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A Survey of Elliptic Cohomology - the derived moduli stack of derived elliptic curves

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Abstract We sketch some basic ideas (Jacob Lurie’s ideas, that is) about higher geometry motivated from the higher version of the moduli stack of elliptic curves: the derived moduli stack of derived elliptic curves. We survey aspects of the theory of generalized schemes and then sketch how the derived moduli stack of derived elliptic curves is an example of a generalized scheme modeled on the formal dual of E-∞ rings.

This is a sub-entry of

see there for background and context.

For fully appreciating the details of the main theorem here the material discussed in the previous sessions (and a little bit more) is necessary, but our exposition of generalized schemes is meant to be relatively self-contained (albeit necessarily superficial).

This are the entries on the previous sessions:


the derived moduli stack of derived elliptic curves

Motivation and Statement

In the context of elliptic cohomology one assigns to every elliptic curve ϕ\phi over a ring RR a cohomology theory represented by an E-∞ ring spectrum E ϕE_\phi.

Since, by definition, we may identify the elliptic curve ϕ\phi over RR with a patch ϕ:SpecR 1,1\phi : Spec R \to \mathcal{M}_{1,1} of the moduli stack 1,1\mathcal{M}_{1,1} of elliptic curves, this assignment

𝒪 Der:ϕE ϕ \mathcal{O}^{Der} : \phi \mapsto E_\phi

looks like an E-∞ ring valued structure sheaf on 1,1\mathcal{M}_{1,1}.

There is a very general theory of higher geometry for generalized spaces with generalized structure sheaves. Using this one may regard the pair

( 1,1,𝒪 Der) (\mathcal{M}_{1,1}, \mathcal{O}^{Der})

as a structured space that is a “derived” Deligne-Mumford stack.

The central theorem about elliptic cohomology of Jacob Lurie, refining the Goerss-Hopkins-Miller theorem says that

the central theorem, first version

The moduli stack 1,1\mathcal{M}_{1,1} of elliptic curves equipped with the E-∞ ring-valued structure sheaf 𝒪 Der\mathcal{O}^{Der} may be regarded as the derived moduli stack of derived elliptic curves in that for any E-∞ ring RR the space of derived stack morphisms

(SpecR,R)( 1,1,𝒪 Der) (Spec R, R) \to (\mathcal{M}_{1,1}, \mathcal{O}^{Der})

is equivalent to the space of derived elliptic curves over RR.

After we have looked at some concepts in higher geometry a bit more closely below, we will restate this in slightly nicer fashion.

References

A sketch of what this theorem means and how it is proven is part of the content of

and goes back to Jacob Lurie’s PhD thesis (listed here).

The general theory for the context of higher geometry invoked here has later been spelled out in

but the special case of the general theory that is needed here, where the coefficient objects of structure sheaves are E-∞ rings has only been announced as

and isn’t available yet. On the other hand, the general theory of E-∞ rings themselves, in the (∞,1)-category theory context needed here, is developed in

Notions of Space

The statement that we are after really lives in the context of higher geometry (often called “derived geometry”). Here is an outline of the central aspects.

The central ingredient which we choose at the beginning to get a theory of higher geometry going is an (∞,1)-category 𝒢\mathcal{G} whose objects we think of as model spaces : the simplest objects exhibiting the geometric structures that we mean to consider.

Examples for categories of model spaces

  • with smooth structure

    • 𝒢=\mathcal{G} = Diff, the category of smooth manifolds;

    • 𝒢=𝕃\mathcal{G} = \mathbb{L}, the category of smooth loci;

  • without smooth structure

    • 𝒢=(CRing fin) op\mathcal{G} = (C Ring^{fin})^{op}, the formal dual of CRing: the category of (finitely generated) algebraic affine schemes;

    • 𝒢=(sCRing fin) op\mathcal{G} = (sC Ring^{fin})^{op}, the formal dual of simplicial objects in CRing;

    • 𝒢=(E Ring fin) op\mathcal{G} = (E_\infty Ring^{fin})^{op}, the formal dual of E-∞ rings: the category of (finitely generated) algebraic derived affine schemes.

These (∞,1)-categories 𝒢\mathcal{G} are naturally equipped with the structure of a site (and a bit more, which we won’t make explicit for the present purpose). Following Jacob Lurie we call such a 𝒢\mathcal{G} a geometry .

