Articles

# “A Teichmüller space for X”

Or: some rambling thoughts on the following questions—in what sense is Outer space “like Teichmüller space for $\mathrm{Out}(F_n)$“? And in what sense is “higher Teichmüller theory” Teichmüller theory?

### The classical / canonical case

For a closed surface of genus $g \geq 2$, Teich(S) is the set of hyperbolic metrics on S, modulo isotopy. (By the uniformization theorem, these are all the constant-curvature metrics on S.) We may specify points in Teich(S) by pairs (X, h) where X is a surface of genus g (the “marked surface”, or “clothing”) with a hyperbolic metric, and $h: S \to X$ (the “marking”, or “instructions on how to wear the clothing”) is a homeomorphism. Alternatively, we may specify points by specifying a (hyperbolic) Riemannian metric m. We may go between the two notions: given a pair (X, h), the metric m in question is the pullback of the metric on X under h.

The construction of the Fenchel-Nielsen coordinates shows that Teich(S) is topologically an open (6g-6)-cell. We can endow it with a topology in various ways (algebraically, etc.)

Teich(S) can be given a Finsler metric, the Teichmüller metric $d_{Teich}$, with respect to which it is complete but not nonpositively-curved; it can also be given a Riemannian metric, the Weil-Petersson metric $d_{WP}$ (which has many reinterpretations—see e.g. Thurston-Wolpert, Bonahon, McMullen), with respect to which it is negatively-curved but not complete. The metric completion with respect to $d_{WP}$ is precisely the compactification of Teich(S) given by augmented Teichmueller space.

The mapping class group Mod(S) acts naturally on Teich(S), by precomposition of markings. This can be used to do things such as classify elements of Mod(S) (Thurston), study deformations of Klenian groups (Minsky-Canary-many others), etc.

### “Slightly elevated Teichmueller theory”

Choi-Goldman constructed the set C(S) of convex real projective geometric structures on a closed surface S [modulo some natural notion of equivalence]. They give it a topology analogous to the algebraic topology on Teich(S), and aso show that it is a (16g-16)-cell, using coordinates analogous to the Fenchel-Nielsen coordinates.

Moreover, this space has the natural structure of a complex variety (see e.g. this Calegari blogpost for an exposition.)

#### Other things called higher Teichmüller theory

… or that have that label attached to them: the Hitchin Component—but this seems like a very much more algebraic generalisation of Teich(S) (whereas Teich(S) may be seen as a connected component of a space of representations from $\pi_1(S)$ to PSL(2,R), Hitchin components are connected components of spaces of representations into PSL(n,R) for more general n.

Ditto more general spaces of representations [albeit representations with nice geometric properties / motivations, e.g. Anosov representations]

Fock-Goncharov have work in a similar vein (representations of $\pi_1(S)$ into a more general semisimple Lie group.) [see reference below]

### The case of outer space

Outer space $CV_n$ may be defined as the set of all metric tree structures on a bouquet of n circles (slightly more poetically, a rose of n petals), modulo free homotopy. Points in outer space may be specified as pairs (X, h) where X is a graph homotopy-equivalent to the bouquet of n circles, and h is a free homotopy class of homotopy equivalences from X to said bouquet.

It has the structure of a simplicial structure (with open simplices—most vertices are missing.)

$\mathrm{Out}(F_n)$, which may be described as the mapping class group of a bouquet of n circles, acts naturally on $CV_n$ by precomposition of markings.

### The common theme

Given a topological space X, “the Teichmüller space of X” is (should be?), roughly speaking, the set of all “natural [geo]metric structures” on X; a priori this may only be a set, but it may turn out that it can be given additional structure, possibly in nice ways.

I’m not sure there is a systematic way to make “natural [geo]metric structure” more precise at the moment, or that we can more systematically specify when and how we can endow what is a priori the set that is Teichmüller space with more structure, or at least not ways of doing this which encompass all the contexts described above.

