Physics offers glimpse into the universe’s dark era

January 13, 1997

TORONTO, ONT.—University of Michigan astrophysicists Fred Adams and Greg Laughlin have seen the future and it is dark.

Far in the future, they say, after the stars have burned themselves out, the galaxies dispersed and the black holes radiated away, the universe will be nothing but a vast sea of electrons, positrons, neutrinos and radiation immersed in nearly complete and total blackness.

Adams and Laughlin are not reading the universe’s future in a crystal ball. “The same fundamental physical laws we study today can be used both to understand how the universe began and to explain how it will end,” Adams said. Until now, scientists have been so focused on unlocking the secrets of the universe’s past that few have bothered to think much about what will happen in the future.

At a press conference held here today (Jan. 15) during the American Astronomical Society meeting, Adams and Laughlin described their vision of the long-term fate of the universe. A detailed analysis of their research on the future evolution of astrophysical objects, titled “A Dying Universe,” also will be published in the April 1997 issue of Reviews of Modern Physics.

Understanding the universe’s future requires a fundamental shift in our thinking about time, which Adams—a U-M associate professor of physics and winner of this year’s AAS Warner Prize—calls the “Copernican Time Principle.” “Copernicus taught us that the Earth is not located at the center of the universe,” Adams explained. “It is equally true that our current cosmological epoch has no central place in time. We live in an important time in the universe’s development, but interesting events will continue to occur as long as the universe exists.”

To describe the immense time scales involved in the future evolution of the universe, Adams and Laughlin introduced a convenient new unit of time in their study, which they call a “cosmological decade.” Each cosmological decade represents a tenfold increase in the number of years which have elapsed since the beginning of time. For example, the universe is currently only about 10 cosmological decades or 10 billion years old (10 multiplied by itself 10 times). The most distant epochs considered in the U-M study will not occur until the 200th cosmological decade (10 multiplied by itself 200 times).

In their study, the U-M scientists divide future development of the universe into several distinct periods or eras, including:

The Stelliferous or Star-Filled Era – Cosmological Decades 6- 14.

We live at the mid-point of the Stelliferous Era, a time in the history of the universe when energy is generated by the birth, life and death of stars. In their study, Adams and Laughlin pay special attention to the longevity of the universe’s most ordinary stars—red and white dwarfs.

“Most red dwarfs have considerably less than half the mass of our sun, but they are so numerous that their combined mass is greater than the mass of all the larger stars in the universe,” said Laughlin, a U-M post-doctoral fellow. “Red dwarfs are real misers at burning hydrogen. They hoard their energy and some will still be around 10 trillion years from now when the last sun- like star has long since exhausted its fuel supply and collapsed into a white dwarf.”

The Degenerate Era – Cosmological Decades 15-37.

When star formation and stellar evolution cease, the universe will move into the Degenerate Era. “The only remaining stellar objects will be degenerate stellar remnants, such as white dwarfs, brown dwarfs, neutron stars and black holes,” Laughlin said. During this era, galaxies will begin to relax dynamically with some stellar remnants moving out to the edge of the galaxy and others falling to the center. “An occasional rare burst of energy will be generated when two brown dwarfs collide to create a new low-mass star,” Laughlin added. “But on average this will happen only once every quadrillion years in a galaxy the size of the Milky Way.”

Capture of dark matter or WIMPs (Weakly Interacting Massive Particles) by white dwarfs will provide another low-level energy source for the dying universe, according to Adams. “WIMPs annihilating through collisions in the centers of white dwarfs will produce small amounts of energy. As they are depleted from the galactic halo, however, the energy level in the universe will continue to decline.”

By cosmological decade 30 or so, the supply of dark-matter particles will be exhausted and matter in the universe will be limited to white and brown dwarfs, neutron stars and a few scattered dead planets. Finally, the mass of white dwarfs and neutron stars will begin to dissipate through a process called proton decay.

“A white dwarf fueled by proton decay generates approximately 400 watts—enough to run a few light bulbs,” Laughlin said. “An entire galaxy of these stars would have a total luminosity smaller than one hydrogen-burning star.”

“As protons decay, a large fraction of the ‘ordinary’ mass in the universe is eventually converted into radiation,” Adams said. “The proton decay epoch thus will initiate the most significant change in the future universe.”

The Black Hole Era – Cosmological Decades 38-100.

Although black holes probably will outlive white dwarfs, brown dwarfs and neutron stars, they won’t last forever. Fed by material falling to the center of galaxies during the Degenerate Era, black holes will grow larger for a long time. But even their enormous mass will eventually dissipate in thermal radiation, photons and other decay products through a quantum mechanical process first proposed by Stephen Hawking. “Even a black hole with the mass of a large galaxy will evaporate on a time scale of about 98 to 100 cosmological decades,” Adams said.

The Dark Era – Cosmological Decades 100 and beyond.

“Once the black holes have radiated away, the universe will consist of a diffuse sea of electrons, positrons, neutrinos and radiation,” Adams said. Even though only a few complex particles will exist at this time in the far future of the universe, Adams maintains there is a possibility that interesting things will continue to happen.

“The apparent poverty of this distant epoch is most likely due to our difficulties in extrapolating far enough into the future, rather than an actual dearth of physical processes,” he said.

The U-M research project was supported by the National Science Foundation, NASA and the U-M Department of Physics. Initial work was developed for use in the Winter 1996 U-M undergraduate theme semester, “Death, Extinction and the Future of Humanity,” which was sponsored by the U-M College of Literature, Science, and the Arts.

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