We want to be talking about generalized spaces modeled on the objects of 𝒢\mathcal{G}. There is a hierarchy of notions of what that may mean:

Hierarchy of generalized spaces modeled on 𝒢\mathcal{G}

𝒢 Spec 𝒢 Sch(𝒢) Top(𝒢) op Sh (,1)(Pro(𝒢)) modelspaces spaceslocallylikemodelspaces concretespacescoprobeablebymodelspaces spacesprobeablebymodelspaces affine𝒢schemes 𝒢schemes 𝒢structured(,1)toposes stackson𝒢 tamebutrestrictive versatilebutpossiblywild \array{ \mathcal{G} &\stackrel{Spec^{\mathcal{G}}}{\hookrightarrow}& Sch(\mathcal{G}) &\hookrightarrow& \mathcal{L}Top(\mathcal{G})^{op} &\hookrightarrow& Sh_{(\infty,1)}(Pro(\mathcal{G})) \\ \\ model spaces && spaces locally like model spaces && concrete spaces coprobeable by model spaces && spaces probeable by model spaces \\ \\ affine\;\mathcal{G}-schemes && \mathcal{G}-schemes && \mathcal{G}-structured\;(\infty,1)-toposes && \infty-stacks\;on\;\mathcal{G} \\ \stackrel{tame\;but\;restrictive}{\leftarrow} & &&&& & \stackrel{versatile\;but\;possibly\;wild}{\to} }

We explain what this means from right to left.

spaces probeable by model spaces: \infty-stacks

An object XX probeable by objects of 𝒢\mathcal{G} should come with an assignment

X:(U𝒢)(X(U)Grpd) X : (U \in \mathcal{G}) \mapsto (X(U) \in \infty Grpd)

of an ∞-groupoid of possible ways to probe XX by UU, for each possible UU, natural in UU. More precisely, this should define an object in the (∞,1)-category of (∞,1)-presheaves on 𝒢\mathcal{G}

XPSh(𝒢)=Funct(𝒢 op,Grpd) X \in PSh(\mathcal{G}) = Funct(\mathcal{G}^{op}, \infty Grpd)

But for XX to be consistently probeable it must be true that probes by UU can be reconstructed from overlapping probes of pieces of UU, as seen by the topology of 𝒢\mathcal{G}. More precisely, this should mean that the (∞,1)-presheaf XX is actually an object in an (∞,1)-category of (∞,1)-sheaves on 𝒢\mathcal{G}

XSh(𝒢)PSh(𝒢). X \in Sh(\mathcal{G}) \stackrel{}{\hookrightarrow} PSh(\mathcal{G}) \,.

Such objects are called ∞-stacks on 𝒢\mathcal{G}. The (∞,1)-category Sh(𝒢)Sh(\mathcal{G}) is called an ∞-stack (∞,1)-topos.

A supposedly pedagogical discussion of the general philosophy of ∞-stacks as probebable spaces is at motivation for sheaves, cohomology and higher stacks.

The ∞-stacks on 𝒢\mathcal{G} that are used in the following are those that satisfy descent on Čech covers. But we will see (∞,1)-toposes of ∞-stacks that may satisfy different descent conditions, in particular with respect to hypercovers. Every ∞-stack (∞,1)-topos has a hypercompletion to one of this form.

For concretely working with hypercomplete (∞,1)-toposes it is often useful to use models for ∞-stack (∞,1)-toposes in terms of the model structure on simplicial presheaves.

Sh (,1) hc(C) lex PSh (,1)(C) abstract nonsense def of (∞,1)-topos Lurie's theorem ([C op,SSet] loc) Bousfieldloc. ([C op,SSet] glob) model category of simplicial presheaves \array{ Sh^{hc}_{(\infty,1)}(C) &\stackrel{\stackrel{\;\;\;\;\;lex\;\;\;\;\;\;}{\leftarrow}} {\hookrightarrow}& PSh_{(\infty,1)}(C) && \text{abstract nonsense def of (∞,1)-topos} \\ \uparrow^{\simeq} && \uparrow^{\simeq} && \text{Lurie's theorem} \\ ([C^{op}, SSet]_{loc})^\circ &\stackrel{\stackrel{Bousfield\;loc.}{\leftarrow}}{\to}& ([C^{op}, SSet]_{glob})^\circ && \text{model category of simplicial presheaves} }
Warning

This discussion here is glossing over all set-theoretic size issues. See StSp, warning 2.4.5.

concrete spaces co-probeable by model spaces: structured (,1)(\infty,1)-toposes

Spaces probeable by 𝒢\mathcal{G} in the above sense can be very general. They need not even have a concrete underlying space , even for general definitions of what that might mean.

