arXiv subjects

Branimir Ćaćić

Publications and source records attributed to Branimir Ćaćić.

7 recordsLinked to original sources

arXiv2026

A reconstruction theorem for Connes-Landi deformations of commutative spectral triples

We formulate and prove an extension of Connes's reconstruction theorem for commutative spectral triples to so-called Connes-Landi or isospectral deformations of commutative spectral triples along the action of a compact Abelian Lie group $G$, also known as toric noncommutative manifolds. In particular, we propose an abstract definition for such spectral triples, where noncommutativity is entirely governed by a deformation parameter sitting in the second group cohomology of the Pontrjagin dual of $G$, and then show that such spectral triples are well-behaved under further Connes-Landi deformation, thereby allowing for both quantisation from and dequantisation to $G$-equivariant abstract commutative spectral triples. We then use a refinement of the Connes-Dubois-Violette splitting homomorphism to conclude that suitable Connes-Landi deformations of commutative spectral triples by a rational deformation parameter are almost-commutative in the general, topologically non-trivial sense.

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arXiv2023

Geometric foundations for classical $\mathrm{U}(1)$-gauge theory on noncommutative manifolds

We systematically extend the elementary differential and Riemannian geometry of classical $\mathrm{U}(1)$-gauge theory to the noncommutative setting by combining recent advances in noncommutative Riemannian geometry with the theory of coherent $2$-groups. We show that Hermitian line bimodules with Hermitian bimodule connection over a unital pre-$\mathrm{C}^\ast$-algebra with $\ast$-exterior algebra form a coherent $2$-group, and we prove that weak monoidal functors between coherent $2$-groups canonically define bar or involutive monoidal functors in the sense of Beggs--Majid and Egger, respectively. Hence, we prove that a suitable Hermitian line bimodule with Hermitian bimodule connection yields an essentially unique differentiable quantum principal $\mathrm{U}(1)$-bundle with principal connection and vice versa; here, $\mathrm{U}(1)$ is $q$-deformed for $q$ a numerical invariant of the bimodule connection. From there, we formulate and solve the interrelated lifting problems for noncommutative Riemannian structure in terms of abstract Hodge star operators and formal spectral triples, respectively; all the while, we account precisely for emergent modular phenomena of geometric nature. In particular, it follows that the spin Dirac spectral triple on quantum $\mathbf{C}\mathrm{P}^1$ does not lift to a twisted spectral triple on $3$-dimensional quantum $\mathrm{SU}(2)$ with the $3$-dimensional calculus but does recover Kaad--Kyed's compact quantum metric space on quantum $\mathrm{SU}(2)$ for a canonical choice of parameters.

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arXiv2021

Classical gauge theory on quantum principal bundles

We propose a conceptually economical and computationally tractable completion of the foundations of gauge theory on quantum principal bundles à la Brzeziński--Majid to the case of general differential calculi and strong bimodule connections. In particular, we use explicit groupoid equivalences to reframe the basic concepts of gauge theory -- gauge transformation, gauge potential, and field strength -- in terms of reconstruction of calculi on the total space (to second order) from given calculi on the structure quantum group and base, respectively. We therefore construct gauge-equivariant moduli spaces of all suitable first- and second-order total differential calculi, respectively, compatible with these choices. As a first illustration, we relate the gauge theory of a crossed product algebra qua trivial quantum principal bundle to lazy Sweedler and Hochschild cohomology with coefficients. As a second illustration, we show that a noncommutative $2$-torus with real multiplication is the base space of a non-trivial $\mathrm{U}_{q^2}(1)$-gauge theory admitting Connes's constant curvature connection as a $q$-monopole connection, where -- in the spirit of Manin's Alterstraum -- one must take $q$ to be the norm-positive fundamental unit of the corresponding real quadratic field.

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arXiv2021

Gauge theory on noncommutative Riemannian principal bundles

We present a new, general approach to gauge theory on principal $G$-spectral triples, where $G$ is a compact connected Lie group. We introduce a notion of vertical Riemannian geometry for $G$-$C^\ast$-algebras and prove that the resulting noncommutative orbitwise family of Kostant's cubic Dirac operators defines a natural unbounded $KK^G$-cycle in the case of a principal $G$-action. Then, we introduce a notion of principal $G$-spectral triple and prove, in particular, that any such spectral triple admits a canonical factorisation in unbounded $KK^G$-theory with respect to such a cycle: up to a remainder, the total geometry is the twisting of the basic geometry by a noncommutative superconnection encoding the vertical geometry and underlying principal connection. Using these notions, we formulate an approach to gauge theory that explicitly generalises the classical case up to a groupoid cocycle and is compatible in general with this factorisation; in the unital case, it correctly yields a real affine space of noncommutative principal connections with affine gauge action. Our definitions cover all locally compact classical principal $G$-bundles and are compatible with $θ$-deformation; in particular, they cover the $θ$-deformed quaternionic Hopf fibration $C^\infty(S^7_θ) \hookleftarrow C^\infty(S^4_θ)$ as a noncommutative principal $\operatorname{SU}(2)$-bundle.

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arXiv2013

Real structures on almost-commutative spectral triples

We refine the reconstruction theorem for almost-commutative spectral triples to a result for real almost-commutative spectral triples, clarifying, in the process, both concrete and abstract definitions of real commutative and almost-commutative spectral triples. In particular, we find that a real almost-commutative spectral triple algebraically encodes the commutative *-algebra of the base manifold in a canonical way, and that a compact oriented Riemannian manifold admits real (almost-)commutative spectral triples of arbitrary KO-dimension. Moreover, we define a notion of smooth family of real finite spectral triples and of the twisting of a concrete real commutative spectral triple by such a family, with interesting KK-theoretic and gauge-theoretic implications.

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arXiv2012

A reconstruction theorem for almost-commutative spectral triples

We propose an expansion of the definition of almost-commutative spectral triple that accommodates non-trivial fibrations and is stable under inner fluctuation of the metric, and then prove a reconstruction theorem for almost-commutative spectral triples under this definition as a simple consequence of Connes's reconstruction theorem for commutative spectral triples. Along the way, we weaken the orientability hypothesis in the reconstruction theorem for commutative spectral triples, and following Chakraborty and Mathai, prove a number of results concerning the stability of properties of spectral triples under suitable perturbation of the Dirac operator.

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arXiv2009

Moduli spaces of Dirac operators for finite spectral triples

The structure theory of finite real spectral triples developed by Krajewski and by Paschke and Sitarz is generalised to allow for arbitrary KO-dimension and the failure of orientability and Poincare duality, and moduli spaces of Dirac operators for such spectral triples are defined and studied. This theory is then applied to recent work by Chamseddine and Connes towards deriving the finite spectral triple of the noncommutative-geometric Standard Model.

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