As we take a longer-term view of our future, a host of astrophysical processes are waiting to unfold as the Earth, the Sun, the Galaxy, and the Universe grow increasingly older. The basic astronomical parameters that describe our universe have now been measured with compelling precision. Recent observations of the cosmic microwave background radiation show that the spatial geometry of our universe is flat (Spergel et al., 2003). Independent measurements of the red-shift versus distance relation using Type Ia supernovae indicate that the universe is accelerating and apparently contains a substantial component of dark vacuum energy (Garnavich et al., 1998; Perlmutter et al., 1999; Riess et al., 1998). 1 This newly consolidated cosmologicalmodel represents an important milestone in our understanding of the cosmos. With the cosmological parameters relatively well known, the future evolution of our universe can now be predicted with some degree of confidence (Adams and Laughlin, 1997). Our best astronomical data imply that our universe will expand forever or at least live long enough for a diverse collection of astronomical events to play themselves out.
Other chapters in this book have discussed some sources of cosmic intervention that can affect life on our planet, including asteroid and comet impacts (Chapter 11, this volume) and nearby supernova explosions with their accompanying gamma-rays (Chapter 12, this volume). In the longer-term future, the chances of these types of catastrophic events will increase. In addition, taking an even longer-term view, we find that even more fantastic events could happen in our cosmological future. This chapter outlines some of the astrophysical events that can affect life, on our planet and perhaps elsewhere, over extremely long time scales, including those that vastly exceed the current age of the universe.
These projections are based on our current understanding of astronomy and the laws of physics, which offer a firm and developing framework for understanding the future of the physical universe (this topic is sometimes called Physical Eschatologgy – see the review of Ćirković, 2003). Notice that as we delve deeper into the future, the uncertainties of our projections must necessarily grow. Notice also that this discussion is based on the assumption that the laws of physics are both known and unchanging; as new physics is discovered, or if the physicalconstants are found to be time dependent, this projection into the future must be revised accordingly.
One issue of immediate importance is the fate of Earth’s biosphere and, on even longer time scales, the fate of the planet itself. As the Sun grows older, it burns hydrogen into helium. Compared to hydrogen, helium has a smaller partial pressure for a given temperature, so the central stellar core must grow hotter as the Sun evolves. As a result, the Sun, like all stars, is destined to grow brighter as it ages. When the Sun becomes too bright, it will drive a runaway greenhouse effect through the Earth’s atmosphere (Kasting et al., 1988). This effect is roughly analogous to that of global warming driven by greenhouse gases (see Chapter 13, this volume), a peril that our planet faces in the near future; however, this later-term greenhouse effect will be much more severe. Current estimates indicate that our biosphere will be essentially sterilized in about 3.5 billion years, so this future time marks the end of life on Earth. The end of complex life may come sooner, in 0.9-1.5 billion years owing to the runaway greenhouse effect (e.g., Caldeira and Kasting, 1992).
The biosphere represents a relatively small surface layer and the planet itself lives comfortably through this time of destruction. Somewhat later in the Sun’s evolution, when its age reaches 11-12 billion years, it eventually depletes its store of hydrogen in the core region and must readjust its structure (Rybicki and Denis, 2001; Sackmann et al., 1993). As it does so, the outer surface of the star becomes somewhat cooler, its colour becomes a brilliant red, and its radius increases. The red giant Sun eventually grows large enough to engulf the radius of the orbit of Mercury, and that innermost planet is swallowed with barely a trace left. The Sun grows further, overtakes the orbit of Venus, and then accretes the second planet as well. As the red giant Sun expands, it loses mass so that surviving planets are held less tightly in their orbits. Earth is able to slip out to an orbit of larger radius and seemingly escape destruction. However, the mass loss from the Sun provides a fluid that the Earth must plough through as it makes its yearly orbit. Current calculations indicate that the frictional forces acting on Earth through its interaction with the solar outflow cause the planet to experience enough orbital decay that it is dragged back into the Sun. Earth is thus evaporated, with its legacy being a small addition to the heavy element supply of the solar photosphere. This point in future history, approximately 7 billion years from now, marks the end of our planet.
