Noncommutative geometry (NCG) is a branch of mathematics concerned with a geometric approach to noncommutative algebras, and with the construction of spaces which are locally presented by noncommutative algebras of functions (possibly in some generalized sense). A noncommutative algebra is an associative algebra in which the multiplication is not commutative, that is, for which xy does not always equal yx; or more generally an algebraic structure in which one of the principal binary operations is not commutative; one also allows additional structures, e.g. topology or norm to be possibly carried by the noncommutative algebra of functions. The leading direction in noncommutative geometry has been laid by French mathematician Alain Connes since his involvement from about 1979.
The main motivation is to extend the commutative duality between spaces and functions to the noncommutative setting. In mathematics, spaces, which are geometric in nature, can be related to numerical functions on them. In general, such functions will form a commutative ring. For instance, one may take the ring C(X) of continuous complex-valued functions on a topological space X. In many cases (e.g., if X is a compact Hausdorff space), we can recover X from C(X), and therefore it makes some sense to say that X has commutative topology.
More specifically, in topology, compact Hausdorff topological spaces can be reconstructed from the Banach algebra of functions on the space (Gel'fand-Neimark). In commutative algebraic geometry, algebraic schemes are locally prime spectra of commutative unital rings (A. Grothendieck), and schemes can be reconstructed from the categories of quasicoherent sheaves of modules on them (P. Gabriel-A. Rosenberg). For Grothendieck topologies, the cohomological properties of a site are invariant of the corresponding category of sheaves of sets viewed abstractly as a topos (A. Grothendieck). In all these cases, a space is reconstructed from the algebra of functions or its categorified version—some category of sheaves on that space.
Functions on a topological space can be multiplied and added pointwise hence they form a commutative algebra; in fact these operations are local in the topology of the base space, hence the functions form a sheaf of commutative rings over the base space.
The dream of noncommutative geometry is to generalize this duality to the duality between
- noncommutative algebras, or sheaves of noncommutative algebras, or sheaf-like noncommutative algebraic or operator-algebraic structures
- and geometric entities of certain kind,
and interact between the algebraic and geometric description of those via this duality.
Regarding that the commutative rings correspond to usual affine schemes, and commutative C*-algebras to usual topological spaces, the extension to noncommutative rings and algebras requires non-trivial generalization of topological spaces, as "non-commutative spaces". For this reason, some talk about non-commutative topology, though the term has also other meanings.
 Applications in mathematical physics
Some applications in particle physics are described on the entries Noncommutative standard model and Noncommutative quantum field theory. Sudden rise in interest in noncommutative geometry in physics, follows after the speculations of its role in M-theory made in 1997.
 Motivation from ergodic theory
Some of the theory developed by Alain Connes to handle noncommutative geometry at a technical level has roots in older attempts, in particular in ergodic theory. The proposal of George Mackey to create a virtual subgroup theory, with respect to which ergodic group actions would become homogeneous spaces of an extended kind, has by now been subsumed.
 Noncommutative C*-algebras, von Neumann algebras
(The formal duals of) non-commutative C*-algebras are often now called non-commutative spaces. This is by analogy with the Gelfand representation, which shows that commutative C*-algebras are dual to locally compact Hausdorff spaces. In general, one can associate to any C*-algebra S a topological space Ŝ; see spectrum of a C*-algebra.
 Noncommutative differentiable manifolds
A smooth Riemannian manifold M is a topological space with a lot of extra structure. From its algebra of continuous functions C(M) we only recover M topologically. The algebraic invariant that recovers the Riemannian structure is a spectral triple. It is constructed from a smooth vector bundle E over M, e.g. the exterior algebra bundle. The Hilbert space L2(M,E) of square integrable sections of E carries a representation of C(M) by multiplication operators, and we consider an unbounded operator D in L2(M,E) with compact resolvent (e.g. the signature operator), such that the commutators [D,f] are bounded whenever f is smooth. A recent deep theorem states that M as a Riemannian manifold can be recovered from this data.
This suggests that one might define a noncommutative Riemannian manifold as a spectral triple (A,H,D), consisting of a representation of a C*-algebra A on a Hilbert space H, together with an unbounded operator D on H, with compact resolvent, such that [D,a] is bounded for all a in some dense subalgebra of A. Research in spectral triples is very active, and many examples of noncommutative manifolds have been constructed.
 Noncommutative affine and projective schemes
In analogy to the duality between affine schemes and commutative rings, we define a category of noncommutative affine schemes as the dual of the category of associative unital rings. There are certain analogues of Zariski topology in that context so that one can glue such affine schemes to more general objects.
There are also generalizations of the Cone and of the Proj of a commutative graded ring, mimicking a Serre's theorem on Proj. Namely the category of quasicoherent sheaves of O-modules on a Proj of a commutative graded algebra is equivalent to the category of graded modules over the ring localized on Serre's subcategory of graded modules of finite length; there is also analogous theorem for coherent sheaves when the algebra is Noetherian. This theorem is extended as a definition of noncommutative projective geometry by Michael Artin and J. J. Zhang, who add also some general ring-theoretic conditions (e.g. Artin-Schelter regularity).
