A Long Night Ahead for the Universe
(From the March/April 1997 issue of StarDate magazine)
One by one, the stars will wink out. The biggest will go first, blowing themselves to bits or collapsing to form black holes. Average stars like our Sun will flare up for awhile, then fade away. Trillions of years later, the tiniest stars of all will end their lives, too. And even then, our universe will be no more than an infant.
Galaxies will disappear, stellar corpses disintegrate, black holes evaporate in great flashes of energy. Eventually, the universe will grow dark and cold. But it will go on.
That's the scenario described recently by University of Michigan astronomers Greg Laughlin and Fred Adams.
"As we approach the end of the millennium, I would like to make the point that the end is not really that near," Adams joked during a January meeting of the American Astronomical Society.
The universe probably began with the Big Bang about 10 billion to 15 billion years ago. The first stars and galaxies appeared soon afterward. Our solar system was born 4.5 billion years ago, and the Sun is expected to remain in its present form for at least five billion years more.
While such time scales are mind-boggling, they may represent less than a single tick on the cosmological clock, the astronomers say. They devised a new scale for describing the life of the universe, based on a "cosmological decade." Each decade is 10 times longer than the preceding decade. The universe has completed about 10 of these decades, or 1010 years -- a one followed by 10 zeroes.
If the universe contains enough matter, and the matter is packed tightly enough, gravity will someday pull the universe inward, resulting in a "big crunch" -- the opposite of the Big Bang. But if the universe is spread thinly enough, it will continue to expand forever, perhaps following the path outlined by the Michigan astronomers.
In a related study, Adams, Laughlin, and P. Bodenheimer (also of the University of Michigan), say the lowest-mass stars will live for about 10 trillion (1013) years -- roughly a thousand times longer than the current age of the universe. Such stars are called red dwarfs, and they outnumber all of the other types of stars combined. Proxima Centauri, the nearest star to the solar system, is a red dwarf. Long after the Milky Way galaxy's other stars have expired, red dwarfs will continue to shine meekly, like cosmic night lights in a great city gone dark.
The Milky Way and the other galaxies will continue, but they will hardly resemble the present-day "island universes" of stars. They will contain only crushed stellar corpses (white dwarfs and neutron stars), black holes, and brown dwarfs -- objects that were too small to form planets. After the "normal" stars all expire, a new star may flare into existence once every million billion years or so as two brown dwarfs ram into each other.
Later still, galaxies will lose their grip on their dead stars. Some of the stars will sail off into space, while others fall into the giant black holes that will dominate the hearts of all the galaxies. By about 1038 years from now, all the stellar corpses will disintegrate as their atoms fall apart.
Even the monstrous black holes in the hearts of galaxies will not last forever. Thanks to a trick of quantum physics, they will evaporate in great explosions of energy, punctuating the dark universe like Fourth-of-July firecrackers. The last of these black holes should disappear by about 10100 years from now.
If the same laws of physics that control the present-day universe remain in effect, the universe will contain only electrons, positrons (the antimatter counterparts to electrons), neutrinos (subatomic particles with little or no mass), and radiation.
And still, without stars to light the eternal dark or galaxies spinning in the void, the universe will go on.
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