Posted by: matheuscmss | August 31, 2014

## Dynamics of the Weil-Petersson flow: rates of mixing

For the last installment of this series, our goal is to discuss the rates of mixing of the Weil-Petersson (WP) geodesic flow on the unit tangent bundle ${T^1\mathcal{M}_{g,n}}$ of the moduli space ${\mathcal{M}_{g,n}}$ of Riemann surfaces of genus ${g\geq 0}$ with ${n\geq 0}$ punctures for ${3g-3+n\geq 1}$.

However, before entering into the mathematical discussion strictly speaking, let me take the opportunity to dedicate this blog post to the memory of two Russian mathematicians who passed away earlier this month: Dmitri Anosov and Nikolai Chernov. Among their several well-known contributions in Dynamical Systems, we can quote:

Of course, the list of contributions of Anosov and Chernov to Dynamical Systems is vast: each of them wrote more than 90 research articles and books about the features of systems with some hyperbolicity (such as geodesic flows on negatively curved manifolds and chaotic billiards) among other topics.

In particular, it is out of the scope of this post to provide detailed descriptions of the works of these two very influential dynamicists.

On the other hand, as a form of “small compensation”, let me say that the second section of this post (about rates of the WP flow on the modular surface) briefly discusses some of the ideas advanced by these two mathematicians.

Concerning the rates of mixing of the WP flow, let us recall that, by Burns-Masur-Wilkinson theorem (cf. Theorem 1 in the first post of this series), the WP flow ${\varphi_t}$ on ${T^1\mathcal{M}_{g,n}}$ is mixing with respect to the Liouville measure ${\mu}$ whenever ${3g-3+n\geq 1}$.

By definition of the mixing property, this means that the correlation function ${C_t(f,g):=\int f \cdot g\circ\varphi_t d\mu - \left(\int f d\mu\right)\left(\int g d\mu\right)}$ converges to ${0}$ as ${t\rightarrow\infty}$ for any given ${L^2}$-integrable observables ${f}$ and ${g}$. (See, e.g., the section “${L^2}$ formulation” in this Wikipedia article about the mixing property.)

Given this scenario, it is natural to ask how fast the correlation function ${C_t(f,g)}$ converges to zero. In general, the correlation function ${C_t(f,g)}$ can decay to ${0}$ (as a function of ${t\rightarrow\infty}$) in a very slow way depending on the choice of the observables (see, e.g., this blog post of Climenhaga for some concrete examples). Nevertheless, it is often the case (for mixing flows with some hyperbolicity) that the correlation function ${C_t(f,g)}$ decays to ${0}$ with a definite (e.g., polynomial, exponential, etc.) speed when restricting the observables to appropriate spaces of “reasonably smooth” functions.

In other words, given a mixing flow (with some hyperbolicity), it is usually possible to choose appropriate functional (e.g., Hölder, ${C^r}$, Sobolev, etc.) spaces ${X}$ and ${Y}$ such that

• ${|C_t(f,g)|\leq C\|f\|_{X} \|g\|_{Y} t^{-n}}$ for some constants ${C>0}$, ${n\in\mathbb{N}}$ and for all ${t\geq 1}$ (polynomial decay),
• or ${|C_t(f,g)|\leq C\|f\|_{X} \|g\|_{Y} e^{-ct}}$ for some constants ${C>0}$, ${c>0}$ and for all ${t\geq 1}$ (exponential decay).

Evidently, the “precise” rate of mixing of the flow (i.e., the sharp values of the constants ${C>0}$, ${n\in\mathbb{N}}$ and/or ${c>0}$ above) depend on the choice of the functional spaces ${X}$ and ${Y}$ (as they might change if we replace ${C^1}$ observables by ${C^2}$ observables say). On the other hand, the qualitative speed of decay of ${C_t(f,g)}$, that is, the fact that ${C_t(f,g)}$ decays polynomially or exponentially as ${t\rightarrow\infty}$ whenever ${f}$ and ${g}$ are “reasonably smooth”, remains unchanged if we select ${X}$ and ${Y}$ from a well-behaved scale of functional (like ${C^r}$ spaces, ${r\in\mathbb{N}}$, or ${H^s}$ spaces, ${s>0}$). In particular, this partly explains why in the Dynamical Systems literature one simply says that a given mixing flow ${\varphi_t}$ has “polynomial decay” or “exponential decay”: usually we are interested in the qualitative behavior of the correlation function for reasonably smooth observables, but the particular choice of functional spaces ${X}$ and ${Y}$ is normally treated as a “technical detail”.

