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# Isomorphic groups via hyperbolic geometry

$\mathrm{PSO}(1, 2)$ is the isometry group of hyperbolic 2-space under the hyperboloid model; $\mathrm{PSL}(2,\mathbb{R})$ is the isometry group of the upper half-plane; $\mathrm{PSU}(1,1)$ is the isometry group of the Poincaré unit disk. Since these are in fact models for the same space, all of these Lie groups are isomorphic.

Question: is there a proof of this isomorphism that doesn’t proceed through this identification as isometry groups? (Probably, via the Lie algebras for instance, but I’m a little too lazy to try and work it out at the moment.)

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# A quick and dirty definition of an arithmetic subgroup?

Or, at least, of an arithmetic subgroup of $G \leq \mathrm{SL}(n,\mathbb{R})$ a semisimple Lie group:

1. The “subgroup of integer points” $G_{\mathbb{Z}} = G \cap \mathrm{SL}(n,\mathbb{Z})$ is an arithmetic group …
2. … if embedded in a reasonable way which doesn’t distort the arithmetic structure—slightly more precisely, if it is [essentially] an algebraic group over the rationals.
3. Anything isomorphic to a finite extension is arithmetic too.

For a more precise definition—Witte Morris’ book is a good place to start; he fudges over a few things too, but actually has references to fill those in …

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# Mostow Rigidity: several proofs

Mostow rigidity (or, technically, the special case thereof, for $\mathrm{PO}(1,3)$) states that if M and N are two closed (i.e. compact and boundary-less) hyperbolic 3-manifolds, and $f: M \to N$ is a homotopy equivalence, then f is homotopic to an isometry $g: M \to N$.

This is a strong rigidity result. If M and N were hyperbolic surfaces, by contrast, there are plenty of homotopy equivalences between them which are not homotopic to isometries; indeed, this is the very subject of Teichmüller theory (technically, we’re looking at moduli space, not Teichmüller space, but eh.)

We can state it in algebraic terms, by identifying the hyperbolic 3-manifolds $M \cong \mathbb{H}^3 / \Gamma_M$ and $N \cong \mathbb{H}^3 / \Gamma_N$ with their respective fundamental groups $\Gamma_M = \pi_1(M)$ and $\Gamma_N = \pi_1(N)$, which are (uniform) lattices in $\mathrm{Isom}^+(\mathbb{H}^3) \cong \mathrm{PO}(1,3)$.

Mostow rigidity then states any isomorphism between the lattices is given by conjugation in $\mathrm{PO}(1,3)$. Again, contrast this with the case of lattices in $\mathrm{Isom}^+(\mathbb{H}^2) \cong \mathrm{PO}(1,2)$ (i.e. fundamental groups of hyperbolic surfaces), which have plenty of outer automorphisms (elements of the [extended] mapping class group.)

Here we describe, at a relatively high level, various proofs of this result.

### Step 1: lift and extend

Let $\tilde{h}: \mathbb{H}^3 \to \mathbb{H}^3$ be a lift of h to the universal cover(s). This map is a quasi-isometry, by a Milnor-Švarc argument:

• The Cayley graphs of $\Gamma_M$ and $\Gamma_N$ are both quasi-isometric to $\mathbb{H}^3$ by Milnor-Švarc, the quasi-isometry from the Cayley graphs into hyperbolic space being the orbit map.
• If we now construct quasi-inverses from hyperbolic space to the Cayley graphs, then $\tilde{h}$ is coarsely equivalent to a map between the Cayley graphs obtained by composing one orbit map with the quasi-inverse of the other. To wit: $\tilde{h}$ sends $\gamma x_0$ to $(h_* \gamma) x_0$, and the induced map $h_*$ is an isomorphism of the fundamental groups

The quasi-isometry $\tilde{h}$ from the hyperbolic space $\mathbb{H}^3$ to itself extends to a self-homeomorphism of the Gromov boundary $\partial \mathbb{H}^3$:

This is a standard construction which holds for any quasi-isometry between hyperbolic spaces in the sense of Gromov: the boundary extension is defined by $\partial\tilde{h}([\gamma]) = [\tilde{h} \circ \gamma]$, and Gromov hyperbolicity is used to show that this map is a well-defined homeomorphism; roughly speaking, the idea is that quasi-isometries are like “biLipschitz maps for far-sighted people”, and at the boundary at infinity, the uniformly bounded distortions become all but invisible.

