manifolds and cobordisms
cobordism theory, Introduction
Definitions
Genera and invariants
Classification
Theorems
cobordism theory = manifolds and cobordisms + stable homotopy theory/higher category theory
Concepts of cobordism theory
homotopy classes of maps to Thom space MO
complex cobordism cohomology theory
flavors of bordism homology theories/cobordism cohomology theories, their representing Thom spectra and cobordism rings:
bordism theoryM(B,f) (B-bordism):
relative bordism theories:
global equivariant bordism theory:
algebraic:
What is known as the Cobordism Hypothesis is, in its most basic form, the conjecture that, roughly, the (∞,n)-category of cobordisms with framing is equivalently the free symmetric monoidal (∞,n)-category with duals on a single object.
The idea is due to Baez & Dolan (1995), who stated it in the further generality of -tuply monoidal -categories (i.e. for ) under the name Tangle Hypothesis (p. 23), before definitions of these higher category theoretical notions were available. The term “cobordism hypothesis” is used by Baez and Dolan only in passing and without definition (p. 25) but was popularized by Lurie (2009) who gave a detailed though still incomplete sketch of a proof by appeal to the notion of -fold complete Segal spaces as a geometric definition of higher categories, thereby breaking the impasse caused by previous attempts (e.g. tac:18-10) which instead tried algebraic definitions of higher categories.
Since a (framed) fully extended topological quantum field theory (TQFT) may be identified with a symmetric monoidal -functor , the cobordism hypothesis implies (and should be implied by, via a higher Yoneda lemma) that fully extended TQFTs are entirely determined by their value on the point (the “n-vector space of states” of the theory), and conversely that fully dualizable objects in equivalently encode such fully extended TQFTs. As such the hypothesis is actually called the “Extended TQFT Hypothesis” by Baez & Dolan (1995) (p. 28).
From the cobordism hypothesis for framing structure it follows fairly readily that the -groupoid of fully dualizable objects in a symmetric monoidal -category carries an action of the orthogonal group and that cobordisms with tangential G-structure for are similarly classified by the homopy fixed loci of the corresponding -action [Lurie (2009, Thm. 2.4.26)]. This yields the statement of the hypothesis for common notions of cobordisms, such as with orientation (SO(n)-structure), spin structure (Spin(n)-structure), etc.
The idea of the cobordism hypothesis is that:
cylinders count as trivial (identity) cobordisms between their boundary components, corresponding to how local spacetime-evolution in a topological quantum field theory is trivial (corresponding to the physics jargon that “their Hamiltonian vanishes”),
so that all the non-trivial information in cobordism and in TQFT-propagation is in the structure of handle body-attachments (in physics known as: “topology change”) which obey the zig-zag rules known from adjunctions, 2-adjunctions and their further higher dimensional analogs,
which means that the point – regarded as a 0-dimensional element of an -dimensional cobordism – is equipped with the structure of, first of all, a dualizable object under the relevant tensor product of cobordisms (which is disjoint union), and next that of a -dualizable object for all .
More generally, the geometric cobordism hypothesis [Grady & Pavlov (2021)] asserts that general non-topological geometric extended functorial field theories (such as conformal field theory as well as the usual relativistic QFT on curved spacetimes) are similarly encoded by (1.) local data depending only on the notion of geometry and (2.) global topological data encoded only in higher dualizability structure.
For example, the FRS-theorem on rational 2d CFT is, at least in spirit, a realization of this idea: It states that rational 2d conformal field theories, when defined on all possible worldsheet surfaces, are fully encoded (1.) by local geometric data encoded in vertex operator algebras or conformal nets or similar and (2.) global purely topological data (a suitable Frobenius algebra object internal to the modular tensor category of representations of that vertex operator algebra/conformal net).
Conversely, the plain (topological) cobordism hypothesis is the special case of the geometric cobordism hypothesis of no geometric structure and in this sense very much captures the idea which in physics is known as “TQFTs are those QFTS without local degrees of freedom”.
