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Symmetry and bifurcations of momentum mappings

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Abstract

The zero set of a momentum mapping is shown to have a singularity at each point with symmetry. The zero set is diffeomorphic to the product of a manifold and the zero set of a homogeneous quadratic function. The proof uses the Kuranishi theory of deformations. Among the applications, it is shown that the set of all solutions of the Yang-Mills equations on a Lorentz manifold has a singularity at any solution with symmetry, in the sense of a pure gauge symmetry. Similarly, the set of solutions of Einstein's equations has a singularity at any solution that has spacelike Killing fields, provided the spacetime has a compact Cauchy surface.

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Abbreviations

(P,ω):

symplectic manifold

\(T_{x_0 } P\) :

tangent space toP atx 0εP

G,g:

Lie group, Lie algebra

g·x g (x):

action ofG onP

ξ P :

infinitesimal generator of the action onP corresponding toξεg

J:P→g*:

momentum mapping

\(dJ\left( {x_0 } \right):T_{x_0 } P \to \mathfrak{g}^ * \) :

differential ofJ atx 0

\(\mathbb{J}\) :

complex structure onP

\(S_{x_0 } \) :

slice for theG-action atx 0

\(I_{x_0 } \) :

isotropy group ofx 0; {g εG|gx 0=x 0}

\(\mathcal{L}_{x_0 } ,\mathfrak{s}_{x_0 } \) :

identity component of\(I_{x_0 } \), its Lie algebra [Eq. (10)]

«,»:

(weak) metric onP

〈,〉:

pairing between g* and g

\((,)_{x_0 } \) :

inner product of g* depending onx 0εP

\(dJ\left( {x_0 } \right)^ * :\mathfrak{g} \to T_{x_0 } P\) :

adjoint ofdJ(x 0) relative to 〈,〉 and «,»

\(dJ\left( {x_0 } \right)^\dag :\mathfrak{g}^ * \to T_{x_0 } P\) :

adjoint ofdJ(x 0) relative to (,) and «,»

\(T_{x_0 } P = Range\left[ {\mathbb{J} \circ dJ\left( {x_0 } \right)^ * } \right] \oplus Range\left[ {dJ\left( {x_0 } \right)^ * } \right] \oplus \left[ {\ker \left( {dJ\left( {x_0 } \right) \circ \mathbb{J}} \right) \cap \ker dJ\left( {x_0 } \right)} \right]\) :

Moncrief's decomposition

ℙ:g*→RangeddJ(x 0):

orthogonal projection

\(\mathcal{C} = J^{ - 1} \left( 0 \right)\) :

zero set ofJ (or constraint set)

\(\mathcal{C}_\mathbb{P} = \left( {\mathbb{P}J} \right)^{ - 1} \left( 0 \right)\) :

zero set of ℙ∘J

\(N_{x_0 } \) :

x's with the same orbit type asx 0

\(\mathcal{N}_{x_0 } \) :

\(N_{x_0 } \cap S_{x_0 } \) (Lemma 1)

\(\mathfrak{g}_{x_0 }^ * \) :

elements in g* with same symmetry asx 0 (Lemma 5)

f :

\(f = \left( {Id - \mathbb{P}} \right) \circ J:\mathcal{C}_\mathbb{P} \cap S_{x_0 } \to \ker dJ\left( {x_0 } \right)^\dag \) (Lemma 8)

Δ=dJ(x 0)∘dJ(x 0) :

“elliptic” operator associated withJ

G−1∘ℙ:

Greens' function for Δ

F(x)=x+dJ(x 0)GQ(h),h=xx 0 :

Kuranishi map (Lemma 9)

\(C_{x_0 } \) :

homogeneous cone associated withd 2 J(x 0) (Theorem 3)

:

orthogonal projection onto kerdJ(x 0) (Theorem 3)

\(\mathfrak{h}\), ℋ:

a Lie subalgebra of\(\mathfrak{s}_{x_0 } \), its Lie group

:

points with symmetry (at least) ℋ (Theorem 4)

:

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Communicated by A. Jaffe

This work was partially supported by the National Science Foundation. The second author was supported by a Killam Visiting fellowship at the University of Calgary during the completion of the paper

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Arms, J.M., Marsden, J.E. & Moncrief, V. Symmetry and bifurcations of momentum mappings. Commun.Math. Phys. 78, 455–478 (1981). https://doi.org/10.1007/BF02046759

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