### Step 2: boundary map is quasiconformal

Now we claim that $\partial\tilde{h}: \hat{\mathbb{C}} \to \hat{\mathbb{C}}$ is quasiconformal.

This follows e.g. from the following geometric lemma: if L is a geodesic and P a totally-geodesic hyperbolic plane in $\mathbb{H}^3$ with $L \perp P$, then $\mathrm{diam}(\pi_{\hat(L)}(\tilde{h}(P)) \leq D$ for some constant D depending only on the quasi-isometry constants of $\tilde{h}$. (Here $\hat{L}$ is the unique geodesic uniformly close to the quasigeodesic $\tilde{h}(L)$.)

The proof of the lemma uses three key ingredients from the geometry of hyperbolic space: thinness of ideal triangles, quasigeodesic stability—which says that the image of any geodesic segment by a quasi-isometry is uniformly (depending only on the quasi-isometry constants) close to the actual geodesic with the same endpoints, which we will call the “straightening” of the quasigeodesic—, and the (1-)Lipschitz property of projection to a geodesic.

Let $x \in L \cap P$ be the unique point of intersection. If $y \in P$, let z be the point in $\partial_\infty P$ corresponding to the ray from x through y. Now consider the ideal triangle with L as one of its sides and as the third vertex. h(x) is uniformly close to the images of the two sides ending at z , and also to straightenings $\bar{M_1},\bar{ M_2}$ thereof; since projection to a geodesic is Lipschitz, the projection of h(x) to the straightening $\hat{L}$ of the quasigeodesic h(L) is uniformly close to the projections of $\bar{M_1}, \bar{M_2}$.

But now the image of the geodesic ray [x, z] lies uniformly close to some geodesic ray, whose projection onto $\bar{L}$ lies between the projections of h(x) and h(z). Hence any point on this geodesic ray has image which projects between h(x) and h(z); again using the Lipschitz property of the projection, we obtain uniform bounds on the diameter we are trying to control, as desired.

Now note that any circle in $\partial\mathbb{H}^3 \cong \hat{\mathbb{C}}$ is the boundary of a totally geodesic hyperbolic plane in $\mathbb{H}^3$, and our lemma then tells us that the image of the circle under $\tilde{h}$ has its outradius/inradius ratio bounded above by $e^D$.

### Step 3: boundary map is conformal

And then we would be done, for this implies $\tilde{h}$ is (up to homotopy) a hyperbolic isometry, which descends to an isometry $h: M \to N$.

#### An argument along the lines of Mostow’s original

Suppose $\tilde{h}$ is not conformal. Consider the $\Gamma_M$-invariant measurable line field $\ell_h$ on $\hat{\mathbb{C}}$ obtained by taking the direction of maximal stretch at each point.

Rotating this line field by any fixed angle $\phi$ yields a new measurable line field $\ell_h^\phi$. But now $\bigcup_{0 \leq \phi \leq \pi/2} \ell_h^\phi$ yields a measurable $\Gamma_M$-invariant proper subset of $T^1M$, which contradicts the ergodicity of the geodesic flow on $T^1 M$.

An argument following Tukia, in the spirit of Sullivan

Suppose $\tilde{h} =: \phi$ is differentiable at z. Let $\gamma$ be a Möbius transformation fixing z and $\infty$, with z an attracting fixed point and with no rotational component. Now, by the north-south dynamics that hyperbolic Möbius transformations exhibit, $\gamma^n \phi \gamma^{-n} \to d\phi_z$ as $n \to\ infty$; on the other hand, $\gamma^n \phi \gamma^{-n}$ conjugates $\gamma^n\Gamma_1\gamma^{-n}$ to $\gamma^n\Gamma_2\gamma^{-n}$.

