Moduli Of Abelian Varieties

Abelian varieties are a natural generalization of elliptic curves, including algebraic tori in higher dimensions. Just as elliptic curves have a natural moduli space $\mathcal_$ over characteristic 0 constructed as a quotient of the upper-half plane by the action of $SL_2(\mathbb)$, there is an analogous construction for abelian varieties $\mathcal_g$ using the Siegel upper half-space and the symplectic group $\operatorname_(\mathbb)$.

Constructions over characteristic 0

Principally polarized Abelian varieties

Recall that the Siegel upper-half plane is given by

$H_g = \ \subseteq \operatorname_g(\mathbb)$

which is an open subset in the $g\times g$ symmetric matrices (since $\operatorname(\Omega) > 0$ is an open subset of $\mathbb$, and $\operatorname$ is continuous). Notice if $g=1$ this gives $1\times 1$ matrices with positive imaginary part, hence this set is a generalization of the upper half plane. Then any point $\Omega \in H_g$ gives a complex torus

$X_\Omega = \mathbb^g/(\Omega\mathbb^g + \mathbb^g)$

with a principal polarization $H_\Omega$ from the matrix $\Omega^$^{page 34}. It turns out all principally polarized Abelian varieties arise this way, giving $H_g$ the structure of a parameter space for all principally polarized Abelian varieties. But, there exists an equivalence where

$X_\Omega \cong X_ \iff \Omega = M\Omega'$ for $M \in \operatorname_(\mathbb)$

hence the moduli space of principally polarized abelian varieties is constructed from the stack quotient

\mathcal_g = operatorname_(\mathbb)\backslash H_g/math>

which gives a Deligne-Mumford stack over $\operatorname(\mathbb)$. If this is instead given by a GIT quotient, then it gives the coarse moduli space $A_g$.

Principally polarized Abelian varieties with level ''n''-structure

In many cases, it is easier to work with the moduli space of principally polarized Abelian varieties with level ''n''-structure because it creates a rigidification of the moduli problem which gives a moduli functor instead of a moduli stack.Level ''n''-structures are used to construct an intersection theory of Deligne–Mumford stacks This means the functor is representable by an algebraic manifold, such as a variety or scheme, instead of a stack. A level ''n''-structure is given by a fixed basis of

: $H_1(X_\Omega, \mathbb/n) \cong \frac\cdot L/L \cong n\text X_\Omega$

where $L$ is the lattice $\Omega\mathbb^g + \mathbb^g \subset \mathbb^$. Fixing such a basis removes the automorphisms of an abelian variety at a point in the moduli space, hence there exists a bona-fide algebraic manifold without a stabilizer structure. Denote

\Gamma(n) = \ker operatorname_(\mathbb) \to \operatorname_(\mathbb)/n/math>

and define

$A_{g,n} = \Gamma(n)\backslash H_g$

as a quotient variety.

References

See also

* Schottky problem
* Siegel modular variety
* Moduli stack of elliptic curves
* Moduli of algebraic curves
* Hilbert scheme
* Deformation Theory

Abelian varieties
Elliptic curves