The Dark Era and beyond

Log (Temperature/ Kelvin)

^ Mars mass black hole

-MS (ooKng Back \ V- \VD tMh„6 track solar mass N 5 ■ ^ Solarmass WD (mfctftn composition^

Minimum mass MS -'___________a '125 Salar! Wo Mmposilion)


_Jhy<itogen composition) Jovian mass {Degeneracy liftcd)

^ Mars mass black hole

The frozen earth

Endpomt of stellar evolution--

Fig. 2.2 This plot shows the long-term evolution of cold degenerate stars in the H-R diagram. After completing the early stages of stellar evolution, white dwarfs and neutron stars cool to an equilibrium temperature determined by proton decay. This figure assumes that proton decay is driven by gravity (microscopic black holes) on a time scale of 10 years. The white dwarf models are plotted at successive twofold decrements in mass. The mean stellar density (in log[p/g]) is indicated by the grey scale shading, and the sizes of the circles are proportional to stellar radius. The relative size of the Earth and its position on the diagram are shown for comparison. The evaporation of a neutron star, starting with one solar mass, is illustrated by the parallel sequence, which shows the apparent radial sizes greatly magnified for clarity. The Hawking radiation sequence for black holes is also plotted. The arrows indicate the direction of time evolution. (Reprinted with permission from Adams, F.C., Laughlin, G., Mbonye, M., and Perry, M.J. (1998). Gravitational demise of cold degenerate stars. Phys. Rev. D, 58, 083003.)

When the cosmic age exceeds 10100 years, the black holes will be gone and the cosmos will be filled with the leftover waste products from previous eras: neutrinos, electrons, positrons, dark matter particles, and photons of incredible wavelength. In this cold and distant Dark Era, physical activity in the universe slows down, almost (but not quite) to a standstill. The available energy is limited and the expanses of time are staggering, but the universe doggedly continues to operate. Chance encounters between electrons and positrons can forge positronium atoms, which are exceedingly rare in an accelerating universe. In addition, such atoms are unstable and eventually decay. Other low-level annihilation events also take place, for example, between any surviving dark matter particles. In the poverty of this distant epoch, the generation of energy and entropy becomes increasingly difficult.

At this point in the far future, predictions of the physical universe begin to lose focus. If we adopt a greater tolerance for speculation, however, a number of possible events can be considered. One of the most significant potential events is that the vacuum state of the universe could experience a phase transition to a lower energy state. Our present-day universe is observed to be accelerating, and one possible implication of this behaviour is that empty space has a nonzero energy associated with it. In other words, empty space is not really empty, but rather contains a positive value of vacuum energy. If empty space is allowed to have a non-zero energy (allowed by current theories of particle physics), then it remains possible for empty space to have two (or more) different accessible energy levels. In this latter case, the universe could make a transition from its current (high energy) vacuum state to a lower-energy state sometime in the future (the possibility of inducing such a phase transition is discussed in Chapter 16). As the universe grows increasingly older, the probability of a spontaneous transition grows as well. Unfortunately, our current understanding of the vacuum state of the universe is insufficient to make a clear predictions on this issue - the time scale for the transition remains enormously uncertain. Nonetheless, such a phase transition remains an intriguing possibility. If the universe were to experience a vacuum phase transition, it remains possible (but is not guaranteed) that specific aspects of the laws of physics (e.g., the masses of the particles and/or the strengths of the forces) could change, thereby giving the universe a chance for a fresh start.

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