Relativity Lite

76 | Relativity Lite (4d) So now we must somehow choose which shape (k = 1, 0, or −1) fits our universe. THE BANG One naturally wonders why the universe is expanding at all. Indeed, we can run the movie backward in our imagination, seeing the galaxies coming closer together as we move back- ward in time, as in figure 4. Their initial, yellow, configuration becomes denser as time goes backward, with galaxies overlapping as the universe shrinks to a third of that size, shown in blue. As time runs down to zero, the spacetime containing the matter that makes up the galaxies turns into an infinitely dense dot that would also be extremely hot due to the friction of the jostled particles as they are forced into a smaller and smaller volume. Figure 4. If we run time backward, the galaxies in their initial, yellow, configuration grow closer together, indeed overlapping as the universe shrinks to a third of that size, shown in blue. As time runs back to zero, the density of matter in galaxies becomes infinite and extremely hot.