Again, using convergence dynamics, we have $\gamma^n\Gamma_i\gamma^{-n} \to \Gamma_i^\infty$ as $n \to \infty$, for i = 1, 2, where both $\Gamma_i^\infty$ are cocompact and of equal covolume.

Now $d\phi_z$ conjugates $\Gamma_1^\infty$ to $\Gamma_2^\infty$; thus $\phi$ is conformal a.e., and hence conformal.

### Alternative perspective I: the Gromov norm

Gromov gives a proof which follows Step 1 as above, but then proceeds differently: he uses the Gromov norm to show that $\tilde{h}$ is conformal, without (separately) showing that it is quasiconformal. We sketch an outline of this here; see Calegari’s blog and/or Lücker’s thesis for details.

The Gromov norm is a measure of complexity for elements of the (singular) homology—more precisely, it is the infimum, over all chains representing the homology element, of $L^1$ norm of the chain.

It is a theorem of Gromov that the Gromov norm of (the fundamental class of) an oriented hyperbolic n-manifold M is equivalent, up to a normalization dependent only on n, to the hyperbolic volume of M. The proof proceeds by

• observing that it suffices to take the infimum over rational chains of geodesic simplices; this suffices to give the lower bound, with a constant given in terms of volume of an ideal simplex;
• using an explicit construction involving almost-ideal simplices with almost-equidistributed vertices to obtain a upper bound.

Then, since $\tilde{h}$ is a homotopy equivalence (and by looking at this last construction carefully), it—or, rather, its extension to $\overline{\mathbb{H}^3}$—sends regular simplices close to regular ideal simplices. Simplices in the chains from the upper bound construction above are (almost) equidistributed, and the error can be made arbitrarily small as we use the construction to get arbitrarily close to the bound.

Translating the vertices of our ideal simplices, we get a dense set of equilateral triangles that are sent by $\partial\tilde{h}$ to equilateral triangles on the boundary. This implies $\partial\tilde{h}$ is conformal a.e., and hence conformal.

(In short: Gromov’s theorem and its proof allows us to control volume in terms of explicitly-constructed singular chains and to control what happens to simplices in those chains under our homotopy equivalence.)

### Alternative perspective II: dynamically characterizing locally-symmetric Riemannian metrics

Besson, Courtois and Gallot have an entirely different proof, which uses a characterization of locally-symmetric Riemannian metrics in terms of entropy to conclude that, since f is a degree-1 map between locally-symmetric spaces, M and N are homothetic. It then follows from e.g. an equal-volume assumption that M and N would be isometric.

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# Arguments using moduli spaces: some examples

This blogpost started as a response to the question “why moduli spaces?” … but then David Ben-Zvi had a good pithy answer to that in his  Princeton Companion to Mathematics article on moduli spaces : “Moduli spaces can be thought of as geometric solutions to geometric classification problems.”

If the geometry of the moduli space is well-chosen, it can tell us things about the geometry of the spaces / objects which points in it correspond to. Ben-Zvi has a thoroughly concrete and persuasive discussion of this in the case of 1-dimensional real projective space, a.k.a. the moduli space of lines through the origin in the plane.

Below we document, somewhat briefly, a couple more examples of how we can say things properties of geometric objects of a certain type by arguing in a moduli space of all such objects.

### An example from (classical) algebraic geometry

A conic (or conic section) is a curve obtained by the intersection of a cone with a plane; familiar examples, known to the Greeks, include circles, ellipses, and hyperbolae.

Conics in 2-dimensional (complex) projective space $\mathbb{P}^2 = \mathbb{P}(\mathbb{C}^3)$ are parametrized by $\mathbb{P}(\mathrm{Sym}^2 k^3) \cong \mathbb{P}^5$; explicitly, we have the correspondence $[a:b:c:d:e] \leftrightarrow ax^2 + bxy + cy^2 + dyz + ez^2 = 0$.

In other words, $\mathbb{P}^5$ is the moduli space of conics in $\mathbb{P}^2$. We will use this to show that given any five distinct points $(p_1, p_2, p_3, p_4, p_5)$ in $\mathbb{P}^5$ such that no four lie on a (projective) line, there is a unique conic (possibly degenerate) passing through $(p_1, p_2, p_3, p_4, p_5)$.

