The spin connection for twisted geometries

In the first part of this section we studied the role of torsion for discrete geometries.

Employing the Minkowski reconstruction theorem (generalised to Minkowski space in subsection 3.5.2) we realised torsion guarantees the geometricity of the elementary

*Note that we do not need a metric to speak about convexity: A setP∈R⁴is said to be convex if for any two pointsX^µ, Y^µ∈P also all elements of the connecting linetX^µ+ (1−t)Y^µwitht∈(0,1) are points inP.

building blocks: If there is no torsion in a four-simplex, then each bone bounding a triangle has a unique length, and all bones close to form a triangle. Every individual four-simplex is geometric, but if we ask how the elementary tetrahedra glue across neighbouring four-simplices we may find a discontinuity. This is what happens in loop gravity. When looking at a spatial slice, the discontinuities induce atwisted geometry.

Twisted geometries were discovered first in the pioneering articles [51, 54, 150]. In this section we compute the torsionless connection for twisted geometries. But let us first explain what we actually mean by a twisted geometry.

A twisted geometry is a generalisation of a three-dimensional Regge geometry. It is an oriented three-dimensional simplicial complex (a triangulation), equipped with a flat Euclidean metric in each tetrahedron, together with the condition that for any two tetrahedra sharing a triangle both metrics agree on the area bivector in between.^* The definition of the area bivectors is as follows. In a locally flat region we can find inertial coordinates (x¹, x², x³) = (x, y, z) to write the area bivector of an oriented triangle τ as the surface integral:

E_i[τ] = 1 2

Z

τ

ǫ_ijkdxⁱ∧dx^k. (3.121)

Any triangle bounds two tetrahedra, and we thus have two metrics to compute its shape. Three numbers determine the shape of a triangle—for example its area and two angles. Twisted geometries preserve the area, but the angles may change across the triangle. If we only match the areas, we get a twisted geometry, if in addition we also match the angles between any two bones, we further reduce to a Regge geometry.

In loop quantum gravity the semi-classic limit leaves us with a twisted geometry.

The fundamental phase space variables are the holonomies of the Ashtekar–Barbero connection along the links between adjacent tetrahedra, and the area bivectors between.

In the continuum, the underlying connection A^(β)i_a neatly splits into two parts.

The Ashtekar connection is, in fact, nothing but the spin connectionΓⁱ_a[e]shifted by the extrinsic curvature tensor Kⁱ_a: A^(β)i_a = Γⁱ_a+βKⁱ_a, where β is the Barbero–

Immirzi parameter. The extrinsic curvature tensorKⁱ_a depends on the embedding of the spatial slice into the spacetime manifold. The spin connection, on the other hand, is fully determined by the intrinsic geometry through Cartan’s first structure equation, namely the condition of vanishing three-torsion.

As pointed out in [150,151], there is no such clean separation of extrinsic and intrinsic contributions for the discrete theory, because the Cartan equation requires continuity of the triad across the triangle. For this reason, a definition of the spin connection for twisted geometries has been an open task just until recently. The solution was found in [159], which I published together with Hal Haggard, Carlo Rovelli and Francesca Vidotto. In the following I will briefly report on this result, thus providing a definition ofΓⁱa[e]that remains meaningful for a twisted geometry.

In a twisted geometry there is a discontinuity of the metric across a triangle. There is one flat metric from the “left”, and another one from the “right”, each of which induce

*This definition is slightly stronger than the one emerging from the classical limit of loop quantum gravity, since it fixes the full triangulation and not just its dual graph. Also, the definition given here refers only to theintrinsic geometry. The full definition of the twisted geometry that appears in quantum gravity includes also theextrinsic curvature, which plays no role here. Finally, for sim-plicity we restrict our attention to triangulations, but the results presented extend to generic cellular decompositions (and therefore to polyhedra other than tetrahedra).

the same bivector on the triangle. This bivector E_i[τ] is nothing but the area of the triangle weighted by its normal:

Ei[τ] =A[τ]ni[τ]. (3.122)

The triangle has both a unique area and a unique normal. The normal is the same from the two sides, therefore the discontinuity of the metric can only be in the induced metric on the plane of the triangle. This is a two-dimensional metric, thus described by three numbers. Three numbers determine a triangle, and the area is one of them. The discontinuity must therefore be in the two remaining degrees of freedom, that describe the shape of the triangle up to an overall scaling.

The triangle looks different from the two sides, yet its area is the same. Given two triangles that have the same area there is a linear change of coordinates that map one to the other. If we embed the two triangles into R² this transformation must be an SL(2,R) element. We can thus use an element e ∈ SL(2,R) to parametrise the discontinuity of the geometry at the plane of the triangle.

Let us now choose a coordinate system {x, y, z} covering the two tetrahedra and align it to the triangle: the triangle should rest atz= 0, while∂_z should be its normal vector. The geometry is locally flat, and we can thus always choose this coordinate system such that it is inertial in the “left” tetrahedron. This means that eⁱ = dxⁱ is a cotriad on the left hand side tetrahedron (i.e. for z <0). There is no discontinuity in the normal direction, and we can thus always find a triad on the right hand side that has the constant form

e¹ =e¹_xdx+e¹_ydy, e²=e²_xdx+e²_ydy, e³ = dz. (3.123) The area is preserved across the triangle, and we thus have the condition dete = 1, whereeis the matrix

e=





e¹_x e¹_y 0 e²x e²y 0

0 0 1



. (3.124)

This is an SL(3,R) element, or, more specifically it is in the SL(2,R) upper block diagonal subgroup ofSL(3,R).

