Simon-Mars tensor in Curzon-Chazy spacetime

This worksheet is based on SageManifolds (version 0.7). It implements the computation of the Simon-Mars tensor of Curzon-Chazy spacetime used in the article arXiv:1412.6542.

It is released under the GNU General Public License version 2.

(c) Claire Somé, Eric Gourgoulhon (2015)

The worksheet file in Sage notebook format is here.

Spacetime manifold

We start by declaring the Curzon-Chazy spacetime as a 4-dimensional manifold:

We introduce the coordinates $(t,r,y,\phi)$ with $y$ related to the standard Weyl-Papapetrou coordinates $(t,r,\theta,\phi)$ by $y=\cos\theta$:

Metric tensor

We declare the only parameter of the Curzon-Chazy spacetime, which is the mass $m$ as a symbolic variable:

Without any loss of generality, we set $m$ to some specific value (this amounts simply to fixing some length scale):

Let us introduce the spacetime metric $g$ and set its components in the coordinate frame associated with Weyl-Papapetrou coordinates:

The Levi-Civita connection $\nabla$ associated with $g$:

As a check, we verify that the covariant derivative of $g$ with respect to $\nabla$ vanishes identically:

Killing vector

The default vector frame on the spacetime manifold is the coordinate basis associated with Weyl-Papapetrou coordinates:

Let us consider the first vector field of this frame:

The 1-form associated to it by metric duality is

Its covariant derivative is

Let us check that the Killing equation is satisfied:

Equivalently, we check that the Lie derivative of the metric along $\xi$ vanishes:

Thank to Killing equation, $\nabla_g \underline{\xi}$ is antisymmetric. We may therefore define a 2-form by $F := - \nabla_g \xi$. Here we enforce the antisymmetry by calling the function antisymmetrize() on nab_xi:

We check that

The squared norm of the Killing vector is

Instead of invoking $g(\xi,\xi)$, we could have evaluated $\lambda$ by means of the 1-form $\underline{\xi}$ acting on the vector field $\xi$:

or we could have used index notation in the form $\lambda = - \xi_a \xi^a$:

Curvature

The Riemann curvature tensor associated with $g$ is

The component $R^0_{\ \, 101} = R^t_{\ \, rtr}$ is

while the component $R^2_{\ \, 323} = R^y_{\ \, \phi y \phi}$ is

The Ricci tensor:

Let us check that the Curzon-Chazy metric is a solution of the vacuum Einstein equation:

The Weyl conformal curvature tensor is

Let us exhibit two of its components $C^0_{\ \, 123}$ and $C^0_{\ \, 101}$:

To form the Mars-Simon tensor, we need the fully covariant (type-(0,4) tensor) form of the Weyl tensor (i.e. $C_{\alpha\beta\mu\nu} = g_{\alpha\sigma} C^\sigma_{\ \, \beta\mu\nu}$); we get it by lowering the first index with the metric:

The (monoterm) symmetries of this tensor are those inherited from the Weyl tensor, i.e. the antisymmetry on the last two indices (position 2 and 3, the first index being at position 0):

Actually, Cd is also antisymmetric with respect to the first two indices (positions 0 and 1), as we can check:

To take this symmetry into account explicitely, we set

Hence we have now

Simon-Mars tensor

The Simon-Mars tensor with respect to the Killing vector $\xi$ is a rank-3 tensor introduced by Marc Mars in 1999 (Class. Quantum Grav. 16, 2507). It has the remarkable property to vanish identically if, and only if, the spacetime $(\mathcal{M},g)$ is locally isometric to a Kerr spacetime.

Let us evaluate the Simon-Mars tensor by following the formulas given in Mars' article. The starting point is the self-dual complex 2-form associated with the Killing 2-form $F$, i.e. the object $\mathcal{F} := F + i \, {}^* F$, where ${}^*F$ is the Hodge dual of $F$:

Let us check that $\mathcal{F}$ is self-dual, i.e. that it obeys ${}^* \mathcal{F} = -i \mathcal{F}$:

Let us form the right self-dual of the Weyl tensor as follows

\[\mathcal{C}_{\alpha\beta\mu\nu} = C_{\alpha\beta\mu\nu} + \frac{i}{2} \epsilon^{\rho\sigma}_{\ \ \ \mu\nu} \, C_{\alpha\beta\rho\sigma}\]

where $\epsilon^{\rho\sigma}_{\ \ \ \mu\nu}$ is associated to the Levi-Civita tensor $\epsilon_{\rho\sigma\mu\nu}$ and is obtained by

The right self-dual Weyl tensor is then:

The Ernst 1-form $\sigma_\alpha = 2 \mathcal{F}_{\mu\alpha} \, \xi^\mu$ (0 = contraction on the first index of $\mathcal{F}$):

Instead of invoking the function contract(), we could have used the index notation to denote the contraction:

The symmetric bilinear form $\gamma = \lambda \, g + \underline{\xi}\otimes\underline{\xi}$:

Final computation leading to the Simon-Mars tensor:

The first part of the Simon-Mars tensor is

\[ S^{(1)}_{\alpha\beta\gamma} = 4 \mathcal{C}_{\mu\alpha\nu\beta} \, \xi^\mu \, \xi^\nu \, \sigma_\gamma\]

The second part is the tensor

\[ S^{(2)}_{\alpha\beta\gamma} = - \gamma_{\alpha\beta} \, \mathcal{C}_{\rho\gamma\mu\nu} \, \xi^\rho \, \mathcal{F}^{\mu\nu}\]

which we compute by using the index notation to denote the contractions:

The Mars-Simon tensor with respect to $\xi$ is obtained by antisymmetrizing $S^{(1)}$ and $S^{(2)}$ on their last two indices and adding them:

We use the index notation for the antisymmetrization:

An equivalent writing would have been (the last two indices being in position 1 and 2):

The Simon-Mars tensor is

Hence the Simon-Mars tensor is not zero: the Curzon-Chazy spacetime is not locally isomorphic to the Kerr spacetime.

Computation of the Simon-Mars scalars

First we form the "square" of the Simon-Mars tensor:

Then we take the real and imaginary part of this compex scalar field. Because this spacetime is spherically symmetric, we expect that the imaginary part vanishes.

Furthermore we scale those scalars by the ADM mass of the Curzon-Chazy spacetime, which corresponds to $m$:

And we take the log of this quantity

Then we plot the value of this quantity as a function of $\rho = x = r \sqrt{1-y^2}$ and $z = r y$, thereby producing Figure 10 of arXiv:1412.6542: