... Part 1: Observations of Global Properties . Part 2: Homogeneity and Isotropy; Many Distances; Scale Factor . ... Tutorial : Part 1 | ... Time dilation causes the proper time measured by an observer to depend on the velocity of the observer, so we specify that the time variable t in the Hubble law is the proper time since the Big Bang for comoving observers . ... The space-time diagram below shows a "zero" (really very low) density cosmological model plotted using the D and t of the Hubble law. ...