ({\mathcal {O}}(d))\}}$

where ${\displaystyle V(f)}$ is the associated projective scheme for the degree ${\displaystyle d}$ homogeneous polynomial ${\displaystyle f}$.

Fine moduli spaces

This is the standard concept. Heuristically, if we have a space M for which each point m ∊ M corresponds to an algebro-geometric object U_m, then we can assemble these objects into a tautological family U over M. (For example, the Grassmannian G(k, V) carries a rank k bundle whose fiber at any point [L] ∊ G(k, V) is simply the linear subspace L ⊂ V.) M is called a base space of the family U. We say that such a family is universal if any family of algebro-geometric objects T over any base space B is the pullback of U along a unique map B → M. A fine moduli space is a space M which is the base of a universal family.

More precisely, suppose that we have a functor F from schemes to sets, which assigns to a scheme B the set of all suitable families of objects with base B. A space M is a fine moduli space for the functor F if M represents F, i.e., there is a natural isomorphism τ : F → Hom(−, M), where Hom(−, M) is the functor of points. This implies that M carries a universal family; this family is the family on M corresponding to the identity map 1_M ∊ Hom(M, M).

Coarse moduli spaces

Fine moduli spaces are desirable, but they do not always exist and are frequently difficult to construct, so mathematicians sometimes use a weaker notion, the idea of a coarse moduli space. A space M is a coarse moduli space for the functor F if there exists a natural transformation τ : F → Hom(−, M) and τ is universal among such natural transformations. More concretely, M is a coarse moduli space for F if any family T over a base B gives rise to a map φ_T : B → M and any two objects V and W (regarded as families over a point) correspond to the same point of M if and only if V and W are isomorphic. Thus, M is a space which has a point for every object that could appear in a family, and whose geometry reflects the ways objects can vary in families. Note, however, that a coarse moduli space does not necessarily carry any family of appropriate objects, let alone a universal one.

In other words, a fine moduli space includes both a base space M and universal family U → M, while a coarse moduli space only has the base space M.

Moduli stacks

It is frequently the case that interesting geometric objects come equipped with many natural automorphisms. This in particular makes the existence of a fine moduli space impossible (intuitively, the idea is that if L is some geometric object, the trivial family L × [0,1] can be made into a twisted family on the circle S¹ by identifying L × {0} with L × {1} via a nontrivial automorphism. Now if a fine moduli space X existed, the map S¹ → X should not be constant, but would have to be constant on any proper open set by triviality), one can still sometimes obtain a coarse moduli space. However, this approach is not ideal, as such spaces are not guaranteed to exist, they are frequently singular when they do exist, and miss details about some non-trivial families of objects they classify.

A more sophisticated approach is to enrich the classification by remembering the isomorphisms. More precisely, on any base B one can consider the category of families on B with only isomorphisms between families taken as morphisms. One then considers the fibred category which assigns to any space B the groupoid of families over B. The use of these categories fibred in groupoids to describe a moduli problem goes back to Grothendieck (1960/61). In general, they cannot be represented by schemes or even algebraic spaces, but in many cases, they have a natural structure of an algebraic stack.

Algebraic stacks and their use to analyze moduli problems appeared in Deligne-Mumford (1969) as a tool to prove the irreducibility of the (coarse) moduli space of curves of a given genus. The language of algebraic stacks essentially provides a systematic way to view the fibred category that constitutes the moduli problem as a "space", and the moduli stack of many moduli problems is better-behaved (such as smooth) than the corresponding coarse moduli space.

Moduli of curves

The moduli stack ${\displaystyle {\mathcal {M}}_{g}}$ classifies families of smooth projective curves of genus g, together with their isomorphisms. When g > 1, this stack may be compactified by adding new "boundary" points which correspond to stable nodal curves (together with their isomorphisms). A curve is stable if it has only a finite group of automorphisms. The resulting stack is denoted ${\displaystyle {\overline {\mathcal {M}}}_{g}}$. Both moduli stacks carry universal families of curves. One can also define coarse moduli spaces representing isomorphism classes of smooth or stable curves. These coarse moduli spaces were actually studied before the notion of moduli stack was invented. In fact, the idea of a moduli stack was invented by Deligne and Mumford in an attempt to prove the projectivity of the coarse moduli spaces. In recent years, it has become apparent that the stack of curves is actually the more fundamental object.

