Approaches to Quantum Gravity

Algebraic approach to Quantum Gravity II 475corresponding double cross sum m⊲⊳g of Lie algebras. Then the differential versionof the equations (24.7) become a matter of a pair of coupled first order differentialequations for families of vector fields α ξ on M and β φ on G labelled by ξ ∈ gand φ ∈ m respectively. We write these vector fields in terms of Lie-algebra valuedfunctions A ξ ∈ C ∞ (M, m) and B φ ∈ C ∞ (G, g) according to left and righttranslation from the tangent space at the identity:α ξ (s) = R s∗ (A ξ (s)), β φ (u) = L u∗ (B φ (u)). (24.9)In these terms the matched pair equations becomeA ξ (st) = A ξ (s) + Ad s (B ξ⊳s (t)), A ξ (e) = 0B φ (uv) = Ad −1v (A v⊲φ(u)) + B φ (v), B φ (e) = 0 (24.10)along with auxiliary data a pair of linear actions ⊲ of G on m and ⊳ of M on g exponentiatingLie algebra actions ⊲, ⊳ of g, m respectively. Finally, (24.10) becomesa pair of differential equations if we let u, t be infinitesimal i.e. elements η ∈ g,ψ ∈ m say of the Lie algebra. Thenψ R (A ξ )(s) = Ad s ((ξ⊳s)⊲ψ), η L (B φ )(v) = Ad v −1(η⊳(v⊲φ)) (24.11)where η L is the left derivative on the Lie group G generated by η and ψ R the rightderivative on M generated by ψ. Note that this impliesψ R (A ξ )(e) = ξ⊲ψ, η L (B φ )(e) = η⊳φ (24.12)which shows how the auxiliary data are determined. These nonlinear equationswere proposed in [13] as a toy model of Einstein’s equations and solved for R⊲⊳Rwhere they were shown to have singularities and accumulation points not unlike ablack-hole event horizon. Such accumulation points are a typical feature of (24.10)when both groups are noncompact. We have flipped conventions relative to [10]inorder to have a left action of the Poincaré quantum group in our applications.One has to solve these equations globally (taking account of any singularities)in order to have honest Hopf–von Neumann or Hopf C ∗ -algebra quantum groups;there are some interesting open problems there. However, for simply a Hopf algebraat an algebraic level one needs only the initial data (24.12) of the matchedpair, namely the Lie algebra actions ⊲, ⊳ corresponding to m ⊲⊳ g. Clearly thenU(m ⊲⊳ g) = U(m) ⊲⊳ U(g) as a Hopf algebra double cross product or factorisationof Hopf algebras [14]. We content ourselves with one theorem from thistheory.Theorem 1 Let (H 1 , H 2 ) be a matched pair of quantum groups with H 1 ⊲⊳ H 2the associated double cross product. Then (i) there is another quantum groupdenoted H = H 2 ⊲◭H1 ∗ called the ‘semidualisation’ of the matched pair. (ii) This