I present a joint work with S. Popa and D. Shlyakhtenko introducing a cohomology theory for quasi-regular inclusions of von Neumann algebras. In particular, we define $L^2$-cohomology and $L^2$-Betti numbers for such inclusions. Applying this to the symmetric enveloping inclusion of a finite index subfactor, we get a cohomology theory and a definition of $L^2$-Betti numbers for finite index subfactors, as well as for arbitrary rigid $C^*$-tensor categories. For the inclusion of a Cartan subalgebra in a $II_1$ factor, we recover Gaboriau's $L^2$-Betti numbers for equivalence relations.[-]

I will discuss a method for constructing a Haar unitary $u$ in a subalgebra $B$ of a $II_1$ factor $M$ that's “as independent as possible” (approximately) with respect to a given finite set of elements in $M$. The technique consists of “patching up infinitesimal pieces” of $u$. This method had some striking applications over the years:
1. vanishing of the 1-cohomology for $M$ with values into the compact operators (1985);
2. reconstruction of subfactors through amalgamated free products and axiomatisation of standard invariants (1990-1994).
3. first positive results on Kadison-Singer type paving (2013);
4. vanishing of the continuous version of Connes-Shlyakhtenko 1-cohomology (with Vaes in Jan. 2014) and of smooth 1-cohomology (with Galatan in June 2014).[-]

The right side of the Baum-Connes conjecture is the $K$-theory of the reduced $C^*$-algebra $C^*_{red} (G)$ of the group $G$. This algebra is the completion of the algebra $L^1(G)$ in the norm of the algebra of operators acting on $L^2(G)$. If we complete the algebra $L^1(G)$ in the norm of the algebra of operators acting on $L^p(G)$ we will get the Banach algebra $C^{*,p}_{red}(G)$. The $K$-theory of this algebra serves as the right side of the $L^p$-version of the Baum-Connes conjecture. The construction of the left side and the assembly map in this case requires a little bit of techniques of asymptotic morphisms for Banach algebras. A useful category of Banach algebras for this purpose includes all algebras of operators acting on $L^p$-spaces (which may be called $L^p$-algebras).
The current joint work in progress with Guoliang Yu aims at proving the following result:
The $L^p$-version of the Baum-Connes conjecture with coefficients in any $L^p$-algebra is true for any discrete group $G$ which admits an affine-isometric, metrically proper action on the space $X = l^p(Z)$, where $Z$ is a countable discrete set, so that the linear part of this action is induced by a measure-preserving action of $G$ on $Z$.
I will discuss the techniques involved in this work.[-]