## Coordinate Dependent Statements in an Expanding Universe

In this Insight we will discuss the general Friedmann–Lemaitre–Robertson–Walker (FLRW) universe, in which a set of comoving observers with proper time ##t## observe the universe to be homogeneous and isotropic. In such a universe, the line element takes the form $$\newcommand{\dd}[2]{\frac{\partial #1}{\partial #2}}ds^2 = dt^2 – a(t)^2 [dr^2 + f(r)^2 d\Omega^2] \equiv dt^2 – a(t)^2 […]