The geometrical interpretation of these groups is straightforward: e is the linear transformation that sends a triangle with the dimensions given by the left metric into the triangle with the dimensions given by the right. In other words,eis the linear trans-formation that makes the two triangles match. Since the triangle is two-dimensional, we can always restrict this linear transformation to an element of SL(2,R).

Once we have a triad eⁱ, we can look at Cartan’s first structure equation:

deⁱ+ǫⁱ_jk ω^j∧e^k= 0. (3.125) For a given triad, there is a unique solution for ωⁱ_a = Γⁱ_a[e], which in turn defines the torsionless spin connection. For a twisted geometry there is a discontinuity in the cotriad, and Cartan’s first structure equation does not make sense any longer. To define the spin connection for a twisted geometry we need a regularisation. We therefore introduce a smeared cotriad which is now continuous all across the triangle, but depends on a regulator ∆. The limit of ∆ → 0 brings us back to a discontinuous triad, and

defines, through Cartan’s first structure equation, the torsionless spin connection for a twisted connection.

Let us now look at the regionR²×[0,∆]around the triangle. We are now searching for a continuous cotriad e(z) such that e(0) = 1 is the cotriad in the left triangle, while e(∆) = e gives the cotriad (3.124) in the right triangle. In the limit of ∆ց 0 this region shrinks to the plane of the triangle, and there appears a discontinuity in the metric. Once we have a continuous triad, this defines the spin connection, and we can compute the parallel transport U(e) ∈ SO(3) across the triangle. The limit

∆ց0then defines the holonomy across the triangle for a twisted geometry. The only missing ingredient is to choose the actual functione(z) interpolating between the two sides of the triangle. This function cannot be arbitrary, for there is a highly nontrivial condition: The resulting parallel transport must transform homogeneously once we rotate the frames at either side of the triangle. In other words:

U(R_seR⁻¹_t ) =R_sU(e)R_t⁻¹ (3.126) for anyRs, Rt∈SO(3). Looking at the polar decomposition ofe:

e= exp(A) exp(S), (3.127)

whereAis antisymmetric and S is symmetric, we can find an interpolating triad

e(z) = exp(zA) exp(zS), (3.128)

that satisfies equation (3.126) for allR_s, R_t∈SO(3). This defines a continuous triad joining the two tetrahedra, differentiable^* in (0,∆). We can now compute the spin connection and take the limit∆ց0. This defines a torsionless spin connection for the twisted geometry.

We will now compute this connection explicitly. From the last equation, we have deⁱ = A+ exp(zA)Sexp(−zA)i

jdz∧e^j. (3.129) Inserting this into the Cartan equation (and lowering an index) we have

A+ exp(zA)Sexp(−zA)

ijdz∧e^j =−ǫ_ijk ω^j∧e^k. (3.130) The solution of this equation is given by

ωⁱ=ωⁱj e^j, (3.131)

where

ωⁱ_j =−ǫ^ikl A+ exp(zA)Sexp(−zA)

jke_l^z+1

2ǫ^klmA_kle_m^zδ_jⁱ, (3.132) ande_i^z for i= 1,2,3are matrix elements of the triad.

*This triad has a discontinuity in its first derivatives at z ∈ {0,∆}, since for z < 0 (z > ∆) e(z) assumes the constant forme(z) =1(e(z) =e). This discontinuity is, however, of little physical importance, since we can always smooth it out by a chance of coordinatesz →z(z). Our resulting˜ holonomy has, however, a coordinate invariant definition, and does therefore not depend on this choice.

What is relevant for us here is only the holonomy of the connection along the transver-sal direction. Consider a pathγ crossing the region at constantxandy. The holonomy of the connection ω=ωⁱje^j⊗τi along this path is given by column are determined only by the last term in (3.132). Therefore

ω^k(∂_z) = 1

2ǫ^kijA_ij. (3.134)

So that

U = expA, (3.135)

that is, the holonomy is precisely the orthogonal matrix in the polar decomposition of e. For the explicit form of the polar decomposition, we have then that

U(e) =e(e^Te)^−1/2, (3.136) wheree^T is the transpose ofe. SinceU(e)is independent of the size of the interpolating region, taking the limit ∆→ 0 is immediate. The resulting distributional torsionless spin connection is concentrated on the faceτ : (σ¹, σ²)7→x^a(σ) and is given by

whereϑis an angle, and the distributional one-form of the triangle is defined by dτ_a(x)≡ Levi-Civita density. From this expression it is easy to verify that (3.126) is satisfied.

In this section we defined the torsionless spin connection for a twisted geometry, and computed the corresponding holonomy along the link dual to the triangle. In our inertial coordinate system, the resulting parallel transport is a function of a single angle ϑ, as defined in (3.137). This angle is one of the three numbers parametrising the twist in the metric across the triangle. We can write it fully in terms of the bivectors of the two adjacent tetrahedra. The original paper, reference [159] contains the explicit expression. This angle is therefore a function solely of the intrinsic discrete data on the spatial hypersurface. The same happens in the continuum theory, where the triad fully determines the torsionless spin connection.

This result should clarify some confusion in the literature. It has often been argued (see for instance [165, 166]) that the distributional nature of twisted geometries hints at the presence of torsion. This section proved this intuition wrong. A metric by itself does not define torsion. Torsion is a property of a metric-compatible connection, and thus requires a connection on top of a metric. There is only one connection which is both metric compatible and torsionless. This is the spin connection, and we saw that the torsionless condition defines a metric compatible connection also for the case of twisted geometries.

3.6

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