There is a paper of Grothendieck which axiomatizes (classical) Teichmüller space using the machinery of analytic (and algebraic) geometry, obtaining a description in terms of principal fibre bundles over complex varieties / algebraic curves, but this does not seem to extend to the other contexts (at least not in a straightforward / natural way.

This level of (im)precision is not unlike that which surrounds the notion of a moduli space at the moment. (See Wikipedia article on moduli spaces: “Moduli spaces are spaces of solutions of geometric classification problems”)

Maybe it’s more productive that way, at least as things stand—to leverage the intuitive power of the metaphor without explicitly worrying about trying to make the metaphor categorically precise.

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Theorems

# Uniformization vs. Classification of Space Forms

Or: one illustration of the relationship between the notions of (Riemannian) isometry and  conformal equivalence.

## Uniformisation Theorem

Any connected, simply-connected Riemann surface X is conformally equivalent to $S^2$, the complex plane, or the unit disk.

General scheme of proof (for details see Donaldson’s book, or hyperlinks in the text of the scheme.)

• Any compact (“elliptic”) connected surface X is conformally equivalent to a Riemann sphere: we can show this by demonstrating [the existence of] a meromorphic map from X with a single simple pole.
• One way of doing this is to study the $H^{0,1}_X$ Dolbeault cohomology of X using the following result (the “Main Theorem” in Donaldson’s book): Let $\rho$ be a 2-form on X. There is a solution f to the equation $\Delta f = \rho$ iff $\int_X \rho = 0$, and the solution is unique up to additive constant.
• More explicit constructions using Dirichlet integrals  (similar to those that appeared in the original proofs, due independently to Poincaré and Koebe) may be found in this senior thesis or these notes.
• For noncompact (“parabolic” or “hyperbolic”)  X, we can use similar, but more careful, arguments to exhibit [the existence of] a meromorphic function from X with a single simple pole, which injectively maps X into an open subset of the Riemann sphere which turns out to be $S^2 - I$, where I is a single closed interval (possibly degenerate, i.e. a single point, but not empty.)
• The version of the “Main Theorem” Donaldson uses here (with extra care taken regarding points at infinity) reads: Let $\rho$ be a 2-form of compact support on X with $\int_X \rho = 0$. There is a function $\phi: X \to \mathbb{R}$ with $\Delta\rho = \phi$ s.t. $\rho \to 0$ at infinity in X.
• As above, one can furnish more explicit constructions, using Dirichlet integrals and Green’s functions. See the same references as above for details.

This may be viewed as a generalisation of the Riemann Mapping Theorem, which states that any (connected, simply-connected, proper) open subset of the complex plane is conformally equivalent to the unit disk.

Note that a Riemann surface is an orientable 2-manifold, and conversely any orientable 2-manifold can be given a Riemann surface structure. This suggests (or, at least, causes me to endlessly confuse uniformization with) a similar result, formulated purely in terms of Riemannian geometry.

## Classification of space forms

Any complete Riemannian manifold with constant sectional curvature has universal cover (isometric to) one of $S^n$ (spherical), $\mathbb{R}^n$ (Euclidean / flat), or $\mathbb{H}^n$ (hyperbolic).

Outline of Proof (for details see e.g. Chapters 7 and 8 of do Carmo’s Riemannian Geometry)

• First we need a Lemma (Cartan) (determination of metric by means of the curvature): Let $M, \tilde{M}$ be Riemannian manifolds of dimension n. Choose $p \in M$ and $\tilde{p} \in \tilde{M}$ and a linear isometry $i: T_pM \to T_{\tilde{p}} \tilde{M}$.

Proof: Let V be a normal neighborhood of p in M (such that the exponential maps required below are defined) and define a map $f: V \to \tilde{M}$ using the exponential maps at p and $\tilde{p}$.

For any $q \in V$ there exists a unique normalized geodesic $\gamma: [0,t] \to M$; define $P_t$ to be parallel transport along $\gamma$, and $\phi_t$ to be transport along this geodesic push forward to $\tilde{M}$.