After this brief description of the notion of rate of mixing (speed of decay of correlation functions), we are ready to state the main result of this post.

Theorem 1 (Burns-Masur-M.-Wilkinson) The rate of mixing of the WP flow ${\varphi_t}$ on ${T^1\mathcal{M}_{g,n}}$ is:

• at most polynomial when ${3g-3+n>1}$;
• rapid (faster than any polynomial) when ${3g-3+n=1}$.

Remark 1 This result was announced as Theorem 2 in the first post of this series and also in this preprint here. Since then, Burns, Masur, Wilkinson and myself found some evidence indicating that the Weil-Petersson geodesic flow on ${T^1\mathcal{M}_{g,n}}$ is actually exponentially mixing when ${3g-3+n=1}$. The details will hopefully appear in the forthcoming paper (currently still in preparation).

Remark 2 An open problem left by Theorem 1 is to determine the rate of mixing of the WP flow on ${T^1\mathcal{M}_{g,n}}$ for ${3g-3+n>1}$. Indeed, while this theorem provides a polynomial upper bound for the rate of mixing in this setting, it does not rule out the possibility that the actual rate of mixing of the WP flow is sub-polynomial (even for reasonably smooth observables). Heuristically speaking, we believe that the sectional curvatures of the WP metric control the time spend by WP geodesics near the boundary of ${\overline{\mathcal{M}}_{g,n}}$. In particular, it seems that the problem of determining the rate of mixing of the WP flow (when ${3g-3+n>1}$) is somewhat related to the issue of finding suitable (polynomial?) bounds for how close to zero the sectional curvatures of the WP metric can be (in terms of the distance to the boundary of ${\overline{\mathcal{M}}_{g,n}}$). Unfortunately, the best available bounds for the sectional curvatures of the WP metric (due to Wolpert) do not rule out the possibility that some of these quantities get extremely close to zero (see Remark 4 of this post here).

The difference in the rates of mixing of the WP flow on ${T^1\mathcal{M}_{g,n}}$ when ${3g-3+n>1}$ or ${3g-3+n=1}$ in Theorem 1 reflects the following simple (yet important) feature of the WP metric near the boundary of the Deligne-Mumford compactification of ${\mathcal{M}_{g,n}}$.

In the case ${3g-3+n=1}$, e.g., ${g=1=n}$, the moduli space ${\mathcal{M}_{1,1}\simeq\mathbb{H}/PSL(2,\mathbb{Z})}$ equipped with the WP metric looks like the surface of revolution of the profile ${\{v=u^3: 0 < u \leq 1\}}$ near the cusp at infinity (see Remark 6 of this post here). In particular, even though a ${\varepsilon}$-neighborhood of the cusp is “polynomially large” (with area ${\sim \varepsilon^4}$), the Gaussian curvature approaches only ${-\infty}$ near the cusp and, as it turns out, this strong negative curvature near the cusp makes that all geodesic not pointing directly towards the cusp actually come back to the compact part in bounded (say ${\leq 1}$) time. In other words, the excursions of infinite WP geodesics on ${\mathcal{M}_{1,1}}$ near the cusp are so quick that the WP flow on ${T^1\mathcal{M}_{1,1}}$ is “close” to a classical Anosov geodesic flow on negatively curved compact surface. In particular, it is not entirely surprising that the WP flow on ${T^1\mathcal{M}_{1,1}}$ is rapid.