Both stacks above have dimension 3g−3; hence a stable nodal curve can be completely specified by choosing the values of 3g−3 parameters, when g > 1. In lower genus, one must account for the presence of smooth families of automorphisms, by subtracting their number. There is exactly one complex curve of genus zero, the Riemann sphere, and its group of isomorphisms is PGL(2). Hence, the dimension of ${\displaystyle {\mathcal {M}}_{0}}$ is

dim(space of genus zero curves) − dim(group of automorphisms) = 0 − dim(PGL(2)) = −3.

Likewise, in genus 1, there is a one-dimensional space of curves, but every such curve has a one-dimensional group of automorphisms. Hence, the stack ${\displaystyle {\mathcal {M}}_{1}}$ has dimension 0. The coarse moduli spaces have dimension 3g−3 as the stacks when g > 1 because the curves with genus g > 1 have only a finite group as its automorphism i.e. dim(a group of automorphisms) = 0. Eventually, in genus zero, the coarse moduli space has dimension zero, and in genus one, it has dimension one.

One can also enrich the problem by considering the moduli stack of genus g nodal curves with n marked points. Such marked curves are said to be stable if the subgroup of curve automorphisms which fix the marked points is finite. The resulting moduli stacks of smooth (or stable) genus g curves with n-marked points are denoted ${\displaystyle {\mathcal {M}}_{g,n}}$ (or ${\displaystyle {\overline {\mathcal {M}}}_{g,n}}$), and have dimension 3g − 3 + n.

A case of particular interest is the moduli stack ${\displaystyle {\overline {\mathcal {M}}}_{1,1}}$ of genus 1 curves with one marked point. This is the stack of elliptic curves, and is the natural home of the much studied modular forms, which are meromorphic sections of bundles on this stack.

Moduli of varieties

In higher dimensions, moduli of algebraic varieties are more difficult to construct and study. For instance, the higher-dimensional analogue of the moduli space of elliptic curves discussed above is the moduli space of abelian varieties, such as the Siegel modular variety. This is the problem underlying Siegel modular form theory. See also Shimura variety.

Using techniques arising out of the minimal model program, moduli spaces of varieties of general type were constructed by János Kollár and Nicholas Shepherd-Barron, now known as KSB moduli spaces.^[4]

Using techniques arising out of differential geometry and birational geometry simultaneously, the construction of moduli spaces of Fano varieties has been achieved by restricting to a special class of K-stable varieties. In this setting important results about boundedness of Fano varieties proven by Caucher Birkar are used, for which he was awarded the 2018 Fields medal.

The construction of moduli spaces of Calabi-Yau varieties is an important open problem, and only special cases such as moduli spaces of K3 surfaces or Abelian varieties are understood.^[5]

Moduli of vector bundles

Another important moduli problem is to understand the geometry of (various substacks of) the moduli stack Vect_n(X) of rank n vector bundles on a fixed algebraic variety X.^[6] This stack has been most studied when X is one-dimensional, and especially when n equals one. In this case, the coarse moduli space is the Picard scheme, which like the moduli space of curves, was studied before stacks were invented. When the bundles have rank 1 and degree zero, the study of coarse moduli space is the study of the Jacobian variety.

In applications to physics, the number of moduli of vector bundles and the closely related problem of the number of moduli of principal G-bundles has been found to be significant in gauge theory.

Volume of the moduli space

Simple geodesics and Weil-Petersson volumes of moduli spaces of bordered Riemann surfaces.

Construction tools

• Hilbert scheme
• Quot scheme
• Deformation theory
• GIT quotient
• Artin's criterion, general criterion for constructing moduli spaces as algebraic stacks from moduli functors

Moduli spaces

• Moduli of algebraic curves
• Moduli stack of elliptic curves
• Moduli spaces of K-stable Fano varieties
• Modular curve
• Picard functor
• Moduli of semistable sheaves on a curve
• Kontsevich moduli space
• Moduli of semistable sheaves

Notes

• Moduli theory
• Moduli stacks in P-adic modular forms and Langlands program

Research articles