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Compact Cosmos

38 ORBITALEVOLUTION and the shape of things to come Ancient stargazers charted the wanderings of skygods across the dome of the firmament. The Earth was at the center of all creation, with celestial spheres encircling the heavens. The Greeks invented epicycles top left to model the dance of the planets. Generations of ponderers refined the cranks and wheels until it was audaciously suggested that the Earth spun about the sun top center. Suddenly planetary motions became much simpler. Nowadays there are many centers,as small things go round bigger ones,all snuggly wrapped in gravitys web and orbiting and dancing around changing common centers of mass top right. Ultimately the fate of the universe rests on the critical density, a balancing act between gravity,holding stuff together,and inflation, pushing things apart. Several possibilities result. An open universe has too little mass for gravity to stop expansion. Like a threedimensional saddle curving out forever, matter dilutes over time and entropy slowly takes its toll, leaving little else but gigantic black holes and fading photons. Where there is more than enough mass to stop expansion, space curls in on itself, resulting in a closed universe which achieves a maximum size and then shrinks,ending its days in a big crunch. Should gravity exactly balance inflation, expansion slows down and a flat universeeventually reaches a steady state. Intriguingly,some theoretical finite flat universes have no edges and wrap back on themselves. Even flying in a perfectly straight line any brave travelers would eventually return home. 38 39
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