On the other hand, in the case ${3g-3+n>1}$, the WP metric on ${\mathcal{M}_{g,n}}$ has some sectional curvatures close to zero near the boundary of the Deligne-Mumford compactification ${\overline{\mathcal{M}}_{g,n}}$ of ${\mathcal{M}_{g,n}}$ (see Theorem 3 and Remark 5 of this post here). By exploiting this feature of the WP metric on ${\mathcal{M}_{g,n}}$ for ${3g-3+n>1}$ (that has no counterpart for ${\mathcal{M}_{1,1}}$ or ${\mathcal{M}_{0,4}}$), we will build a non-neglegible set of WP geodesics spending a long time near the boundary of ${\overline{\mathcal{M}}_{g,n}}$ before eventually getting into the compact part. In this way, we will deduce that the WP flow on ${\mathcal{M}_{g,n}}$ takes a fair (polynomial) amount of time to mix certain parts of the boundary of ${\overline{\mathcal{M}}_{g,n}}$ with fixed compact subsets of ${\mathcal{M}_{g,n}}$.

In the remainder of this post, we will give some details of the proof of Theorem 1. In the next section, we give a fairly complete proof (assuming the results in this previous post, of course) of the polynomial upper bound on the rate of mixing of the WP flow on ${T^1\mathcal{M}_{g,n}}$ when ${3g-3+n>1}$. After that, in the final section, we provide a sketch of the proof of the rapid mixing property of the WP flow on ${T^1\mathcal{M}_{1,1}}$. In fact, we decided (for pedagogical reasons) to explain some key points of the rapid mixing property only in the toy model case of a negatively curved surface with one cusp corresponding exactly to a surface of revolution of a profile ${\{v=u^r\}}$, ${r\geq 2}$. In this way, since the WP metric near the cusp of ${\mathcal{M}_{1,1}\simeq \mathbb{H}/PSL(2,\mathbb{Z})}$ can be thought as a “perturbation” of the surface of revolution of ${\{v=u^3\}}$ (thanks to Wolpert’s asymptotic formulas), the reader hopefully will get a flavor of the main ideas behind the proof of rapid mixing of the WP flow on ${\mathcal{M}_{1,1}}$ without getting into the (somewhat boring) technical details needed to check that the arguments used in the toy model case are “sufficiently robust” so that they can be “carried over” to the “perturbative setting” of the WP flow on ${T^1\mathcal{M}_{1,1}}$.

Posted by: matheuscmss | June 24, 2014

## Dynamics of the Weil-Petersson flow: proof of Burns-Masur-Wilkinson ergodicity criterion II

Last time, we showed the first part of Burns-Masur-Wilkinson ergodicity criterion:

Theorem 1 (Burns-Masur-Wilkinson) Let ${N}$ be the quotient ${N=M/\Gamma}$ of a contractible, negatively curved, possibly incomplete, Riemannian manifold ${M}$ by a subgroup ${\Gamma}$ of isometries of ${M}$ acting freely and properly discontinuously. Denote by ${\overline{N}}$ the metric completion of ${N}$ and ${\partial N:=\overline{N}-N}$ the boundary of ${N}$.Suppose that:

• (I) the universal cover ${M}$ of ${N}$ is geodesically convex, i.e., for every ${p,q\in M}$, there exists an unique geodesic segment in ${M}$ connecting ${p}$ and ${q}$.
• (II) the metric completion ${\overline{N}}$ of ${(N,d)}$ is compact.
• (III) the boundary ${\partial N}$ is volumetrically cusplike, i.e., for some constants ${C>1}$ and ${\nu>0}$, the volume of a ${\rho}$-neighborhood of the boundary satisfies

$\displaystyle \textrm{Vol}(\{x\in N: d(x,\partial N)<\rho\})\leq C \rho^{2+\nu}$

for every ${\rho>0}$.

• (IV) ${N}$ has polynomially controlled curvature, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that the curvature tensor ${R}$ of ${N}$ and its first two derivatives satisfy the following polynomial bound

$\displaystyle \max\{\|R(x)\|,\|\nabla R(x)\|,\|\nabla^2 R(x)\|\}\leq C d(x,\partial N)^{-\beta}$

for every ${x\in N}$.

• (V) ${N}$ has polynomially controlled injectivity radius, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that

$\displaystyle \textrm{inj}(x)\geq (1/C) d(x,\partial N)^{\beta}$

for every ${x\in N}$ (where ${inj(x)}$ denotes the injectivity radius at ${x}$).