In general, where the notion of extended (geometric) functorial field theory is one candidate for a definition of (non-topological) non-perturbative quantum field theory, the (geometric) cobordism hypothesis may be regarded as the first step in establishing the construction of such theories, reducing the problem entirely to the local behaviour of the theory.
Notice that the local behaviour of quantum field theory is still immensely rich: It keeps busy quantum field theorists most of whom never encounter a topological effect in their life. For example the original statement of the Wightman axioms/Haag-Kastler axioms are entirely concerned only with the local behaviour of Lorentzian QFTs (namely on Minkowski spacetime) and yet the construction of any non-trivial examples in dimensions remains wide open.
In this vein, much in the spirit of Freyd’s dictum on the role of categorical algebra in general, one may read the (geometric) cobordism hypothesis as trivializing the trivial part of non-perturbative quantum field theory.
A proof of the geometric cobordism hypothesis is thus the important first step towards the currently wide-open and yet crucial problem of mathematically constructing non-perturbative quantum field theories.
In [Lurie 2009] a sketch of an approach to the formalization and proof of the cobordism hypothesis is described.
For a symmetric monoidal (∞,n)-category with duals write for its core (the maximal ∞-groupoid in ).
For , two symmetric monoidal (∞,n)-categories, write for the (∞,n)-category of symmetric monoidal (∞,n)-functors between them.
Let be the symmetric monoidal (∞,n)-category of cobordisms with n-framing.
Evaluation of any such functor on the point
induces an (∞,n)-functor
such that
this factors through the core of ;
the map
is an equivalence of (∞,n)-categories.
This is (Lurie, theorem 2.4.6).
The proof is based on
the Galatius-Madsen-Tillmann-Weiss theorem, which characterizes the geometric realization in terms of the suspension of the Thom spectrum;
Igusa’s connectivity result which he uses to show that putting “framed Morse functions” on cobordisms doesn’t change their homotopy type (theorem 3.4.7, page 73)
In fact, the Galatius-Madsen-Weiss theorem is now supposed to be a corollary of Lurie’s result.
One of the striking consequences of theorem is that it implies that
Every ∞-groupoid
of fully dualizable objects in a symmetric monoidal (∞,n)-category carries a canonical ∞-action of (the ∞-group structure on the homotopy type of) the orthogonal group , induced by the action of on the n-framing of the point in .
for : the action given by passing to dual objects;
for : the -action of the Serre automorphism.
for : the -action on n-fold loop spaces (see e.g. Gaudens-Menichi 07, section 5) (see also at orthogonal spectra).
(Lurie, examples 2.4.12, 2.4.14. 2.4.15)
The action on a connective spectrum factors through the J-homomorphism
But on homotopy groups the image of J is pure torsion which means that for the induced actions on homotopy groups are all trivial. From this and using the long exact sequence of homotopy groups it follows that the -action itself is trivial.
We discuss the cobordism hypothesis for cobordisms that are equipped with the extra structure of maps into some topological space equipped with a vector bundle. This is the case for which an extended TQFT is (the local refinement of) what has also been called an HQFT.
Let be a topological space and a real vector bundle on of rank . Let be a smooth manifold of dimension . An -structure on consists of the following data
An isomorphism of vector bundles
between the fiberwise direct sum of the tangent bundle with the trivial rank bundle and the pullback of along .
This is (Lurie, notation 2.4.16).
The two extreme cases of def. are the following
For the point and , then an -structure is the same as an -framing, hence
For the classifying space of real vector bundles of rank (the delooping of the ∞-group underlying the orthogonal group) and for the vector bundle associated to the -universal bundle, then -structure on -dimensional manifolds is essentially no-structure (the maximal compact subgroup-inclusion is a weak homotopy equivalence). Cobordisms with this structure will also be called unoriented cobordisms
Accordingly, for the delooping of the special orthogonal group, the corresponding -structure makes oriented manifolds
Generally:
For a topological group mapping via a homomorphism to , then and , the -structure is G-structure.
