We have mentioned Hubble’s estimate that matter is distributed in space at an average rate of 1·5 × 10⁻³¹ grammes per cubic centimetre. The annihilation of a gramme of matter liberates 9 × 10²⁰ ergs of energy, so that the annihilation of 1·5 × 10⁻³¹ grammes of matter liberates 1·35 × 10⁻¹⁰ ergs of energy. It follows that the total annihilation of all the substance of the existing universe would only fill space with energy at the rate of 1·35 × 10⁻¹⁰ ergs per cubic centimetre. This amount of energy is only enough to raise the temperature of space from absolute zero to a temperature far below that of liquid air; it would only raise the temperature of the earth’s surface by a 6000th part of a degree Centigrade. The reason why the effect of annihilating a whole universe is so extraordinarily slight is of course that space is so extraordinarily empty of matter; trying to warm space by annihilating all the matter in it is like trying to warm a room by burning a speck of dust here and a speck of dust there. As compared with any amount of radiation that is ever likely to be poured into it, the capacity of space is that of a bottomless pit. Indeed, so far as scientific observation goes, it is entirely possible that the radiation of thousands of dead universes may even now be wandering round space without our suspecting it. Such is the final end of things to which, so far as present-day science can see, the material universe must inevitably come in some far-off age, unless the course of nature is changed in the meantime. Let us now try to peer back towards the beginnings of things. THE BEGINNINGS OF THE UNIVERSE As we go forwards in time, material weight continually changes into radiation. Conversely, as we go backwards in time, the total material weight of the universe must continually increase. We have seen how the present weights of the stars are incompatible with their having existed for more than some 5 or 10 million million years, and that they would need approximately the whole of this enormous period to acquire certain signs of age which their present arrangement and motions reveal. We have seen that the break-up of the huge extra-galactic nebulae must result in the birth of stars, and have found that the most consistent account of the origin of the galactic system of stars is provided by the supposition that the whole system originated out of the break-up of a single huge nebula some 5 to 10 million million years ago.