• (VI) The first derivative of the geodesic flow ${\varphi_t}$ is polynomially controlled, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that, for every infinite geodesic ${\gamma}$ on ${N}$ and every ${t\in [0,1]}$:

$\displaystyle \|D_{\stackrel{.}{\gamma}(0)}\varphi_t\|\leq C d(\gamma([-t,t]),\partial N)^{\beta}$

Then, the Liouville (volume) measure ${m}$ of ${N}$ is finite, the geodesic flow ${\varphi_t}$ on the unit cotangent bundle ${T^1N}$ of ${N}$ is defined at ${m}$-almost every point for all time ${t}$, and the geodesic flow ${\varphi_t}$ is non-uniformly hyperbolic (in the sense of Pesin’s theory) and ergodic.

Actually, the geodesic flow ${\varphi_t}$ is Bernoulli and, furthermore, its metric entropy ${h(\varphi_t)}$ is positive, finite and ${h(\varphi_t)}$ is given by Pesin’s entropy formula (i.e., it is equal to the sum of positive Lyapunov exponents of ${\varphi_t}$ counted with multiplicities).

More precisely, we proved in the previous post of this series that a geodesic flow ${\varphi_t}$ satisfying the assumptions (II), (III) and (VI) above is non-uniformly hyperbolic with respect to the volume probability measure, and, furthermore, we identified the Oseledets stable and unstable subspaces (cf. the last theorem of this post):

Theorem 2 Under the assumptions (II), (III) and (VI) in Theorem 1 above, the geodesic flow ${\varphi_t}$ is non-uniformly hyperbolic: more concretely, there exists a subset ${\Lambda_0\subset T^1N}$ of full ${m}$-measure such that the ${D\varphi_t}$-invariant splitting

$\displaystyle T_vT^1N=E^s(v)\oplus E^0(v)\oplus E^u(v)$

into the flow direction ${E^0(v)=\mathbb{R}\dot{\varphi}(v)}$ and the spaces ${E^s(v)}$ and ${E^u(v)}$ of stable and unstable Jacobi fields along ${\gamma(t)=\varphi_t(v)}$ have the property that

$\displaystyle 0<\lim\limits_{t\rightarrow\infty}\frac{1}{t}\log\|D_v\varphi_t(\xi^u)\|<\infty \quad \textrm{and} \quad -\infty<\lim\limits_{t\rightarrow\infty}\frac{1}{t}\log\|D_v\varphi_t(\xi^s)\|<0$

for all ${\xi^u\in E^u(v)-\{0\}}$ and ${\xi^s\in E^s(v)-\{0\}}$.

Today, we want to exploit the non-uniform hyperbolicity of ${\varphi_t}$ (and the assumptions (I) to (VI) above) in order to deduce the ergodicity of ${\varphi_t}$ via Hopf’s argument.

For this sake, we organize this post as follows. In the first section, we discuss the geometry of stable and unstable manifolds of ${\varphi_t}$: in particular, we will see that these invariant manifolds form global laminations with useful absolute continuity properties. After that, we describe Hopf’s argument in the second section: from the nice properties of the invariant laminations, we deduce that Birkhoff averages are constant almost everywhere, and, hence, ${\varphi_t}$ is ergodic. Finally, we conclude this post with a remark (inspired by conversations with Y. Coudène and B. Hasselblatt last November 2013) about the deduction of the mixing property for ${\varphi_t}$ from Hopf’s argument.

Posted by: matheuscmss | May 23, 2014

## What is … the Kontsevich-Zorich cocycle?

In this post (with title inspired by the “What is …” column in Notices of the AMS), I would like to record some conversations I had with Jean-Christophe Yoccoz (mostly by the time we wrote our joint paper with David Zmiaikou) about a little technical issue arising when one tries to see the so-called Kontsevich-Zorich cocycle as a linear cocycle (in the usual sense of Dynamical Systems) over the Teichmüller flow (and/or ${SL(2,\mathbb{R})}$-action) on moduli spaces of translation surfaces.

