The era of degenerate remnants

After the stars burn out and star formation shuts down, a significant fraction of the ordinary mass will be bound within the degenerate remnants that remain after stellar evolution has run its course. For completeness, however, one should keep in mind that the majority of the baryonic matter will remain in the form of hot gas between galaxies in large clusters (Nagamine and Loeb, 2004). At this future time, the inventory of degenerate objects includes brown dwarfs, white dwarfs, and neutron stars. In this context, degeneracy refers to the state of the high-density material locked up in the stellar remnants. At such enormous densities, the quantum mechanical exclusion principle determines the pressure forces that hold up the stars. For example, when most stars die, their cores shrink to roughly the radial size of Earth. With this size, the density of stellar material is about one million times greater than that of the Sun, and the pressure produced by degenerate electrons holds up the star against further collapse. Such objects are white dwarfs and they will contain most of the mass in stellar bodies at this epoch. Some additional mass is contained in brown dwarfs, which are essentially failed stars that never fuse hydrogen, again owing to the effects of degeneracy pressure. The largest stars, those that begin with masses more than eight times that of the Sun, explode at the end of their lives as supernovae. After the explosion, the stellar cores are compressed to densities about one quadrillion times that of the Sun. The resulting stellar body is a neutron star, which is held up by the degeneracy pressure of its constituent neutrons (at such enormous densities, typically a few x 1015 g/cm3, electrons and protons combine to form neutrons, which make the star much like a gigantic atomic nucleus). Since only three or four out of every thousand stars are massive enough to produce a supernova explosion, neutron stars will be rare objects.

During this Degenerate Era, the universe will look markedly different from the way it appears now. No visible radiation from ordinary stars will light up the skies, warm the planets, or endow the galaxies with the faint glow they have today. The cosmos will be darker, colder, and more desolate. Against this stark backdrop, events of astronomical interest will slowly take place. As dead stars trace through their orbits, close encounters lead to scattering events, which force the galaxy to gradually readjust its structure. Some stellar remnants are ejected beyond the reaches of the galaxy, whereas others fall in toward the centre. Over the next 1020 years, these interactions will enforce the dynamical destruction of the entire galaxy (e.g., BinneyandTremaine, 1987; Dyson, 1979).

In the meantime, brown dwarfs will collide and merge to create new low-mass stars. Stellar collisions are rare because the galaxy is relentlessly empty. During this future epoch, however, the universe will be old enough so that some collisions will occur, and the merger products will often be massive enough to sustain hydrogen fusion. The resulting low-mass stars will then burn for trillions of years. At any given time, a galaxy the size of our Milky Way will harbour a few stars formed through this unconventional channel (compare this stellar population with the ~100 billion stars in the Galaxy today).

Along with the brown dwarfs, white dwarfs will also collide at roughly the same rate. Most of the time, such collisions will result in somewhat larger white dwarfs. More rarely, white dwarf collisions produce a merger product with a mass greater than the Chandrasekhar limit. These objects will result in a supernova explosion, which will provide spectacular pyrotechnics against the dark background of the future galaxy.

White dwarfs will contain much of the ordinary baryonic matter in this future era. In addition, these white dwarfs will slowly accumulate weakly interacting dark matter particles that orbit the galaxy in an enormous diffuse halo. Once trapped within the interior of a white dwarf, the particles annihilate with each other and provide an important source of energy for the cosmos. Dark matter annihilation will replace conventional nuclear burning in stars as the dominant energy source. The power produced by this process is much lower than that produced by nuclear burning in conventional stars. White dwarfs fuelled by dark matter annihilation produce power ratings measured in quadrillions of Watts, roughly comparable to the total solar power intercepted by Earth (~1017 Watts). Eventually, however, white dwarfs will be ejected from the galaxy, the supply of the dark matter will get depleted, and this method of energy generation must come to an end.

Although the proton lifetime remains uncertain, elementary physical considerations suggest that protons will not live forever. Current experiments show that the proton lifetime is longer than about 1O33 years (Super-Kamiokande Collaboration, 1999), and theoretical arguments (Adams et al., 1998; Ellis et al., 1983; Hawking et al., 1979; Page, 1980; Zeldovich, 1976) suggest that the proton lifetime should be less than about 1045 years. Although this allowed range of time scales is rather large, the mass-energy stored within white dwarfs and other degenerate remnants will eventually evaporate when their constituent protons and neutrons decay. As protons decay inside a white dwarf, the star generates power at a rate that depends on the proton lifetime. For a value near the centre of the (large) range of allowed time scales (specifically 1037 years), proton decay within a white dwarf generates approximately 400 Watts of power - enough to run a few light bulbs. An entire galaxy of these stars will appear dimmer than our present-day Sun. The process of proton decay converts the mass energy of the particles into radiation, so the white dwarfs evaporate away. As the proton decay process grinds to completion, perhaps 1040 years from now, all of the degenerate stellar remnants disappear from the universe. This milestone marks a definitive end to life as we know it as no carbon-based life can survive the cosmic catastrophe induced by proton decay. Nonetheless, the universe continues to exist, and astrophysical processes continue beyond this end of known biology.

Continue reading here: The era of black holes

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