# HITCHIN HARMONIC MAPS ARE â€؛ ~andysan â€؛ آ HITCHIN HARMONIC MAPS ARE IMMERSIONS ANDREW

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HITCHIN HARMONIC MAPS ARE IMMERSIONS

ANDREW SANDERS

Abstract. In [Hit92], Hitchin used his theory of Higgs bundles to con-

struct an important family of representations ρ : π1(Σ) → Gr where Σ is a closed, oriented surface of genus at least two, and Gr is the split

real form of a complex adjoint simple Lie group G. These Hitchin rep-

resentations comprise a component of the space of conjugacy classes of

representations of π1(Σ) into G r and are deformations of the irreducible

Fuchsian representations π1(Σ) → SL2R → Gr which uniformize the surface Σ. For any choice of marked complex structure on the surface

and any Hitchin representation, we show that the corresponding equi-

variant harmonic map is an immersion. Pulling back the Riemannian

metric on the symmetric space G/K, we construct a map from the space

of Hitchin representations to the space of isotopy classes of Riemannian

metrics on the surface Σ. As an application of this procedure, we obtain

a new lower bound on the exponential growth rate of orbits in G/K

under the action of the image of a Hitchin representation.

1. Introduction

Equivariant harmonic maps became an important tool to study properties

of representations of fundamental groups of closed manifolds some time af-

ter the foundations of the theory were laid by Eells-Sampson [ES64]. Many

of the more remarkable applications were to rigidity theorems, of which the

work of Toledo [Tol89], Gromov-Schoen [GS92], and Corlette [Cor95] are just

a few important examples, with a primary tool being the Bochner formula for

harmonic maps. At around the same time, the theory of Higgs bundles was

introduced by Hitchin [Hit87] and further developed by Simpson [Sim92],

with important analytic foundations laid by Donaldson [Don87] and Cor-

lette [Cor88], showing that equivariant harmonic maps could be used as a

bridge between representation varieties for fundamental groups of compact,

Kähler manifolds (of which Riemann surfaces are the most well developed,

2010 Mathematics Subject Classification. Primary: 53C42(Immersions), 53C43 (Differ-

ential geometric aspects of harmonic maps), 53A10 (Minimal surfaces), 53C35 (Symmetric

spaces), 37C35 (Orbit growth); Secondary: 28D20 (Entropy and other invariants), 30F60

(Teichmüller theory), .

Key words and phrases. Hitchin representations, Higgs bundles, Harmonic maps, Min-

imal immersions, Entropy.

Sanders gratefully acknowledges partial support from the National Science Foundation

Postdoctoral Research Fellowship 1304006 and from U.S. National Science Foundation

grants DMS 1107452, 1107263, 1107367 ”RNMS: GEometric structures And Representa-

tion varieties” (the GEAR Network).

1

2 ANDREW SANDERS

and rich, examples) and the complex analytic theory of Higgs bundles. This

dictionary came to be known as the non-abelian Hodge correspondence, and

while the theory provides an amazing connection between seemingly dis-

parate subjects, perhaps as a result of this unexpected relationship, it is

very difficult to precisely translate information from one side of the story

to the other. The aim of the present paper is to show that some informa-

tion about the basic properties of equivariant harmonic maps can be rather

easily read off from the data of Higgs bundles.

A harmonic map f : X → M from a Riemann surface to a Riemannian manifold M is an extremizer of the (conformally invariant) Dirichlet energy,

E(f) = 1

2

∫ X ‖df‖2dV.

Just as the theory of harmonic functions in real dimension two is inextricably

linked to the theory of holomorphic functions in complex dimension one,

harmonic maps f : X →M also give rise to holomorphic objects. Precisely, the (1, 0)-part of the differential df : TX → f∗(TM) is a holomorphic section of the canonical bundle K of X twisted by the pull-back of the complexified

tangent bundle f∗(TM)⊗C of M with a suitable complex structure arising from the Levi-Civita connection on M. The theory of Higgs bundles, seen

in this guise, gives an integrability condition: given a holomorphic section φ

of K ⊗ (TM ⊗ C), it describes when this holomorphic section is the (1, 0)- part of the differential of a harmonic map f : X → M. Thus, this allows us to determine properties of the differential of harmonic maps (such as the

rank), by analyzing this associated holomorphic section. We aim to apply

this process in the case of harmonic maps which are equivariant for Hitchin

representations.