Of course, there are several ways to come around this little technical subtlety (from the dynamical point of view) in the definition of Kontsevich-Zorich cocycle and this is the main purpose of this post. Evidently, the content of this post is well-known (especially among experts), but I hope that this post will benefit the reader with some background in Dynamical Systems wishing to know the answer to the following question:

Does the Kontsevich-Zorich cocycle (as it is classically defined) qualifies as a genuine example of linear cocycle in the usual sense in Dynamical Systems?

Disclaimer. Even though this post benefited from my conversations with Jean-Christophe Yoccoz, all errors and mistakes below are my sole responsibility.

Posted by: matheuscmss | May 12, 2014

## Dynamics of the Weil-Petersson flow: proof of Burns-Masur-Wilkinson ergodicity criterion I

Last time, we reduced the proof of Burns-Masur-Wilkinson theorem on the ergodicity (and mixing) of the Weil-Petersson geodesic flow to a certain estimate for the first derivative of a geodesic flow on negatively curved manifolds (cf. Theorem 11 in this post) and Burns-Masur-Wilkinson ergodicity criterion for geodesic flows on some negatively curved manifolds (cf. Theorem 1 in this post).

The plan for this post is the following. After quickly reviewing in Section 1 below some basic features of the geometry of tangent bundles of Riemannian manifolds, we will estimate the first derivative of geodesic flows on certain negatively curved manifolds in terms its sectional curvatures (as promised last time). Finally, we will complete today’s discussion by proving the first part of Burns-Masur-Wilkinson ergodicity criterion (i.e., we will show that any geodesic flow verifying the assumptions of Burns-Masur-Wilkinson is non-uniformly hyperbolic in the sense of Pesin’s theory), while leaving the second part of Burns-Masur-Wilkinson ergodicity criterion (i.e., the verification of ergodicity via Hopf’s argument) for the next post of this series.

Posted by: matheuscmss | May 12, 2014

## Summer School on Dynamical Systems (2014) and Dynamics and Geometry in Teichmueller Spaces (2015)

It is a pleasure to announce in this short post the following two interesting forthcoming events in Dynamical Systems:

The first event is aimed at graduate students interested in learning some recent topics in Dynamics and also undergraduate students with some background in Dynamical Systems wishing to pursue her/his studies in Dynamics. The details for this event are being uploaded at the summer school webpage and the organizers (Vadim Kaloshin and Yuri Lima) will be happy to provide extra information for all potential participants.

The second event is a research conference around Teichmueller and moduli spaces from both the geometrical and dynamical points of view. The details for this conference are still being defined (as far as I know) and one is encouraged to write to the organizers (Pascal Hubert, Erwan Lanneau and/or Anton Zorich) for more informations.

Posted by: matheuscmss | March 17, 2014

## Holder regularity of solutions of the cohomological equation for interval exchange transformations

Last Wednesday, Jean-Christophe Yoccoz gave a talk (in French) entitled “Regularité holdérienne des solutions de l’équation cohomologique pour les échanges d’intervalle”.

This was the second talk of a new “flat surfaces” seminar organised by himself, Anton Zorich and myself at Instut Henri Poincare (IHP) in Paris. The details about this seminar (such as current schedule, previous and next talks, abstracts, etc.) can be found at this website here.

For the time being, this seminar is an experiment in the sense that IHP allows us to use their rooms from March to June 2014. Of course, if the experiment is a success (i.e., if it manages to gather a non-trivial number of participants interested in flat surfaces and Teichmueller dynamics), then we plan to continue it.

Below the fold, I will reproduce my notes of Jean-Christophe’s talk about a new result together with Stefano Marmi on the cohomological equation for interval exchange transformations of restricted Roth type. Logically, it goes without saying that any errors/mistakes are my entire responsibility.

Posted by: matheuscmss | February 5, 2014

## On the continuity of Lyapunov spectrum for random products

Last week, the conference “Random walks on groups” took place at IHP as part of the activities of a trimester on random walks and asymptotic geometry of groups (organized by Indira Chatterji, Anna Erschler, Vadim Kaimanovich, and Laurent Saloff-Coste) from January to March 2014.

Given the very interesting program of this conference, it was not surprising that Amphithéâtre Hermite (where the talks were delivered) was always full.