This we get to below.
Let be a topological space and an -dimensional vector bundle. The (∞,n)-category is defined analogously to but with all manifolds equipped with -structure, def. .
This is (Lurie, def. 2.4.17).
Let be a symmetric monoidal (∞,n)-category with duals, let be a CW-complex, let be an -dimensional vector bundle over equipped with an inner product, and let be the associated O(n)-principal bundle of orthonormal frames in .
There is an equivalence in ∞Grpd
where on the right we regard as a topological space carrying the canonical -action discussed above.
This is (Lurie, theorem. 2.4.18). The following is some aspects of the idea of the proof in (Lurie, p. 57).
In the language of ∞-actions (as discussed there), the space is that of horizontal maps fitting into
where the left map is the classifying map for and the right one is the canonical one out of the homotopy quotient.
Notice that for each point there is an induced inclusion
of the framed cobordisms, def. , into those of -structure, def. , including those cobordisms whose map to is constant on , and observing that for these an -structure is equivalently an -framing. Moreover, by corollary the induced point evaluation is -equivariant, hence yielding a morphism of ∞-groupoids
where denotes the -principal bundle to which is associated.
More generally, this is true for the pullback structure of along along any map , yielding
By the previous comment, observe that is an equivalence for .
Now the codomain of this natural transformation sends (∞,1)-colimits in over to (∞,1)-limits. (Lurie, theorem 3.1.8) shows that the same is true for the domain. Hence is an equivalence for all that appear as (∞,1)-colimits of the point. But this is the case for all ∞-groupoids , by this proposition.
We consider now some special cases of the general definition of local structure-topological field theory
We discuss the special case of the cobordism hypothesis for -cobordisms (def. ) for the case that the vector bundle is the trivial vector bundle .
In this case . Write
Write ∞Grpd for the fundamental ∞-groupoid of .
There is an equivalence in ∞Grpd
This is a special case of the above theorem.
Notice that one can read this as saying that is roughly like the free symmetric monoidal (∞,n)-category on the fundamental ∞-groupoid of (relative to -categories of fully dualizable objects at least).
We discuss the special case of the cobordism hypothesis for -bundles (def. ) for the special case of G-structure (example ), hence for the case that is the classifying space of a topological group.
Let be a topological group equipped with a homomorphism to the orthogonal group. Notice that via the canonical linear representation Vect of on , this induces accordingly a representation of on ..
Let then
be the classifying space for ;
be the corresponding associated vector bundle to the universal principal bundle .
We say
is the -category of cobordisms with -structure.
See (Lurie, notation 2.4.21)
We have
For the trivial group, a -structure is just a framing and so
reproduces the -category of framed cobordisms, def. .
For the special orthogonal group equipped with the canonical embedding a -structure is an orientation
For the orthogonal group itself equipped with the identity map a -structure is no structure at all,
See (Lurie, example 2.4.22).
Then we have the following version of the cobordism hypothesis for manifolds with -structure.
For an ∞-group equipped with a homomorphism to the orthogonal group (regarded as an ∞-group in ∞Grpd), then evaluation on the point induces an equivalence
between extended TQFTs on -dimensional manifolds with G-structure and the ∞-groupoid homotopy invariants of the infinity-action of on (which is induced by the evaluation on the point).
This is (Lurie, theorem 2.4.26).
Hence it remains to see that the right hand side are equivalently the homotopy invariants of the -∞-action. This follows for instance with the discussion at ∞-action, by which
This yields
By the discussion at dependent product
which are the homotopy invariants.
The case that is the identity is at the other extreme of the framed case, and turns out to be similarly fundamental.
For an (∞,1)-topos, write for the (∞,n)-category of correspondences in . For an (∞,n)-category with duals internal to , write for the (∞,n)-category of correspondences over and equipped with the phased tensor product. There is the forgetful monoidal (∞,n)-functor
By the discussion at (∞,n)-category of correspondences these are (∞,n)-categories with duals and the canonical -∞-action on them, corollary , is trivial for . This means that an -homotopy fixed point in is just an object of equipped in turn with an -∞-action. Therefore
Local unoriented-topological field theory
are equivalent to objects equipped with an -∞-action.
At least for ∞Grpd, then given such, the corresponding field theory sends a cobordism to the space of maps
hence
In particular this means that the assignment to the point is again itself.
This is a slight rephrasing of the paragraph pp 58-59 in (Lurie).
At least for ∞Grpd, with an object equipped with an -∞-action, then horizontal lifts in
are equivalent to
This is (Lurie, prop. 3.2.8).
Via the interpretation of local field theories with coefficients in as Local prequantum field theory, the statement of prop. translates in quantum field theory jargon to the statement that “All background structures are fields.” This is essentially the slogan of general covariance.
Let be an abelian ∞-group object, regarded as a (∞,n)-category with duals internal to .
At least if ∞Grpd, then local unoriented-topological field theories of the form
are equivalent to a choice
is equivalent to an ∞-action
Therefore by prop. is equivalent to
Finally, by theorem and in view of remark , this is equivalent to maps of the form
By the discussion at Local prequantum field theory, these statements hold also for fields with moduli spaces in more general -toposes (one sufficient condition is that has an (infinity,1)-site of definition all whose objects are etale contractible).
Some examples are discussed at prequantum field theory in the section Higher Chern-Simons field theory – Levels.
If in def. one chooses for any topological space , and the pullback of the canonical vector bundle bundle on to , then an -functor is similar to what Turaev calls an HQFT over .
There is a vast generalization of the plain -category of cobordisms (with topological structure) considered above given by allowing the cobordisms to be equipped with various types of singularities (Lurie 09, Definition Sketch 4.3.2).
Each type of singularity in dimension now corresponds to a new generator k-morphisms, and the (framed) -category of cobordisms with singularities is now no longer the free symmetric monoidal -category freely generated from just a point (a 0-morphisms), but freely generated from these chosen generators. This general version is (Lurie 09, Theorem 4.3.11).
For instance if the generator on top of the point is a 1-morphism of the form , then this defines a type of codimension -boundary; and hence extended TQFTs with such boundary data and with coefficients in some symmetric monoidal -category with all dual are equivalent to choices of morphisms , where is the fully dualizable object assigned to the point, as before, and now equipped with a morphism from the tensor unit into it. Indeed, this is the usual datum that describes branes in QFT (see for instance at FRS formalism).
For more on this see at QFT with defects.
One important variant of the category of cobordisms is obtained by discarding all those morphisms which have non-empty incoming (say, dually one could use outgoing) boundary component. Then a representation of this category imposes on its values “cups but no caps”, hence only half of the data of a dualizable object in the given degree.
Accordingly, in this case the cobordism hypothesis says that such a functor is given not quite by a fully dualizable object, but by a weaker structure called a Calabi-Yau object (see there for more).
2-dimensional TQFT of this form is known as TCFT, see there for more
A non-topological quantum field theory is a representation of a cobordism category for cobordisms equipped with extra stuff, structure, property that is “not just topological”, meaning roughly not of the form of def. .
The theory for this more general case is not as far developed yet.
steps towards classification of quantum field theories with super-Euclidean structure are discussed at
a general definition of a cobordism category of cobordisms equipped with geometric structure given by a morphism into, roughly, a smooth infinity-groupoid of structure is discussed in (Ayala).
A full-blown geometric cobordism statement is due to Grady & Pavlov 2021.
In particular this means that is itself an -category, i.e. an ∞-groupoid.
When interpreting symmetric monoidal functors from bordisms to as TQFTs this means that TQFTs with given codomain form a space, an ∞-groupoid. In particular, any two of them are either equivalent or have no morphism between them.
