As framed by a well-known poem by Robert Frost, the world could end either in fire or in ice. In the astronomical context considered here, Earth has only a small chance of escaping the fiery wrath of the red giant Sun by becoming dislodged from its orbit and thrown out into the icy desolation of deep space. Our particular world is thus likely to end its life in fire. Given that humanity has a few billion years to anticipate this eventuality, one can hope that migration into space could occur, provided that the existential disasters outlined in other chapters of this book can be avoided. One alternative is for a passing star to wander near the inner portion of our solar system. In this unlikely event, the disruptive gravitational effects of the close encounter could force Earth to abandon its orbit and be exiled from the solar system. In this case, our world would avoid a scalding demise, but would face a frozen future.

A similar fate lies in store for the Sun, the Galaxy, and the Universe. At the end of its life as an ordinary star, the Sun is scheduled to become a white dwarf. This stellar remnant will grow increasingly cold and its nuclei will atrophy to lower atomic numbers as the constituent protons decay. In the long run, the Sun will end up as a small block of hydrogen ice. As it faces its demise, our Galaxy will gradually evaporate, scattering its stellar bodies far and wide. The effective temperature of a stellar system is given by the energies of its stellar orbits. In the long term, these energies will fade to zero and the galaxy will end its life in a cold state. For the universe as a whole, the future is equally bleak, but far more drawn out. The currently available astronomical data indicate that the universe will expand forever, or at least for long enough that the timeline outlined above can play itself out. As a result, the cosmos, considered as a whole, is likely to grow ever colder and face an icy death.

In the beginning, starting roughly 14 billion years ago, the early universe consisted of elementary particles and radiation - essentially because the background was too hot for larger structures to exist. Here we find that the universe of the far future will also consist of elementary particles and radiation - in this case because the cosmos will be too old for larger entities to remain intact. From this grand perspective, the galaxies, stars, and planets that populate the universe today are but transient phenomena, destined to fade into the shifting sands of time. Stellar remnants, including the seemingly resilient black holes, are also scheduled to decay. Even particles as fundamental as protons will not last forever. Ashes to ashes, dust to dust, particles to particles -such is the ultimate fate of our universe.

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