The conics through any given point in $\mathbb{P}^2$ are parametrized by a hyperplane in the moduli space $\mathbb{P}^5$; conics through 5 distinct points are given by the intersection of five hyperplanes, which is non-empty by considerations of dimension. This already gives us existence, even without the “no four on a line” condition; it remains only to show uniqueness given this additional condition.

Suppose this intersection consists of a single point (in $\mathbb{P}^5$, i.e. a unique conic in $\mathbb{P}^2$.) Then it is clear that we cannot have any four of our $p_i$ on a line $\ell$, for taking the union of this line and a line between the remaining point and any one of the four yields a conic containing the five given points, contradicting uniqueness.

Conversely, suppose the intersection is larger, i.e. it contains a $\mathbb{P}^1$ (a “pencil”) of conics.

We parametrize this pencil by $\{sF + tG | [s:t] \in \mathbb{P}^1\}$ for some $F, G$. Then, for any $q \in \mathbb{P}^2$ on the line containing $p_i, p_j$, $sF(q) + tG(q) = 0$ has a solution (in $s, t$), i.e. there is a conic in our pencil going through $q$. We may verify that any conic in $\mathbb{P}^2$ containing three collinear points is a pair of lines (or a double line); by the pigeonhole principle, at least one of these lines $\ell$ must contain at least three of the $p_i$.

Now if $\ell$ contains exactly three of the $p_i$, then there is a unique conic containing all five $p_i$—the pair of lines consisting of $\ell$ and the line through the remaining two $p_i$. Since by assumption we do not have uniqueness, $\ell$ must contain (at least) four of the $p_i$.

### An example from geometric topology

The genus g Teichmüller space Teich(g) is the space of all complex / conformal structures on a closed Riemann surface of genus g, up to some natural notion of equivalence. Equivalently, via uniformization, it is a space of all constant-curvature Riemannian metrics on a topological surface of genus g.

We will focus on the case of surfaces of genus at least two; for these surfaces, uniformization tells us that the constant-curvature metrics are hyperbolic. There is an analogous theory for genus one which gives us the moduli space of flat metrics on the torus, and many of the same general arguments that appear below can be applied there as well, but the specifics there can have a slightly different flavour, due to the lack of negative curvature (in the objects, that is.)

A point in Teich(g) can be specified by a pair (S, h), where S is a surface with a hyperbolic metric, and $h: \pi_1(F) \to \pi_1(S)$ is a marking, a choice of isomorphism from the fundamental group of a “naked” topological (reference) surface F without a metric and our metrized surface S. To steal Dick Canary’s metaphor: S is the hyperbolic clothing, and h provides instructions for how to wear it.

Technically, a point in Teich(g) is an equivalence class of such pairs, where two marked hyperbolic metrics $(S_1, h_1)$ and $(S_2, h_2)$ are considered equivalent if there is an isometry $j: S_1 \to S_2$ such that $j \circ h_1 \simeq h_2$, i.e. the two maps are homotopic to each other. In terms of the metaphor: jiggling the clothing around a little doesn’t give an essentially different way of wearing it.

We have specified Teich(g) as a set. We can give it a topology in (at least) two ways:

1. By identifying each equivalence class of marked hyperbolic structures $[(S,h)]$ with a holonomy representation $h_*: \pi_1(F) \to \mathrm{Isom}^+(\mathbb{H}^2) \cong \mathrm{PSL}(2,\mathbb{R})$ and thus Teich(g) with a subspace of the representation variety $X_2(F) := \mathrm{Hom}(\pi_1(F), \mathrm{PSL}(2,\mathbb{R})) / \mathrm{PSL}(2,\mathbb{R})$, where the quotient identifies representations that are conjugate in $\mathrm{PSL}(2,\mathbb{R})$—this corresponds in the algebra to the identification of homotopic marked metrics in the geometry.$X_2(F)$ isn’t actually a variety, in the algebraic geometry sense, but it does have natural topology. The induced topology on Teich(g) is called the algebraic topology.
2. By defining a metric on Teich(g), the Teichmüller metric $d_{Teich}$, and giving it the metric topology. $d_{Teich}([(S_1, h_1)], [(S_2, h_2)])$ is defined as (the logarithm of) the minimum quasiconformal distortion over all quasiconformal maps between the two marked hyperbolic structures.(For full definitions see e.g. Chapter 11 of Farb and Margalit’s Primer on Mapping Class Groups.)