Today, we will discuss one of the talks of this conference, namely, the talk “On the continuity of Lyapunov spectrum for random products” of Alex Eskin about his joint work (in preparation) with Artur Avila and Marcelo Viana.

As usual, all mistakes/errors in this post are entirely my responsibility.

Remark 1 A video of a talk of Artur Avila on the same subject can be found here.

Update [February 11, 2014]: Last Friday, I was lucky enough to get some extra explanations concerning “costs of couplings” directly from Alex. At the end of this post (see the “Epilogue”), I will try to briefly summarize what I could understand from this conversation.

Posted by: matheuscmss | December 31, 2013

## Dynamics of the Weil-Petersson flow: basic geometry of the Weil-Petersson metric II

In the first post of this series, we planned to discuss in the third and fourth posts the proof of the following ergodicity criterion for geodesic flows in incomplete negatively curved manifolds of Burns-Masur-Wilkinson:

Theorem 1 (Burns-Masur-Wilkinson) Let ${N}$ be the quotient ${N=M/\Gamma}$ of a contractible, negatively curved, possibly incomplete, Riemannian manifold ${M}$ by a subgroup ${\Gamma}$ of isometries of ${M}$ acting freely and properly discontinuously. Denote by ${\overline{N}}$ the metric completion of ${N}$ and ${\partial N:=\overline{N}-N}$ the boundary of ${N}$.Suppose that:

• (I) the universal cover ${M}$ of ${N}$ is geodesically convex, i.e., for every ${p,q\in M}$, there exists an unique geodesic segment in ${M}$ connecting ${p}$ and ${q}$.
• (II) the metric completion ${\overline{N}}$ of ${(N,d)}$ is compact.
• (III) the boundary ${\partial N}$ is volumetrically cusplike, i.e., for some constants ${C>1}$ and ${\nu>0}$, the volume of a ${\rho}$-neighborhood of the boundary satisfies

$\displaystyle \textrm{Vol}(\{x\in N: d(x,\partial N)<\rho\})\leq C \rho^{2+\nu}$

for every ${\rho>0}$.

• (IV) ${N}$ has polynomially controlled curvature, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that the curvature tensor ${R}$ of ${N}$ and its first two derivatives satisfy the following polynomial bound

$\displaystyle \max\{\|R(x)\|,\|\nabla R(x)\|,\|\nabla^2 R(x)\|\}\leq C d(x,\partial N)^{-\beta}$

for every ${x\in N}$.

• (V) ${N}$ has polynomially controlled injectivity radius, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that

$\displaystyle \textrm{inj}(x)\geq (1/C) d(x,\partial N)^{\beta}$

for every ${x\in N}$ (where ${inj(x)}$ denotes the injectivity radius at ${x}$).

• (VI) The first derivative of the geodesic flow ${\varphi_t}$ is polynomially controlled, i.e., there are constants ${C>1}$ and ${\beta>0}$ such that, for every infinite geodesic ${\gamma}$ on ${N}$ and every ${t\in [0,1]}$:

$\displaystyle \|D_{\stackrel{.}{\gamma}(0)}\varphi_t\|\leq C d(\gamma([-t,t]),\partial N)^{\beta}$

Then, the Liouville (volume) measure ${m}$ of ${N}$ is finite, the geodesic flow ${\varphi_t}$ on the unit cotangent bundle ${T^1N}$ of ${N}$ is defined at ${m}$-almost every point for all time ${t}$, and the geodesic flow ${\varphi_t}$ is non-uniformly hyperbolic (in the sense of Pesin’s theory) and ergodic.

Actually, the geodesic flow ${\varphi_t}$ is Bernoulli and, furthermore, its metric entropy ${h(\varphi_t)}$ is positive, finite and ${h(\varphi_t)}$ is given by Pesin’s entropy formula (i.e., it is equal to the sum of positive Lyapunov exponents of ${\varphi_t}$ counted with multiplicities).

However, since the second post of this series was dedicated to the discussion of items (I), (II) and (III) above for the Weil-Petersson (WP) metric, we think it is natural that this third post provides a discussion of items (IV), (V) and (VI) for the Weil-Petersson metric (thus completing the proof of Burns-Masur-Wilkinson theorem of ergodicity of the Weil-Petersson geodesic flow modulo the proof of their ergodicity criterion).

