# Which time is there in the FRW metric?

The FRW metric is given by $$ds^2=dt^2-a^2(t)\Big[\frac{dr^2}{1-kr^2}+r^2(d\theta)^2+(r\sin\theta)^2(d\phi)^2\Big].$$ There is a time $t$ sitting in this metric. In which frame is this time measured?

• it's a coordinate time. It's the same as comoving time in comoving frame.
– Kosm
Jun 11 '17 at 15:17
• Is the time $t$ same for all comoving observers?
– SRS
Jun 12 '17 at 6:12
• yes, but there is no difference between any two comoving observers, they are at rest wrt each other
– Kosm
Jun 12 '17 at 6:24
• Well, I would not say that they are at rest which each other (even if in a sense it is true) as they experience for instance the redshift phenomenon from other co-moving observers... therefore there is relative motion also described as the expansion of space. Jun 12 '17 at 12:03
• @Valter Moretti, I agree. They are at rest in a sense that they have 0 peculiar velocity, but redshift is still there
– Kosm
Jun 12 '17 at 18:50