Even though the universe is only 13.8 billion years old, more or less, the era of star formation is nearly complete. Because our universe expands so rapidly, the effects of gravity continue to lessen and therefore matter will soon cease to clump. Existing stars and galaxies will become more and more isolated. In a hundred billion years, the universe will be a thousand times larger than it is now. After a trillion years, all the stars will have burnt themselves out, except for slow-burning, feeble red dwarfs. In a hundred trillion years the Degenerate Era arrives and the universe will be populated only by white dwarfs and brown dwarfs, along with neutron stars and black holes, all fainter and fainter with time. It will all be dark, except when a rare supernova erupts. After 1034 years, atoms themselves will decay into protons and neutrons. In 1040 years, all of what we think of as matter will have become photons and leptons. Later, black holes themselves will leak photons and disappear. It's the Dark Era: just random electrons and positrons that rarely encounter each other – an inactive universe that suffers from Heat Death. It's all dark, dead, and cold. This sorry state will continue for all eternity.
it's a gloomy scenario, but I don't believe it. It doesn't make sense. And it doesn't account for Dark Matter and Dark Energy, which no one knows anything about except that they must exist perhaps in the form of neutrinos. Most physicists seem to believe that the visible, palpable universe, which contains the galaxies and the stars and the planets and you and me, is only about 6% of the whole kit and kaboodle.
My own theory, unsupported by observation, evidence, or calculation, is that the universe will continue to expand for billions of years, and then Dark Matter will kick in, gravity will plump up and spring to new life, and the universe will start to contract. A few thousand billion years further on and here comes the Big Crunch, when all matter will contract to a point and then, mirabile dictu, another Big Bang and it's off to the races. Expand. Wait a few thousand trillion years. Contract. More years — expand. Contract. Expand. Again and again. My universe pulses, for all eternity. Et saecula saeculorum; in aeternum et ultra. No beginning, no end.
The scientific consensus universe is this: start with the Big Bang, end with the eternal Dark Era. It's one-way ticket. My universe is round-trip. Multiple, infinite round-trips. The astronomers have the data and the numbers and theory on their side, but they can't explain how it all happened, or why? Why the Big Bang? What was going on before all the Banging began? They don't have a glimmer of an idea, not a clue; in fact, they punt and claim that it's impossible to know what happened in the first few fractions of seconds. But my universe has no start and no stop. It just is.
Neither their theory nor mine answers the implicit question, which some might call philosophical or theological: "why is there something rather than nothing?" But my idea has the advantage in that it defers that question indefinitely. "What happened at the Big Bang? "Well, what happened is what always happens at Big Bangs."
(Information about the end of the universe is adapted from David J. Eicher, The New Cosmos (Cambridge University Press, 2015), an excellent introduction that is written so lucidly that I understood maybe 60% to 70% of the whole. Well, perhaps a little less. It's not Eicher's fault if I've gotten anything wrong.)
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