2.2 The Big Bang 14 2.2 The Big Bang In 1929, Edwin Hubble discovered what is known as Hubble’s Law , which states that there exists a linear relationship between the distance of a galaxy from us and its recession velocity, with the constant of proportionality being Hubble’s Constant , H$_{0}$ , which has been observed using a variety of sources, with the combined data value [40] being Therefore, the further away a galaxy is from our Milky Way, the faster it is traveling away from us. As a result of the Cosmological Principle, Hubble’s Law implies that, over large scales, each galaxy is moving away from every other galaxy. If we consider this fact in reverse time, it is clear that galaxies will get ever closer to each other. At some point in the past, the Universe will be sufficiently dense, hot and energetic that the equations of GR will break down and leave us with a spacetime singularity. It is this point that we label as the beginning of our Universe, which occurred around 13 . 8Gyr ago. (It is not clear whether such a singularity actually existed. Given the extremely high energies, small scales and small time intervals that existed in the very early Universe, we are required to obtain a theory of Quantum Gravity in order to fully explain the physics of this time, which is not yet fully available to us.) We define the Hot Big Bang , hereafter referred to as HBB, to begin at the time when the reheating process from inflation (discussed in Section 3.2.3) is complete. This process must leave us with a radiation-dominated Universe H$_{0}$ = 67 . 74 ± 0 . 46 km s − 1 Mpc − 1 (2.31)