- #1
center o bass
- 560
- 2
When working with light-propagation in the FRW metric
$$ds^2 = - dt^2 + a^2 ( d\chi^2 + S_k(\chi) d\Omega^2)$$
most texts just set $$ds^2 = 0$$ and obtain the equation
$$\frac{d\chi}{dt} = - \frac{1}{a}$$
for a light-ray moving from the emitter to the observer.
Question1: Do we not strictly speaking also have to check that the above equation actually specifies a geodesic?
Setting ##ds^2 = 0## does not automatically guarantee that the obtained relation specifies a geodesic, right?
Question2: Is there a quick way to verify that the above curve indeed is a null-geodesic?
$$ds^2 = - dt^2 + a^2 ( d\chi^2 + S_k(\chi) d\Omega^2)$$
most texts just set $$ds^2 = 0$$ and obtain the equation
$$\frac{d\chi}{dt} = - \frac{1}{a}$$
for a light-ray moving from the emitter to the observer.
Question1: Do we not strictly speaking also have to check that the above equation actually specifies a geodesic?
Setting ##ds^2 = 0## does not automatically guarantee that the obtained relation specifies a geodesic, right?
Question2: Is there a quick way to verify that the above curve indeed is a null-geodesic?