A moment’s thought and some fiddling shows that these two approaches produce the same topology.

Intuitively, neighborhoods in this topology correspond to sets of marked hyperbolic metrics which don’t differ from each other too much—the clothing has similar dimensions, and the instructions are relatively similar.

### A space to act on

The mapping class group Mod(F) of a surface (or more generally of any space) F is defined as the group of all isotopy classes of homeomorphisms from to itself. Mod(F) may be thought of as the group of symmetries of a surface. The notation Mod comes by way of analogy with SL(2, Z), which is the mapping class group of the torus—the mapping class group for a surface is sometimes also called the Teichmüller modular group.

The mapping class group of a genus g surface acts naturally on the genus g Teichmüller space by change of marking: $\varphi \cdot [(S,h)] = [(S, \varphi_* h)]$, i.e. by changing the layout of the naked topological surface, it yields effectively different instructions for how to wear the hyperbolic clothing.

It can be shown that this action is properly discontinuous (see e.g. section 12.3 of Farb and Margalit), and isometric if we give Teich(g) the Teichmüller metric. The quotient of Teich(g) is $\mathcal{M}_g$, the moduli space of genus g surfaces—which we discuss slightly more below,

Using this action, we may obtain Thurston’s classification of elements of Mod(S), analogous to classification of elements of SL(2, R) by their action on hyperbolic 2-space: define the translation length $\tau(\phi) := \inf_{\mu \in \mathrm{Teich}(g)} d_{Teich}(\mu, \phi(\mu))$; then

• periodic elements $\phi$ have fixed points in Teichmüller space, and hence have $\tau(\phi) = 0$ and achieve this inf (analogous to elliptic elements of SL(2, R));
• pseudo-Anosov elements $\phi$ have positive translation length, and always achieve this translation length on a Teichmüller geodesic (analogous to hyperbolic elements of SL(2, R));
• reducible elements $\phi$ have zero translation length, but do not achieve it (analogous to parabolics.)

A fuller description of this classification and the work needed to obtain it can be found in Chapter 14 of Farb and Margalit’s Primer.

### A space to explore

It would perhaps been possible to rephrase the above classification and obtain it without the use of Teichmüller space, but the use of Teichmüller space certainly helps illuminate the (geometric) structure of the argument and how it is analogous to the classification of elements of SL(2, R) essentially using hyperbolic geometry.

Similarly, moduli spaces can provide a new light in which to consider / describe possible geometric structures.

For instance, we might ask: how could we imagine or describe the various hyperbolic metrics can we put on a genus g surface?

One possible approach to answering this could involve putting coordinates on the genus g Teichmüller space and attempting to interpret those coordinates in terms of the genus 2 surface. Indeed, we can put a set of (global!) 6– 6 coordinates on Teichmüller space, the Fenchel-Nielsen coordinates, which we obtain by considering a set of 3g – 3 disjoint geodesics on our surface (a pants decomposition, so called because it divides our surface into spheres with three boundary components, i.e.  pairs of pants) and taking length and twist parameters along those geodesics—for a full description see Danny Calegari’s post (linked right above).

The Fenchel-Nielsen coordinates (together with a little hyperbolic geometry) give us a very concrete way of answering our question: to obtain some hyperbolic metric on a genus 2 surface, say, we

1. take two pairs of pants,
2. identify pairs of boundary curves that we will glue to obtain our genus 2 surface,
3. specify how long we want each pair of boundary curves to be—this uniquely determines the hyperbolic metric on each pair of pants—,
4. specify how much we twist the curves in each pair relative to each other when we glue them together.

To obtain a different metric, we change our specifications for the lengths and/or the twist parameters.

Or, jumping up a dimension, we might ask: are there any hyperbolic metrics on closed 3-manifolds that we could describe similarly concretely?

One way to answer this would be to consider a corresponding representation variety $\mathrm{Hom}(\pi_1(M), \mathrm{PSL}(2,\mathbb{C})) // \mathrm{PSL}(2,\mathbb{C})$. Unfortunately, 3-manifold fundamental groups are much more complicated than surface groups, and these representation varieties have been much harder to study (although, using machinery I do not [yet?] understand, they are actually algebraic varieties.)

### Deformation, degeneration, and the totality

Putting the totality of geometric objects together as a moduli space highlights ways of varying the geometry on them which may be less obvious or natural from a non-moduli-space perspective:

• Infinitesimal deformations of a particular geometric structure: e.g. we might ask, how many degrees of freedom are there if we start with a given hyperbolic metric and try to vary the lengths of the geodesics? Can we concretely describe or characterize these degrees of freedom? Barry Mazur has an excellent overview article which talks more about deformations.
• Degeneration of particular aspect/s of our geometric structures—e.g. what happens to our hyperbolic metric if we pick a geodesic loop and shrink its length towards zero, or expand it towards infinity?

Having such a structured totality also allows us to answer with some degree of precision questions such as “how special is such a feature (say, the presence of many short disjoint geodesics) for this type of geometric structure?” With a moduli space in hand, we might be able to answer “it is a high-codimension feature” (so rather special), or perhaps “it is true except in small balls of finite total volume” (so not very special.)

### Aside: coarse and fine moduli; universal properties

The distinction between Teichmüller space Teich(g) and its quotient moduli space $\mathcal{M}_g$, briefly mentioned above, more generally reflects a distinction between fine and coarse moduli spaces. The emphasis on marked hyperbolic metrics may not seem all that natural: we may not care so much about different instructions for how to wear the clothing, but are only interested in essentially different sorts of hyperbolic clothing—which is, untranslating the metaphor, what $\mathcal{M}_g$ is describing.

$\mathcal{M}_g$, though, is not as nice as Teich(g)—for one thing, Teich(g) has the universal property that any continuously-varying family of marked genus g surfaces parametrized by a topological space S is described by a continuous map from S into Teich(g). Indeed, we can put a complex structure on Teich(g), and then we can replace “continuous” above with “complex analytic”.

$\mathcal{M}_g$, however, does not have this property, essentially because any higher-genus surface has non-trivial automorphisms, which kill any hope of obtaining the universal property; to address this problem, we “rigidify” the surface (or, from another perspective, “kill off the automorphisms”) by specifying a marking.

Such a universal property provides one way of specifying, categorically, what a moduli space is, and is one way of more precisely expressing the notion that the moduli space captures the geometry that we are interested in.

For more on this circle of ideas—really quite key to the history of Teichmüller space—, introduced by Teichmüller in his 1944 paper and expanded upon by Grothendieck in a series of lectures at the Séminaire Henri Cartan, see this article of A’Campo, Ji, and Παπαδόπουλος.

### Invariants from moduli space

Moduli spaces also allow for a novel way of defining topological invariants, which, again, Ben-Zvi’s article describes much better than I could:

‘an important application of moduli spaces in geometry and topology is inspired by quantum field theory, where a particle, rather than follow the “best” classical path between two points, follows all paths with varying probabilities. Classically, one calculates many topological invariants by picking a geometric structure (such as a metric) on a space, calculating some quantity using this structure, and finally proving that the result of the calculation did not depend on the structure we chose. The new alternative is to look at all such geometric structures, and integrate some quantity over the space of all choices. The result, if we can show convergence, will manifestly not depend on any choices. String theory has given rise to many important applications of this idea, in particular by giving a rich structure to the collection of integrals obtained in this way. Donaldson and Seiberg-Witten theories use this philosophy to give topological invariants of four-manifolds. Gromov-Witten theory applies it to the topology of symplectic manifolds, and to counting problems in algebraic geometry.’

We remark here that Vassiliev invariants in knot theory can be seen as another example of this approach; this is perhaps clearer from the point of view of the Kontsevich integral, rather than from the combinatorial perspective.

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