A. L. Rosenberg has created a rather general relative concept of noncommutative quasicompact scheme (over a base category), abstracting the Grothendieck's study of morphisms of schemes and covers in terms of categories of quasicoherent sheaves and flat localization functors. There is also another interesting approach via localization theory, due to Fred Van Oystaeyen, Luc Willaert and Alain Verschoren, where the main concept is that of a schematic algebra.
 Invariants for noncommutative spaces
Some of the motivating questions of the theory are concerned with extending known topological invariants to formal duals of noncommutative (operator) algebras and other replacements and candidates for noncommutative spaces. One of the main starting points of the Alain Connes' direction in noncommutative geometry is his spectacular discovery (and independently by Boris Tsygan) of a very important new homology theory associated to noncommutative associative algebras and noncommutative operator algebras, namely the cyclic homology and its relations to the algebraic K-theory (primarily via Connes-Chern character map).
The theory of characteristic classes of smooth manifolds has been extended to spectral triples, employing the tools of operator K-theory and cyclic cohomology. Several generalizations of now classical index theorems allow for effective extraction of numerical invariants from spectral triples. The fundamental characteristic class in cyclic cohomology, the JLO cocycle, generalizes the classical Chern character.
 Examples of noncommutative spaces
- In Weyl quantization, the symplectic phase space of classical mechanics is deformed into a non-commutative phase space generated by the position and momentum operators.
- The standard model of particle physics is another example of a noncommutative geometry, cf noncommutative standard model.
- The noncommutative torus, deformation of the function algebra of the ordinary torus, can be given the structure of a spectral triple. This class of examples has been studied intensively and still functions as a test case for more complicated situations.
- Noncommutative algebras arising from foliations.
- Examples related to dynamical systems arising from number theory, such as the Gauss shift on continued fractions, give rise to noncommutative algebras that appear to have interesting noncommutative geometries.
 See also
- Alain Connes, Michael R. Douglas, Albert Schwarz, Noncommutative geometry and matrix theory: compactification on tori. J. High Energy Phys. 1998, no. 2, Paper 3, 35 pp. doi, hep-th/9711162
- Connes, Alain, On the spectral characterization of manifolds, arXiv:0810.2088v1
- M. Artin, J. J. Zhang, Noncommutative projective schemes, Adv. Math. 109 (1994), no. 2, 228--287, doi
- Amnon Yekutieli, James J. Zhang, Serre duality for noncommutative projective schemes, Proc. Amer. Math. Soc. 125, n. 3, 1997, 697-707, pdf
- A. L. Rosenberg, Noncommutative schemes, Compositio Math. 112 (1998) 93--125, doi; Underlying spaces of noncommutative schemes, preprint MPIM2003-111, dvi, ps; MSRI lecture Noncommutative schemes and spaces (Feb 2000): video
- Freddy van Oystaeyen, Algebraic geometry for associative algebras, ISBN 0-8247-0424-X - New York: Dekker, 2000.- 287 p. - (Monographs and textbooks in pure and applied mathematics , 232); F. van Oystaeyen, L. Willaert, Grothendieck topology, coherent sheaves and Serre's theorem for schematic algebras, J. Pure Appl. Alg. 104 (1995), p. 109--122
- H. S. Snyder, Quantized Space-Time, Phys. Rev. 71 (1947) 38
- Connes, Alain (1994), Non-commutative geometry, Boston, MA: Academic Press, ISBN 978-0-12-185860-5
- Connes, Alain; Marcolli, Matilde (2008), "A walk in the noncommutative garden", An invitation to noncommutative geometry, World Sci. Publ., Hackensack, NJ, pp. 1–128, arXiv:math/0601054, MR 2408150
- Connes, Alain; Marcolli, Matilde (2008), Noncommutative geometry, quantum fields and motives, American Mathematical Society Colloquium Publications 55, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-4210-2, MR 2371808
- Gracia-Bondia, Jose M; Figueroa, Hector; Varilly, Joseph C (2000), Elements of Non-commutative geometry, Birkhauser, ISBN 978-0-8176-4124-5
- Landi, Giovanni (1997), An introduction to noncommutative spaces and their geometries, Lecture Notes in Physics. New Series m: Monographs 51, Berlin, New York: Springer-Verlag, arXiv:hep-th/9701078, ISBN 978-3-540-63509-3, MR 1482228
- Van Oystaeyen, Fred; Verschoren, Alain (1981), Non-commutative algebraic geometry, Lecture Notes in Mathematics 887, Springer-Verlag, ISBN 978-3-540-11153-5
- Introduction to Quantum Geometry by Micho Đurđevich
- Lectures on Noncommutative Geometry by Victor Ginzburg
- Very Basic Noncommutative Geometry by Masoud Khalkhali
- Lectures on Arithmetic Noncommutative Geometry by Matilde Marcolli
- Noncommutative Geometry for Pedestrians by J. Madore
- An informal introduction to the ideas and concepts of noncommutative geometry by Thierry Masson (an easier introduction that is still rather technical)
- Noncommutative geometry on arxiv.org
- MathOverflow, Theories of Noncommutative Geometry
- S. Mahanta, On some approaches towards non-commutative algebraic geometry, math.QA/0501166