Now if $(x,y,u,v) = (\phi_t(x), \phi_t(y), \phi_t(u), \phi_t(v))$ for all x, y, u, v and t (i.e. if the curvature along R and $\tilde{R}$ is equal and compatible with our notion/s of parallel transport) then $f: V \to f(V)$ is a local isometry and $df_p = \mathrm{id}$ (then our manifolds are in fact isometric.)

• Now consider the map from $\tilde{M}$ to model space as in the Cartan lemma above, which is globally defined here by completeness. Invoke the Cartan lemma to show it is a local isometry; then show it is a covering map, hence a diffeomorphism and so a global isometry.
• The concrete details differ slightly between the case of non-positive curvature and the case of positive curvature.
• For NPC: map from $\tilde{M}$ to model space ($\mathbb{H}^n$ or $\mathbb{R}^n$) is well-defined due to NPC (by a Theorem of Hadamard, 7.3.1 in do Carmo), and is a covering map from the following Lemma (here, the inequality is satisfied a fortiori because the exponential maps are diffeomorphisms)

Lemma NPC: If f is a local diffeomorphism from a complete Riemannian manifold M onto a Riemannian manifold N which is expanding in the sense that $|df_p(v)| \geq |v|$ for all $p \in M, v \in T_pM$, then f is a covering map

Sketch of prooff has path lifting property for curves in N: since f is a local diffeomorphism, we can lift small initial segments of paths. By the non-contracting property of the map, paths upstairs are no shorter, and so entire path must remain within compact neighborhood by completeness—but then we can argue that we can lift the entire path.

• In positive curvature: map from $\tilde{M} to S^n$ as in Cartan Theorem (with more careful choice of q) is a local isometry; pick another such map, and observe from the Lemma below that the maps are equal where they agree (here we need two charts, not just one.)
• Combine the two to obtain a map g on all of $S^n$; then argue that g is a covering map by compactness of codomain (proper local diffeomorphisms are covering maps.)

Lemma PC: If f and g are local isometries from a connected Riemmanian manifold M to a Riemannian manifold N s.t. $f(p) = g(p)$ and $df_p = dg_p$ for some $p \in M$, then $f = g$

Sketch of proof: Given hypotheses imply f = g in some normal neighborhood V of p; now use connectedness to propagate this identity outwards along paths in M

This suggests an alternative proof of uniformisation, via the classification of (smooth orientable) surfaces and Riemannian geometry: any connected and simply-connected Riemann surface, being a connected and simply-connected orientable topological manifold of real dimension 2, is [can be given a Riemannian metric] isometric to the 2-sphere, Euclidean 2-space, or hyperbolic 2-space; we may put conformal structures on these conformally equivalent to the sphere, the complex plane, and the unit disk.

Unfortunately, the remaining gap is the hardest bit of the argument. Conformal equivalence is a coarser equivalence than Riemannian isometry—a Riemannian metric yields a conformal structure; equivalent metrics yield equivalent conformal structures, but a conformal equivalence need not be an isometry. e.g. $\mathbb{R}^2_+$ (with a flat metric) and $\mathbb{H}^2$ are conformally equivalent. It might be possible to show that any connected, simply-connected Riemann surface is conformally equivalent to a surface of constant sectional curvature, but a priori this does not seem any easier than the other proofs of uniformisation.

Coarser though conformal equivalence may be as an equivalence relation, it is also surprisingly (?) more subtle. Coarsely speaking, the additional information which the Riemannian metric records beyond the conformal structure seems to provide a nice organizing principle which makes things easier. Slightly more precisely: the set of all Riemann surfaces of a fixed genus g, modulo conformal equivalences, is exactly the moduli space $\mathcal{M}_g$, while the same set, modulo [something only slightly stronger than] metric equivalence, is Teichmüller space Teich(g). The former is a quotient of the latter, and is much more difficult to study.

(Offline) References

• Simon Donaldson, Riemann Surfaces
• Manfredo P. do Carmo, Riemannian Geometry
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