3+1 formalism and bases of numerical relativity - LUTh ...
3+1 formalism and bases of numerical relativity - LUTh ...
3+1 formalism and bases of numerical relativity - LUTh ...
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90 Conformal decomposition<br />
6.3.1 General formula relating the two Ricci tensors<br />
The starting point <strong>of</strong> the calculation is the Ricci identity (2.34) applied to a generic vector field<br />
v ∈ T (Σt):<br />
(DiDj − DjDi)v k = R k lij vl . (6.39)<br />
Contracting this relation on the indices i <strong>and</strong> k (<strong>and</strong> relabelling i ↔ j) let appear the Ricci<br />
tensor:<br />
Rijv j = DjDiv j − DiDjv j . (6.40)<br />
Expressing the D-derivatives in term <strong>of</strong> the ˜D-derivatives via formula (6.29), we get<br />
Rijv j = ˜ Dj(Div j ) − C k ji Dkv j + C j<br />
jk Div k − ˜ Di(Djv j )<br />
= ˜ Dj( ˜ Div j + C j<br />
ik vk ) − C k ji( ˜ Dkv j + C j<br />
kl vl ) + C j<br />
jk ( ˜ Div k + C k il vl ) − ˜ Di( ˜ Djv j + C j<br />
jk vk )<br />
= ˜ Dj ˜ Div j + ˜ DjC j<br />
ik vk + C j<br />
ik ˜ Djv k − C k ji ˜ Dkv j − C k ji Cj<br />
kl vl + C j<br />
jk ˜ Div k + C j<br />
jk Ck il vl<br />
− ˜ Di ˜ Djv j − ˜ DiC j<br />
jk vk − C j<br />
jk ˜ Div k<br />
= ˜ Dj ˜ Div j − ˜ Di ˜ Djv j + ˜ DjC j<br />
ik vk − C k jiC j<br />
kl vl + C j<br />
jk Ck il vl − ˜ DiC j<br />
jk vk . (6.41)<br />
We can replace the first two terms in the right-h<strong>and</strong> side via the contracted Ricci identity similar<br />
to Eq. (6.40) but regarding the connection ˜D:<br />
˜Dj ˜ Div j − ˜ Di ˜ Djv j = ˜ Rijv j<br />
Then, after some relabelling j ↔ k or j ↔ l <strong>of</strong> dumb indices, Eq. (6.41) becomes<br />
(6.42)<br />
Rijv j = ˜ Rijv j + ˜ DkC k ij v j − ˜ DiC k jk vj + C l lk Ck ijv j − C k li Cl kj vj . (6.43)<br />
This relation being valid for any vector field v, we conclude that<br />
where we have used the symmetry <strong>of</strong> C k ij<br />
Rij = ˜ Rij + ˜ DkC k ij − ˜ DiC k kj + Ck ijC l lk − Ck il Cl kj<br />
in its two last indices.<br />
, (6.44)<br />
Remark : Eq. (6.44) is the general formula relating the Ricci tensors <strong>of</strong> two connections, with<br />
the Ck ij ’s being the differences <strong>of</strong> their Christ<strong>of</strong>fel symbols [Eq. (6.30)]. This formula<br />
does not rely on the fact that the metrics γ <strong>and</strong> ˜γ associated with the two connections are<br />
conformally related.<br />
6.3.2 Expression in terms <strong>of</strong> the conformal factor<br />
Let now replace Ck ij in Eq. (6.44) by its expression in terms <strong>of</strong> the derivatives <strong>of</strong> Ψ, i.e.<br />
Eq. (6.33). First <strong>of</strong> all, by contracting Eq. (6.33) on the indices j <strong>and</strong> k, we have<br />
<br />
= 2 ˜Di ln Ψ + 3 ˜Di ln Ψ − ˜Di ln Ψ , (6.45)<br />
i.e.<br />
C k ki<br />
C k ki = 6 ˜ Di ln Ψ, (6.46)