(Counter-)Example For 𝒢=\mathcal{G} = Diff, for every nn \in \mathbb{N} we have the ∞-stack Ω cl n()\Omega_{cl}^n(-) (which happens to be an ordinary sheaf) that assigns to each manifold UU the set of closed n-forms on UU. This is important as a generalized space: it is something like the rational version of the Eilenberg-MacLane space K(,n)K(\mathbb{Z}, n). But at the same time this is a “wild” space that has exotic properties: for instance for n=3n=3 this space has just a single point, just a single curve in it, just a single surface in it, but has many nontrivial probes by 3-dimensional manifolds.

In the classical theory for instance of ringed spaces or diffeological spaces a concrete underlying space is taken to be a topological space. But this in turn is a bit too restrictive for general purposes: a topological space is the same as a localic topos: a category of sheaves on a category of open subsets of a topological space. The obvious generalization of this to higher geometry is: an n-localic (∞,1)-topos 𝒳\mathcal{X}.

This makes us want to say and make precise the statement that

An concrete ∞-stack XX is one which has an underlying (∞,1)-topos 𝒳\mathcal{X}:

the collection of UU-probes of XX is a subobject of the collection of (∞,1)-topos-morphisms from UU to 𝒳\mathcal{X}:

X(U)Top(𝒢) op(Sh (U),𝒳) X(U) \subset \mathcal{L}Top(\mathcal{G})^{op}(Sh_{\infty}(U),\mathcal{X})

We think of 𝒳\mathcal{X} as the (∞,1)-topos of ∞-stacks on a category of open subsets of a would-be space XX, only that this would be space XX might not have an independent existence as a space apart from 𝒳\mathcal{X}. The available entity closest to it is the terminal object * 𝒳𝒳{*}_{\mathcal{X}} \in \mathcal{X}.

To say that 𝒳\mathcal{X} is modeled on 𝒢\mathcal{G} means that among all the ∞-stacks on the would-be space a structure sheaf of functions with values in objects of 𝒢\mathcal{G} is singled out: for each object V𝒢V \in \mathcal{G} there is a structure sheaf 𝒪(,V)𝒳\mathcal{O}(-,V) \in \mathcal{X}, naturally in VV.

This yields an (∞,1)-functor

𝒪:𝒢𝒳. \mathcal{O} : \mathcal{G} \to \mathcal{X} \,.

We think of XX as being a concrete space co-probebale by 𝒢\mathcal{G} (we can map from the concrete XX into objects of 𝒢\mathcal{G}).

Such an 𝒪\mathcal{O} is a consistent collection of coprobes if coprobes with values in VV may be reconstructed from co-probes with values in pieces of VV.

More precisely:

Definition

(𝒢\mathcal{G}-structure, StSp, def. 1.2.8)

An (∞,1)-functor 𝒪:𝒢𝒳\mathcal{O} : \mathcal{G} \to \mathcal{X} is a 𝒢\mathcal{G}-valued structure sheaf on the (∞,1)-topos if

A pair (𝒳,𝒪)(\mathcal{X}, \mathcal{O}) of an (∞,1)-topos 𝒳\mathcal{X} equipped with 𝒢\mathcal{G}-valued structure sheaf 𝒪:𝒢𝒳\mathcal{O} : \mathcal{G} \to \mathcal{X} we call a structured (∞,1)-topos.

In summary:

A concrete ∞-stack XX modeled on 𝒢\mathcal{G} is

  • an (∞,1)-topos 𝒳\mathcal{X} (“of \infty-stacks on XX”)

  • equipped with a 𝒢\mathcal{G}-valued structure sheaf 𝒪\mathcal{O} in the form of a finite limits and cover preserving functor 𝒪:𝒢𝒳\mathcal{O} : \mathcal{G} \to \mathcal{X}.

The fundamental example for structured (∞,1)-toposes are provided by the objects of 𝒢\mathcal{G} themselves, which are canonically equipped with a 𝒢\mathcal{G}-structure as follows.

Theorem

(StSp, thm. 2.1.1)

Let f:𝒢𝒢f : \mathcal{G} \to \mathcal{G}' be a morphism of geometries, then the obvious (∞,1)-functor f *:Top(𝒢)Top(𝒢)f^* : \mathcal{L}Top(\mathcal{G}) \to \mathcal{L}Top(\mathcal{G}') admits a left adjoint

f *:Top(𝒢)Top(𝒢):Spec 𝒢 𝒢 f^* : \mathcal{L}Top(\mathcal{G}') \stackrel{\leftarrow}{\to} \mathcal{L}Top(\mathcal{G}) : Spec_{\mathcal{G}}^{\mathcal{G}'}

called the relative spectrum functor.

