# Homework Help: Initial data/Cauchy problem

1. Dec 15, 2011

### PhyPsy

1. The problem statement, all variables and given/known data
Show that $R_{00}= -\frac{1}{2}g^{\alpha\beta}g_{\alpha\beta,00}+ M_{00}$, where M can be expressed solely in terms of the initial data, which is the metric tensor and its first derivatives.

2. Relevant equations
$R_{ab}= R^c_{acb}= \Gamma^a_{bd,c}- \Gamma^a_{bc,d}$ in geodesic coordinates
$\Gamma^a_{bc}= \frac{1}{2}g^{ad}(g_{dc,b}+ g_{db,c}- g_{bc,d})$

3. The attempt at a solution
$R_{00}= R^a_{0a0}= \Gamma^a_{00,a}- \Gamma^a_{0a,0}$
$R_{00}= (g_{b0,0}- \frac{1}{2}g_{00,b})g^{ab}_{,a}+ g^{ab}g_{b0,a0}- \frac{1}{2}g^{ab}g_{00,ab}- \frac{1}{2}(g_{ba,0}+ g_{b0,a}- g_0a,b)g^{ab}_{,0}- \frac{1}{2}g^{ab}g_{ba,00}- \frac{1}{2}g^{ab}g_{b0,0a}+ \frac{1}{2}g^{ab}g_{0a,0b}$
a and b are summations from 0 to 3, while $\alpha$ and $\beta$ are summations for space coordinates, so they are 1 to 3. Since the one second-derivative term in the expression I am trying to reach is summed over the space coordinates, I need to split one of the terms above. I split $-\frac{1}{2}g^{ab}g_{ba,00}= -\frac{1}{2}(g^{\alpha\beta}g_{\beta\alpha,00}+ g^{a0}g_{0a,00}+ g^{0\beta}g_{\beta0,00})$.
So I define $M_{00}= (g_{b0,0}- \frac{1}{2}g_{00,b})g^{ab}_{,a}- \frac{1}{2}(g_{ba,0}+ g_{b0,a}- g_0a,b)g^{ab}_{,0}$, as all the rest of the terms have second derivatives of the metric tensor, which are not in the initial data.
$R_{00}= -\frac{1}{2}(g^{\alpha\beta}g_{\beta\alpha,00}+ g^{a0}g_{0a,00}+ g^{0\beta}g_{\beta0,00}- g^{ab}g_{b0,a0}+ g^{ab}g_{00,ab}- g^{ab}g_{0a,0b})+ M_{00}$
So now I have to show that $-\frac{1}{2}(g^{a0}g_{0a,00}+ g^{0\beta}g_{\beta0,00}- g^{ab}g_{b0,a0}+ g^{ab}g_{00,ab}- g^{ab}g_{0a,0b})=0$
I have simplified those five terms down to three: $\frac{1}{2}(g^{a\beta}g_{0a,0\beta}- g^{a\beta}g_{00,a\beta}+ g^{\alpha\beta}g_{\beta0,\alpha0})$
I do not know how to simplify that any further. Can someone help?

Last edited: Dec 15, 2011
2. Dec 21, 2011

### PhyPsy

I've got it simplified to $R_{00}= -\frac{1}{2}(g^{\alpha\beta}g_{\beta\alpha,00}- g^{\alpha\beta}g_{0\alpha,0\beta}+ g^{\alpha\beta}g_{00,\alpha\beta}- g^{\alpha\beta}g_{\beta0,\alpha0})+ M_{00}$, but I still have not figured out what to do about those three extra terms. Will anybody help, please?