This entry is about the isomorphisms in cohomology induced by Thom classes. For the Pontrjagin-Thom isomorphism in cobordism theory see at Thom's theorem.
manifolds and cobordisms
cobordism theory, Introduction
group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
For $H$ denoting ordinary cohomology with coefficients in some ring, and $V \to X$ a vector bundle of rank $n$ over a simply connected CW-complex, the Thom isomorphism is the morphism
from the cohomology of $X$ to the reduced cohomology of the Thom space $Th(V)$, given by pullback to the Thom space followed by cup product with a Thom class $c \in H^n(Th(V))$.
That this is indeed an isomorphism follows via the Leray-Hirsch theorem (see e.g. Ebert 12, 2.3,2.4) or from running a Serre spectral sequence (e.g. Kochman 96, section 2.6).
In the special case that the vector bundle is trivial of rank $n$, then its Thom space coincides with the $n$-fold suspension of the base space (exmpl.) and the Thom isomorphism coincides with the suspension isomorphism. In this sense the Thom isomorphism may be regarded as a twisted suspension isomorphism.
More generally, for $E$ a generalized multiplicative cohomology theory, and $V \to X$ a vector bundle of rank $n$, which is $E$-orientable, there is a generalization to a Thom-Dold isomorphism
(e.g. Rudyak 98, chapter V, 1.3, Kochman 96, prop. 4.3.6)
One may think of the Thom isomorphism from left to right as cupping with a generalized volume form on the fibers, and from right to left as performing fiber integration against this volume form.
Let $V \to B$ be a topological vector bundle of rank $n \gt 0$ over a simply connected CW-complex $B$. Let $R$ be a commutative ring.
There exists an element $c \in H^n(Th(V);R)$ (in the ordinary cohomology, with coefficients in $R$, of the Thom space of $V$, called a Thom class) such that forming the cup product with $c$ induces an isomorphism
of degree $n$ from the unreduced cohomology group of $B$ to the reduced cohomology of the Thom space of $V$.
(of Thom isomorphism via fiberwise Thom spaces)
Choose an orthogonal structure on $V$. Consider the fiberwise cofiber
of the inclusion of the unit sphere bundle into the unit disk bundle of $V$ (def.).
Observe that this has the following properties
$E \overset{p}{\to} B$ is an n-sphere fiber bundle (spherical fibration), hence in particular a Serre fibration;
the Thom space $Th(V)\simeq E/B$ is the quotient of $E$ by the base space, because of the pasting law applied to the following pasting diagram of pushout squares:
hence the reduced cohomology of the Thom space is (def.) the relative cohomology of $E$ relative to $B$:
$E \overset{p}{\to} B$ has a global section $B \overset{s}{\to} E$ (given over any point $b \in B$ by the class of any point in the fiber of $S(V) \to B$ over $b$; or abstractly: induced via the above pushout by the commutation of the projections from $D(V)$ and from $S(V)$, respectively).
In the following we write $H^\bullet(-)\coloneqq H^\bullet(-;R)$, for short.
By the first point, there is the Thom-Gysin sequence, an exact sequence running vertically in the following diagram
By the second point above this is split, as shown by the diagonal isomorphism in the top right. By the third point above there is the horizontal exact sequence, as shown, which is the exact sequence in relative cohomology $\cdots \to H^\bullet(E,B) \to H^\bullet(E) \to H^\bullet(B) \to \cdots$ induced from the section $B \hookrightarrow E$.
Hence using the splitting to decompose the term in the middle as a direct sum, and then using horizontal and vertical exactness at that term yields
and hence an isomorphism
To see that this is the inverse of a morphism of the form $c \cup (-)$, inspect the proof of the Gysin sequence. This shows that $H^{\bullet-n}(B)$ here is identified with elements that on the second page of the corresponding Serre spectral sequence are cup products
with $\iota$ fiberwise the canonical class $1 \in H^n(S^n)$ and with $b \in H^\bullet(B)$ any element. Since $H^\bullet(-;R)$ is a multiplicative cohomology theory (because the coefficients form a ring $R$), cup producs are preserved as one passes to the $E_\infty$-page of the spectral sequence, and the morphism $H^\bullet(E) \to B^\bullet(B)$ above, hence also the isomorphism $\tilde H^\bullet(Th(V)) \to H^\bullet(B)$, factors through the $E_\infty$-page (see towards the end of the proof of the Gysin sequence). Hence the image of $\iota$ on the $E_\infty$-page is the Thom class in question.
Let $E$ be a generalized (Eilenberg-Steenrod) cohomology theory. First observe that an E-orientation on $V \to X$ induces an $H \pi_0(E)$-orientation, i.e. in ordinary cohomology with coefficients in the degree-0 ground ring.
To see this, let’s assume $E$ is connective. Consider the relative Atiyah-Hirzebruch spectral sequence
Since $(D(V), S(V))$ is $(n-1)$-connected for a rank $n$ vector bundle, then $E_2^{p \lt n, q} = 0$. Hence the edge homomorphism
is an isomorphism, and one checks that it sends Thom classes to Thom classes.
For a fully detailed account see (Pedrotti 16).
A general abstract discussion is around page 30, 31 of (ABGHR).
(…)
The original proof that the Thom isomorphism is indeed an isomorphism is due to
The argument via a Serre spectral sequence for a relative fibration seems to be due to
Textbook accounts:
Discussion in the generality of complex oriented cohomology theory:
Pierre Conner, Edwin Floyd, Section 6 of: The Relation of Cobordism to K-Theories, Lecture Notes in Mathematics 28 Springer 1966 (doi:10.1007/BFb0071091, MR216511)
Stanley Kochman, Section 2.6 of: Bordism, Stable Homotopy and Adams Spectral Sequences, AMS 1996
Dai Tamaki, Akira Kono, Section 3.6 in: Generalized Cohomology, Translations of Mathematical Monographs, American Mathematical Society, 2006 (ISBN: 978-0-8218-3514-2)
Lecture notes:
Johannes Ebert, sections 2.3, 2.4 of A lecture course on Cobordism Theory, 2012 (pdf)
Michael Hopkins (notes by Akhil Mathew), Lecture 21 in: Spectra and stable homotopy theory, 2012 (pdf, pdf)
Riccardo Pedrotti, Complex oriented cohomology, generalized orientation and Thom isomorphism, 2016, 2018 (pdf)
A discussion in differential geometry with fiberwise compactly supported differential forms is around theorem 6.17 of
A comprehensive general abstract account for multiplicative cohomology theories in terms of E-infinity ring spectra is in
An alternative simple formulation in terms of geometric cycles as in bivariant cohomology theory is in
See also
Albrecht Dold, Relations between ordinary and extraordinary homology, Colloq. Algebraic Topology, August 1–10, 1962 , Inst. Math. Aarhus Univ. (1962) pp. 2–9
Yuli Rudyak, On the Thom–Dold isomorphism for nonorientable bundles Soviet Math. Dokl. , 22 (1980) pp. 842–844 Dokl. Akad. Nauk. SSSR , 255 : 6 (1980) pp. 1323–1325
Robert Switzer, Algebraic topology - homotopy and homology , Springer (1975)
PlanetMath, Thom space, Thom class, Thom isomorphism theorem
Formalization in homotopy type theory:
Last revised on January 22, 2021 at 02:49:58. See the history of this page for a list of all contributions to it.