group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
A lifting gerbe is a cocycle/gerbe – or equivalently the principal infinity-bundle that it classifies – arising as the obstruction to the lift of a $G$-principal bundle or more generally of any $G$-principal infinity-bundle, through a shifted central extension
of $G$, for some abelian group $A$.
Such an extension is classified (or rather its delooping is) by a cocycle
in group cohomology.
For $X \to \mathbf{B}G$ the cocycle in nonabelian cohomology classifying a $G$-principal bundle or principal infinity-bundle, the corresponding lifting $A$-gerbe is the thing classified by the composite cocycle
In the literature the following simplest case of the general situation is usually considered exclusively:
For $G$ an ordinary topological or Lie group and
a $U(1)$-central group extension, classified by a 2-cocycle $\mathbf{B}G \to \mathbf{B}^2 U(1)$ in ordinary group cohomology, and for $X \to \mathbf{B}G$ the cocycle of a $G$-principal bundle, the corresponding lifting gerbe is given by the cocycle
In the literature this is often discussed in terms of the model for gerbes given by $U(1)$-principal bundle gerbes: let $P \to X$ be the total space of the bundle classified by $X \to \mathbf{B}G$. Regard this as the surjective submersion that is part of the data of the bundle gerbe to be constructed. By principality of $P$, there is a canonical morphism
that sends two elements in the fiber of the bundle to the unique group element that takes the first to the second. Along this morphism, form the pullback of the extension $\hat G \to G$ to get $L$ in
This $L$ is the line bundle ingredient in the lifting bundle gerbe. There is canocically a multiplication map that completes the definition.
Let $\mathcal{H}$ be a separable infinite-dimensiona Hilbert space, $U(\mathcal{H})$ its unitary group and $P U(\mathcal{H})$ its projective unitary group. Then the extension
is universal with the above property: every $U(1)$-bundle gerbe/circle 2-bundle is the lifting gerbe of some $P U(\mathcal{H})$-principal bundle through this extension.
See projective unitary group for details.
To contrast, in (Roberts 2021), finite dimensional lifting gerbes are very often those gerbes with torsion Dixmier-Douady class. This is in particular true if the fundamental group of the base is finite, or the Lie group $G$ is compact.
The archetypical example of a lifting 2-gerbe is a Chern-Simons gerbe.
A review is for instance in section 2.2 of
Alan Carey, Peter Bouwknegt, Varghese Mathai, Michael Murray and Danny Stevenson, K-theory of bundle gerbes and twisted K-theory , Commun Math Phys, 228 (2002) 17-49 (arXiv)
David Roberts, Many finite-dimensional lifting bundle gerbes are torsion (2021), arXiv:2104.07936.
Last revised on April 27, 2021 at 21:11:47. See the history of this page for a list of all contributions to it.