For this reason, we will continue the discussion of the geometry of the Weil-Petersson metric in this post while leaving the proof of Burns-Masur-Wilkinson ergodicity criterion for the next two posts of this series.

The organization of today’s post is very simple: it is divided in three sections where the items (IV), (V) and (VI) for the Weil-Petersson metric are discussed.

Posted by: matheuscmss | December 23, 2013

## Rates of mixing of Weil-Petersson geodesic flows

Keith Burns, Howard Masur, Amie Wilkinson and I have just upload to ArXiv our research announcement note “Rates of mixing of Weil-Petersson geodesic flows”.

This research announcement has 6 pages and it is divided into two parts:

• a) we present a short (complete) proof of a polynomial upper bound for the rate of mixing of Weil-Petersson (WP) flow on the unit cotangent bundles of moduli spaces $\mathcal{M}_{g,n}$ of surfaces of genus $g\geq 0$ and $n\geq 0$ punctures with $3g-3+n>1$, and
• b) we give a sketch of proof of the rapid (i.e., faster than polynomial) mixing property for the Weil-Petersson (WP) flow on the unit cotangent bundles of moduli spaces of four-times punctered spheres $\mathcal{M}_{0,4}$ and once-punctured torii  $\mathcal{M}_{1,1}$ (that is, in the cases when $3g-3+n=1$).

As we explain in our note, the speed of mixing of the WP flow on the unit cotangent bundle of $\mathcal{M}_{g,n}$ is polynomial at most when $3g-3+n>1$ because the (strata of the) boundary of the moduli spaces $\mathcal{M}_{g,n}$ (in Deligne-Mumford compactification) looks like a non-trivial product of WP metrics on non-trivial moduli spaces of surfaces of lower “complexity” (smaller genus and/or less punctures) when $3g-3+n>1$.

More concretely, using this geometrical information on the WP metric near the boundary of $\mathcal{M}_{g,n}$, one can produce lots of geodesics spending a lot of time near the boundary of the moduli spaces traveling almost-parallel to one of the factors of the products of “lower complexity boundary moduli spaces” (so that the neighborhoods of the moduli spaces take a long time to see the compact parts, and, a fortiori, the rates of mixing between the compact parts and neighborhoods of the boundary are not very fast). Formally, for each $\varepsilon>0$, one can produce a subset of volume $\approx \varepsilon^8$ of vectors leading to WP geodesics traveling in the $\varepsilon$-thin part of $\mathcal{M}_{g,n}$(= $\varepsilon$-neighborhood of the boundary of $\mathcal{M}_{g,n}$) for a time $\geq 1/\varepsilon$. In other words, there is a subset of volume $\approx \varepsilon^8$ of geodesics taking time $1/\varepsilon$ at least to visit the $\varepsilon$-thick part of $\mathcal{M}_{g,n}$ (= complement of the $\varepsilon$-thin part of $\mathcal{M}_{g,n}$). Hence, there are certain non-negligible (volume $\approx\varepsilon^8$) subsets of the $\varepsilon$-thin part of $\mathcal{M}_{g,n}$ taking longer and longer (time $1/\varepsilon$ at least) to mix with the $\varepsilon$-thick part of $\mathcal{M}_{g,n}$ as $\varepsilon\to 0$, so that the rate of mixing of the WP flow can not be very fast (i.e., the rate of mixing is not exponential, and, actually, not even a high degree polynomial).

This argument is not hard to formalize once one dispose of adequate estimates on the geometry of the WP metric near the boundary of $\mathcal{M}_{g,n}$ (for instance, it occupies just 1 page of our note), and we will see it again in this blog in the last post of our series “Dynamics of the Weil-Petersson flow”.

In particular, it is not surprising that we were aware of this argument since 2010: in fact, as far as I can remember, this argument showed up in one of the discussions we had during the intervals of the talks of this conference in honor of Wolpert’s 60th birthday.