For 𝒢\mathcal{G} any geometry, write 𝒢 disc\mathcal{G}_{disc} for the geometry obtained from this by forgetting its Grothendieck topology and instead using the discrete topology where only equivalences cover.

Notice that we may identify 𝒢 disc\mathcal{G}_{disc}-structures on the archetypical (∞,1)-topos ∞Grpd, being finite limit-preserving functors 𝒢 disc opGrpd\mathcal{G}_{disc}^{op} \to \infty Grpd with ind-objects in 𝒢 op\mathcal{G}^{op}, hence with the opposite of pro-objects in 𝒢\mathcal{G}. This gives a canonical inclusion

Pro(𝒢)Top(𝒢) op. Pro(\mathcal{G}) \hookrightarrow \mathcal{L}Top(\mathcal{G})^{op} \,.
Definition

(StSp, def. 2.1.2)

The composite (∞,1)-functor

Spec 𝒢:Pro(𝒢) opTop(𝒢 disc)Spec 𝒢 𝒢 discTop(𝒢) Spec^{\mathcal{G}} : Pro(\mathcal{G})^{op} \hookrightarrow \mathcal{L}Top(\mathcal{G}_{disc}) \stackrel{Spec_{\mathcal{G}}^{\mathcal{G}_{disc}}}{\to} \mathcal{L}Top(\mathcal{G})

we call the absolute spectrum functor

This abstract nonsense is reassuring, but we want a more concrete definition of what such Spec 𝒢USpec^{\mathcal{G}} U is like:

Definition

(StSp, def. 2.2.9)

For every U𝒢U \in \mathcal{G} there is naturally induced a topology on the over category Pro(𝒢)/UPro(\mathcal{G})/U. Define the (∞,1)-topos

SpecU:=Sh (,1)(Pro(𝒢)/U), Spec U := Sh_{(\infty,1)}(Pro(\mathcal{G})/U) \,,

naturally to be thought of as the (∞,1)-topos of ∞-stacks on UU .

This is canonically equipped with a (∞,1)-functor

𝒪 SpecX:𝒢SpecX. \mathcal{O}_{Spec X} : \mathcal{G} \to Spec X \,.

And this is indeed the concrete underlying space produced by the absolute spectrum functor:

Theorem

StSp, prop. 2.2.11, thm. 2.2.12)

For every U𝒢U \in \mathcal{G} the pair (SpecU,𝒪 SpecU)(Spec U, \mathcal{O}_{Spec U}) is indeed a structured (∞,1)-topos and is indeed equivalent to the Spec 𝒢USpec^{\mathcal{G}} U defined more abstractly above.

Example For 𝒢=(CRing fin) op\mathcal{G} = (C Ring^{fin})^{op} with the standard topology we have that 0-localic 𝒢\mathcal{G}-structured spaces are locally ringed spaces , while 𝒢 disc\mathcal{G}_{disc}-structured 0-localic spaces are just arbitrary ringed spaces. Applying the above machinery to this situaton gives a spectrum functor that takes a ring RR first to the ringed space (*,R)({*,R}) and this then to the locally ringed space (SpecR,R)(Spec R, R).

Spaces locally like model spaces: generalized schemes

We have seen that 𝒢\mathcal{G}-structured (∞,1)-toposes are those general spaces modeled on 𝒢\mathcal{G} that are well-behaved in that at least they do have an “underlying topological structure” in the form of an underlying (∞,1)-topos. But such concrete spaces may still be very different from the model objects in 𝒢\mathcal{G}.

In parts this is desireable: many objects that one would naturally build out of the objects in 𝒢\mathcal{G}, such as mapping spaces [Σ,X][\Sigma,X], are much more general than objects in 𝒢\mathcal{G} but do live happily in Top(𝒢) op\mathcal{L}Top(\mathcal{G})^{op}.

But in many situations one would like to regard 𝒢\mathcal{G}-structured (∞,1)-toposes that are not globally but locally equivalent to objects in 𝒢\mathcal{G}. This is supposed to be captured by the following definition.

