Plugging in a given metric ansatz

[Claire]

Hello,

I am trying to write a simple notebook to compute the Kubo formula in an isotropic background. This computation involves making use of a metric ansatz, however, in the final expression for the energy momentum tensor I am left with symbolic inverse metrics, and  this computation takes a long time (due to these terms, I suspect) such that I have to break up the evaluation into several lines. 

In other projects I have, I successfully used the 'ReplaceMetric' function in the notebook to plug in a given metric for more complicated (e.g. black hole) backgrounds and compute the Einstein equations, and I never had this issue of being left with symbolic inverse metric functions, so I am not sure what's going wrong here. 

Best,

Claire