On the other hand, this argument breaks down in the case of the moduli spaces $\mathcal{M}_{0,4}$ of four-times punctured spheres and $\mathcal{M}_{1,1}$ once-punctured torii because the boundary of these particular moduli spaces consists of a single-point (and, thus, it is impossible to travel “almost-parallel” to the boundary in these particular cases).

This leads us to item b) above: as we announce in our note, the WP flow on the unit cotangent bundles of the particular moduli spaces $\mathcal{M}_{0,4}$ and $\mathcal{M}_{1,1}$ is actually rapid (faster than any given polynomial) mixing.

Geometrically, this is intuitively explained by the fact that the WP metric near the boundary of $\mathcal{M}_{0,4}$ and $\mathcal{M}_{1,1}$ looks like the metric of the surface of revolution of the profile $\{y=x^3\}$. In particular, one can check that any WP-geodesic not going straight into the cusp (i.e., hitting the single boundary point of these particular moduli spaces) comes back to the $\varepsilon$-thick part in time $\leq 1$. Since the WP metric has uniformly bounded negative curvature in the $\varepsilon$-thick part and the geodesic flows on negatively curved surfaces tend to be exponentially mixing, it is reasonable to expect rapid mixing (and maybe even exponential mixing) in these particular cases.

Nevertheless, the implementation of this idea is technically subtle because the curvature of the WP metric near the boundary point of $\mathcal{M}_{0,4}$ and $\mathcal{M}_{1,1}$ converges to $-\infty$ so that the standard dynamical tools (such as “bounded distortion”) can not be employed directly.

As the reader can imagine, the previous paragraph partly explains why we are taking our time to write down our article.

This being said, the reason why we decided to release this research announcement note is now more or less clear. Indeed, after the 2010 paper of Burns, Masur and Wilkinson on the ergodicity of the WP flow, it is natural to ask about rates of mixing of this flow, and, for instance, several colleagues asked us about this property during a workshop on the WP metric in 2012 at Palo Alto. Here, while the argument for the proof of the result in item a) is extremely simple (and it is written since 2010), we kept (more or less) silent about this project until now only because we thought that the details of item b) could be filled out before the end of 2013 (thus allowing us to upload to ArXiv a more complete paper). Of course, since it is now clear that our initial plan of filling in the details of item b) before Christmas 2013 was too optimistic (on one hand) and it is not reasonable to write a paper just with the simple argument showing item a) (on the other hand), we decided that a research announcement note was the best solution to inform our friends of the results on the rates of mixing of the WP flow that we could get so far.

Closing this short post, let us point out that the end of our note contains a remark that item a) above gives a polynomial upper bound on the rates of mixing of the WP flow but no polynomial lower bound for these rates. Heuristically speaking, we think that one of the difficulties in showing such lower bounds is the absence of precise asymptotic estimates on how the curvatures of the WP metric approach zero (because such estimates would control how close to a product metric is the WP metric near the strata of the boundary of the moduli spaces, and, thus for how long can a geodesic travel almost-parallel to the strata of the boundary of the moduli spaces).

Posted by: matheuscmss | November 28, 2013

## Dynamics of the Weil-Petersson flow: basic geometry of the Weil-Petersson metric I

Today we will define the Weil-Petersson (WP) metric on the cotangent bundle of the moduli spaces of curves and, after that, we will see that the WP metric satisfies the first three items of the ergodicity criterion of Burns-Masur-Wilkinson (stated as Theorem 3 in the previous post).

In particular, this will “reduce” the proof of the Burns-Masur-Wilkinson theorem (of ergodicity of WP flow) to the verification of the last three items of Burns-Masur-Wilkinson ergodicity criterion for the WP metric and the proof of the Burns-Masur-Wilkinson ergodicity criterion itself.

We organize this post as follows. In next section we will quickly review some basic features of the moduli spaces of curves. Then, in the subsequent section, we will start by recalling the relationship between quadratic differentials on Riemann surfaces and the cotangent bundle of the moduli spaces of curves. After that, we will introduce the Weil-Petersson and the Teichmüller metrics. Finally, the last section of this post will concern the verification of the first three items of the Burns-Masur-Wilkinson ergodicity criterion for the WP metric.