In [Hit92], Hitchin discovered a special component of the space of conju-

gacy classes of representations π1(Σ)→ Gr where Σ is a closed, oriented sur- face of genus at least two, and Gr is the split real form of a complex adjoint

simple Lie group G. As a fundamental example consider G = PSLn(C) and Gr = PSLn(R). This discovery was made via choosing a complex structure (a polarization) σ on Σ and identifying a component of the space of poly-stable

G-Higgs bundles on (Σ, σ), which via the non-abelian Hodge correspondence

Hitchin [Hit87] and Simpson [Sim92] had developed earlier, yielded repre-

sentations π1(Σ)→ G. Hitchin used gauge theoretic techniques to conclude that this component of representations consisted of ρ : π1(Σ) → Gr taking values in the split real form of G. Moreover, this component was charac-

terized by the fact that it contained all of the Fuchsian uniformizing repre-

sentations ρ : π1(Σ) → SL2(R) → Gr where the latter arrow indicates the unique, irreducible representation arising from exponentiating the principal

3-dimensional sub-algebra inside the Lie algebra of G. It was originally dis-

covered by Goldman [Gol90] for Gr = SL3(R) (before the paper of Hitchin),

HITCHIN HARMONIC MAPS ARE IMMERSIONS 3

and then later, by Labourie in general, that every such Hitchin representa-

tion was discrete and faithful, with every element ρ(γ) diagonalizable with

distinct real eigenvalues (purely loxodromic).

Most of the progress in understanding the geometry of Hitchin repre-

sentations has since arisen from dynamical and topological methods, most

notably through the important introduction by Labourie of the notion of

an Anosov representation [Lab06]. The purpose of this paper is to indicate

that the theory of Higgs bundles, which was the original genesis of Hitchin

representations, can be used to reveal some of the geometry in this con-

text. In particular, given any Hitchin representation ρ : π1(Σ) → Gr, a foundational result of Corlette [Cor88] implies the unique existence of an

equivariant harmonic map,

f : (Σ̃, σ)→ G/K,

where K ⊂ G is the maximal compact subgroup. Using the Higgs bundle parameterization our main theorem is the following basic property of all

these harmonic maps:

Theorem 1.1. Let G be a complex adjoint simple Lie group and ρ ∈ Hit(G) a Hitchin representation. For any marked complex structure σ on Σ, let

fρ : (Σ̃, σ)→ G/K be the corresponding ρ-equivariant harmonic map. Then fρ is an immersion.

In the case G = PSL2(C), this was observed by Hitchin [Hit87] in his original paper introducing Higgs bundles through an application of a gen-

eral theorem of Schoen-Yau [SY78] about harmonic maps between Riemann

surfaces which are homotopic to diffeomorphisms.

There is an interesting, and not thoroughly explored, relationship be-

tween the immersiveness of equivariant harmonic maps Σ̃ → G/K and the algebra/geometry of the corresponding representations π1(Σ) → G. When G = PSL2(R), one can use the well-developed harmonic map theory to assert that an equivariant harmonic map Σ̃→ H2 is an immersion if and only if the associated representation if Fuchsian. A theorem of Goldman [Gol80] veri-

fies that this is equivalent to the representation being discrete and faithful.

Though we do not analyze the situation in this paper, this line of ideas can

not be carried over without change in the higher rank case. In particular,

when Gr = PSp4(R), Bradlow, Garcia-Prada, and Gothen [BGPG12] have discovered many components of representations π1(Σ) → PSp4(R) which are not Hitchin, but are nonetheless entirely composed of discrete, faithful

representations. It seems very likely that the techniques of the current paper

can be used to show that the corresponding equivariant harmonic maps in

those cases are also immersions. It is unclear in what generality the relation-

ship between discrete and faithful representations and equivariant harmonic

maps being immersions can be pushed, mostly due to the highly transcen-

dental nature of constructing harmonic maps. We state the following as a

question:

4 ANDREW SANDERS

Question: Let ρ : π1(Σ)→ G be a discrete, faithful, reductive represen- tation where G is a complex, semi-simple Lie group. Under what conditions

are the corresponding equivariant harmonic maps immersions?

The penultimate section of the paper puts forward a dynamical appli-

cation of Theorem 1.1. The primary result concerns the study of the ex-

ponential growth rate of orbits in the symmetric space G/K for the group

Γ = ρ(π1(Σ)) acting vi

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