According to Chris Schommer-Pries interesting morphisms of TQFTs arise when looking at transformations only on sub-categories on all of . This is described at QFT with defects .
The theorem does say that the invariant attached by an extended TQFT to the point determines all the higher invariants – however it is important to notice that there are strong constraints on what is assigned to the point. For an -dimensional framed theory one needs to assign a fully dualizable object, and the meaning of the term “fully dualizable” depends on , and gets increasingly hard to satisfy as n grows..
For an -dimensional unoriented theory, the object assigned to the point has to be a fixed point for the - action on fully dualizable objects that is obtained from the framed case of the theorem.
In the 1d case, this action on dualizable objects takes every object to its dual, and an fixed point is indeed a vector space with a nondegenerate symmetric inner product.
For an oriented theory -dimensional theory need an -fixed point, which for is nothing but for ends up meaning a Calabi-Yau category (in the case the target 2-category is that of categories).
In fact something more general is true: if one wants a theory that takes values on manifolds equipped with a -structure, for any group mapping to (such as for instance orientation already discussed or its higher versions Spin structure or String structure or Fivebrane structure or …) one needs to assign to the point a -fixed point in dualizable objects in your category (with acting through ).
This beautifully includes all the above plus for example manifolds with maps (up to homotopy) to some auxiliary (connected) space – here we take to be the loop space of (mapping trivially to ), so that a reduction of the structure group of the manifold to involves a map to the delooping .
Such theories are classified by -families of fully dualizable objects.
Notice that there is an important subtlety of Lurie’s theorem in the case of manifolds with -structure which is easy to confuse. The general version of the theorem about TFTs does not say that they are the -fixed points for the -action on fully dualizable objects, but rather they are the homotopy fixed points. This is very important because a homotopy fixed point is not just a property. It is additional structure. Depending on , this additional structure is often encoded in the higher dimensional portion of the field theory.
One can see this in the 1 dimensional case: there is no property of vector spaces which automatically endows them with an inner product, but it is extra structure.
duality between algebra and geometry
in physics:
The original hypothesis is formulated in
A sketch of an approach to the formalization and proof of the cobordism hypothesis is described in:
The comparatively simple case of is spelled out in detail in
and aspects of the case (see also at 2d TQFT) are further discussed in
Lecture notes and reviews on the topic of the cobordisms hypothesis:
Jacob Lurie, TQFT and the Cobordism Hypothesis (video, notes)
Jacob Lurie, The Classification of Extended Topological Field Theories (video)
Julie Bergner, UC Riverside Seminar on Cobordism and Topological Field Theories (2009).
Julie Bergner, Models for -Categories and the Cobordism Hypothesis , in Hisham Sati, Urs Schreiber (eds.) Mathematical Foundations of Quantum Field and Perturbative String Theory, AMS 2011
Daniel Freed, The cobordism hypothesis, Bulletin of the American Mathematical Society 50 (2013), pp. 57-92, (arXiv:1210.5100, doi:10.1090/S0273-0979-2012-01393-9)
Chris Schommer-Pries, Dualizability in Low-Dimensional Higher Category Theory (arXiv:1308.3574)
Constantin Teleman, Five lectures on topological field theory, 2014 (pdf)
An approach to the proof of the cobordism hypothesis via factorization homology is in
Discussion of the canonical -action on n-fold loop spaces (which may be thought of as a special case of the cobordism hypothesis) includes
Higher cobordisms with geometric structure (i.e. for non-topological extended quantum field theory) are discussed in
David Ayala, Geometric cobordism categories (arXiv:0811.2280)
Daniel Grady, Dmitri Pavlov, Extended field theories are local and have classifying spaces [arXiv:2011.01208]
and full proof of the cobordism hypothesis in this geometric generality (hence for non-topological extended FQFT such as conformal field theory) is claimed in:
based on Grady & Pavlov 2020.
Review:
Last revised on May 16, 2024 at 23:53:04. See the history of this page for a list of all contributions to it.