Definition

StSp, def. 2.3.9

A structured (∞,1)-topos (𝒳,𝒪)(\mathcal{X}, \mathcal{O}) is a 𝒢\mathcal{G}-generalized scheme if

  • there exists a collection {V i𝒳}\{V_i \in \mathcal{X}\}

  • such that

    • this covers 𝒳\mathcal{X} in that the canonical morphism

      ( iV i)* 𝒳 (\coprod_i V_i) \to {*}_{\mathcal{X}}

      to the terminal object in 𝒳\mathcal{X} is an effective epimorphism

    • the structured (∞,1)-toposes
      (𝒳/V i,𝒪| V i)(\mathcal{X}/V_i, \mathcal{O}|_{V_i}) induced by the V iV_i are model spaces in that there exists {U i𝒢}\{U_i \in \mathcal{G}\} and equivalences

      (𝒳/V i,𝒪| V i)Spec 𝒢U i (\mathcal{X}/V_i, \mathcal{O}|_{V_i}) \simeq Spec^{\mathcal{G}} U_i

Examples

warning the following statements really pertain to pregeometries, not geometries. for the moment this here is glossing over the difference between the two. See geometry (for structured (∞,1)-toposes) for the details.

The derived moduli space of elliptic curves

With the above machinery for higher geometry in hand, we now set out to describe the particular application that we are interested in: the study of the derived moduli stack of derived elliptic curves.

Derived elliptic curves

AE(A) A \mapsto E(A)

The derived moduli stack

Lurie’s discussion of the derived moduli stack ( 1,1,𝒪 Der)(\mathcal{M}_{1,1}, \mathcal{O}^{Der}) is more than a re-interpretation of the Goerss-Miller-Hopkins theorem. It is in particular a re-derivation of this result, from the following perspective

the central statement, conceptually

Input We have the (E Ring) op(E_\infty Ring)^{op}-probeable space

(E:R{derivedellipticcurvesoverR})Sh(E Ring op). (E : R \mapsto \{derived\;elliptic\;curves\;over\;R\}) \in Sh(E_\infty Ring^{op}) \,.

Question: Does this happen to even be a E Ring opE_\infty Ring^{op}-generalized scheme?

Answer Yes. It is actually a derived Deligne-Mumford stack.

Let 1,1\mathcal{M}_{1,1} be the ordinary moduli stack of elliptic curves.

Using constructions in elliptic cohomology we may associate to each elliptic curve over RR, i.e. each morphism ϕ:SpecR𝒳\phi : Spec R \to \mathcal{X}, an E-infinity ring E ϕE_\phi – the multiplicative spectrum that represents the elliptic cohomology theory given by TT.

This gives an E E_\infty-ring valued structure sheaf

𝒪 Der:(ϕ:SpecR𝒳)E ϕ. \mathcal{O}^{Der} : (\phi : Spec R \to \mathcal{X}) \mapsto E_\phi \,.

Question What, if anything, is this derived stack a derived moduli stack of?

the classification of derived elliptic curves

The big theorem is that the derived space (𝒳,𝒪 Der)(\mathcal{X}, \mathcal{O}^{Der}) classifies derived elliptic curves over E E_\infty-rings

This is the theorem that we said above we wanted to consider, stated now a little bit more precisely.

Theorem

(J. Lurie)

For

we have an equivalence

Hom(SpecA,(𝒳,𝒪 Der))E(A) Hom(Spec A, (\mathcal{X}, \mathcal{O}^{Der})) \simeq E(A)

naturally in AA.

Proof

Jacob Lurie writes that the proof proceeds alonmg these steps. Details will be discussed in the next session.

  1. consider the presheaf of preoriented ellitptic curves E(A)E'(A) first

  2. observe that this restricted to ordinary rings produces the ordinary moduli stack

  3. notice that every oo-stack with good deformation theory that restricts this way is a derived Deligne-Mumford stack (𝒳,𝒪)(\mathcal{X}, \mathcal{O}') that assigns connective E E_\infty-rings over affines

  4. let ω\omega be the line bundle on 1,1\mathcal{M}_{1,1} regarded as a coheren sheaf. There is then from the preorientation of the universal curve over (,𝒪)(\mathcal{M}, \mathcal{O}') a morphism

    β:ωπ 2𝒪 \beta : \omega \to \pi_2 \mathcal{O}'
  5. let 𝒪\mathcal{O} be the sheaf obtained from 𝒪\mathcal{O}' by inverting β\beta

  6. show that

    1. for n=2kn = 2 k we have an isomorphism ω kπ 2k𝒪\omega^k \to \pi_{2 k}\mathcal{O}

    2. for n=2k+1n = 2 k + 1 we have an isomorphism 0π 2k+1𝒪0 \to \pi_{2k +1}\mathcal{O}

      strategy: reduce to neighbourhood of a point

  7. notice that this implies the desired statement

Revised on November 6, 2013 02:07:28 by Urs Schreiber (77.251.114.72)