- #1
Frank Castle
- 580
- 23
I’m hoping to clear up some confusion I have over what the Lie derivative of a metric determinant is.
Consider a 4-dimensional (pseudo-) Riemannian manifold, with metric ##g_{\mu\nu}##. The determinant of this metric is given by ##g:=\text{det}(g_{\mu\nu})##. Given this, now consider the quantity ##\sqrt{-g}##. Is it correct that the Lie derivative of this, with respect to a vector field ##X##, is given by $$\mathcal{L}_{X}(\sqrt{-g})=X^{\mu}\partial_{\mu}(\sqrt{-g})$$ The reason I’m unsure about this is that ##\sqrt{-g}## is technically a scalar density rather than just a simple scalar, and so I’m unsure whether or not it behaves like a scalar as far as the Lie (and covariant for that matter) derivative is concerned.
Consider a 4-dimensional (pseudo-) Riemannian manifold, with metric ##g_{\mu\nu}##. The determinant of this metric is given by ##g:=\text{det}(g_{\mu\nu})##. Given this, now consider the quantity ##\sqrt{-g}##. Is it correct that the Lie derivative of this, with respect to a vector field ##X##, is given by $$\mathcal{L}_{X}(\sqrt{-g})=X^{\mu}\partial_{\mu}(\sqrt{-g})$$ The reason I’m unsure about this is that ##\sqrt{-g}## is technically a scalar density rather than just a simple scalar, and so I’m unsure whether or not it behaves like a scalar as far as the Lie (and covariant for that matter) derivative is concerned.