  where \(ct = x^{0}, x^{0}\) is called the  cosmic time, and \(t\) is the proper time of any galaxy. The spacelike hypsersurfaces are therefore the surfaces of  simultaneity with respect to the cosmic time \(x^{0}\).  The Cosmological Principle in turn tells us that these hypersurfaces  of simultaneity are homogeneous and isotropic. 