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If not enough dark matter exists to stop and reverse the expansion, our universe will end up either open or flat, surviving in some form forever. Of course, stars will eventually run out of fuel and die, and the gas clouds that give birth to new generations of stars will likewise exhaust their resources. But the smallest stars, a little less than one-tenth the mass of the Sun, stay much cooler than the Sun and forge helium from hydrogen at a much slower pace. Astronomers expect these stars to keep shining for about 100 trillion years, against which our 15-billion-year-old universe is just a blink of an eye. After the stars die, the major components of the universe will be the collapsed remnants of stars—white dwarfs, neutron stars, and black holes. The white dwarfs should capture any WIMPs, so much of the dark matter surrounding galaxies may disappear. Meanwhile, black holes will grow in size as they suck in other objects. Roughly 1040 (10 thousand trillion trillion trillion) years from now, theory predicts that all protons will have decayed, and that implies all the white dwarfs and neutron stars will evaporate. Physicists can go even further than that, however, out to a googol years, or 10100 years! Black holes dominate this period of time, but even they are not forever. As Stephen Hawking has shown, black holes also eventually evaporate, in a time that depends on their mass. By 10100 years, black holes as large as a galaxy will be gone. That leaves us in an era of darkness, with no objects left to ponder. The universe will be just a vast sea of electrons, positrons, photons, and neutrinos—not much to look at even if you could imagine life existing under those conditions. After that, who knows? The physics gets pretty murky at that stage. But it would be a safe bet that the universe won’t end with the same tumult that would mark a Big Crunch.
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