1 stop of DR -how important it is and for what kind of photography

Search Wikipedia for FLRW. In the first formula, for light, tau=0. Then by simply changing the letter used for the space component with r, in the direction of light we get:

c^2 * dt^2 = a(t)^2 * dr^2

Assuming everything is positive, the speed of light at different times of the universe expansion is:

dr/dt = c / a(t)

Where c is the speed of light at the arbitrary present moment when the scale factor a(t) = 1 by definition. However, the above result shows that the actual speed of light depends on time. This is contradictory to the meaning of the constancy of the speed of light as space-time symmetry. Therefore the Friedman metric is incorrect. It was derived under the most general assumptions of spherical spatial symmetry, but it failed to take the space-time symmetry into account.

Another way to state this symmetry is to say that the constancy go the speed of light is a reflection of the space isotropy. You can google for references on this.

I am questioning the metric itself before it is used to solve the equations of general relativity. Thanks!