Given that the biosphere has at most only 3.5 billion years left on its schedule, and Earth itself has only 7 billion years, it is interesting to ask what types of ‘planet-saving’ events can take place on comparable time scales. Although the odds are not good, the Earth has some chance of being ‘saved’ by being scattered out of the solar system by a passing star system (most of which are binary stars). These types of scattering interactions pose an interesting problem in solar system dynamics, one that can be addressed with numerical scattering experiments. A large number of such experiments must be run because the systems are chaotic, and hence display sensitive dependence on their initial conditions, and because the available parameter space is large. Nonetheless, after approximately a half million scattering calculations, an answer can be found: the odds of Earth being ejected from the solar system before it is accreted by the red giant Sun is a few parts in 105 (Laughlin and Adams, 2000).
Although sending the Earth into exile would save the planet from eventual evaporation, the biosphere would still be destroyed. The oceans would freeze within a few million years and the only pockets of liquid water left would be those deep underground. The Earth contains an internal energy source – the power produced by the radioactive decay of unstable nuclei. This power is about 10,000 times smaller than the power that Earth intercepts from the present-day Sun, so it has little effect on the current operation of the surface biosphere. If Earth were scattered out of the solar system, then this internal power source would be the only one remaining. This power is sufficient to keep the interior of the planet hot enough for water to exist in liquid form, but only at depths 14 km below the surface. This finding, in turn, has implications for present-day astronomy: the most common liquid water environments may be those deep within frozen planets, that is, those that have frozen water on their surfaces and harbour oceans of liquid water below. Such planets may be more common than those that have water on their surface, like Earth, because they can be found in a much wider range of orbits about their central stars (Laughlin and Adams, 2000).
In addition to saving the Earth by scattering it out of the solar system, passing binaries can also capture the Earth and thereby allow it to orbit about a new star. Since most stars are smaller in mass than our Sun, they live longer and suffer less extreme red giant phases. (In fact, the smallest stars with less than one-fourth of the mass of the Sun will never become red giants – Laughlin et al., 1997.) As a result, a captured Earth would stand a better chance of long-term survival. The odds for this type of planet-saving event taking place while the biosphere remains intact are exceedingly slim – only about one in three million (Laughlin and Adams, 2000), roughly the odds of winning a big state lottery.
For completeness, we note that in addition to the purely natural processes discussed here, human or other intentional intervention could potentially change the course of Earth’s orbit given enough time and other resources. As a concrete example, one could steer an asteroid into the proper orbit so that gravitational scattering effectively transfers energy into the Earth’s orbit, thereby allowing it to move outward as the Sun grows brighter (Korycansky et al., 2001). In this scenario, the orbit of the asteroid is chosen to encounter both Jupiter and Saturn, and thereby regain the energy and angular momentum that it transfers to Earth. Many other scenarios are possible, but the rest of this chapter will focus on physical phenomena not including intentional actions.
Because the expansion rate of the universe is starting to accelerate (Garnavich et al., 1998; Perlmutter et al., 1999; Riess et al., 1998), the formation of galaxies, clusters, and larger cosmic structures is essentially complete. The universe is currently approaching a state of exponential expansion and growing cosmological fluctuations will freeze out on all scales. Existing structures will grow isolated. Numerical simulations illustrate this trend (Fig. 2.1) and show how the universe will break up into a collection of ‘island universes’, each containing one bound cluster or group of galaxies (Busha et al., 2003; Nagamine and Loeb, 2003). In other words, the largest gravitationally bound structures that we see in the universe today are likely to be the largest structures that ever form. Not only must each group of galaxies (eventually) evolve in physical isolation, but the relentless cosmic expansion will stretch existing galaxy clusters out of each others’ view. In the future, one will not even be able to see the light from galaxies living in other clusters. In the case of the Milky Way, only the Local Group of Galaxies will be visible. Current observations and recent numerical studies clearly indicate that the nearest large cluster-Virgo – does not have enough mass for the Local Group to remain bound to it in the future (Busha et al., 2003; Nagamine and Loeb, 2003). This local group consists of the Milky Way, Andromeda, and a couple of dozen dwarf galaxies (irregulars and spheroidals). The rest of the universe will be cloaked behind a cosmological horizon and hence will be inaccessible to future observation.
Fig. 2.1 Numerical simulation of structure formation in an accelerating universe with dark vacuum energy. The top panel shows a portion of the universe at the present time (cosmic age 14 Gyr). The boxed region in the upper panel expands to become the picture in the central panel at cosmic age 54 Gyr. The box in the central panel then expands to become the picture shown in the bottom panel at cosmic age 92 Gyr. At this future epoch, the galaxy shown in the centre of the bottom panel has grown effectively isolated. (Simulations reprinted with permission from Busha, M.T., Adams, F.C., Evrard, A.E., and Wechsler, R.H. (2003). Future evolution of cosmic structure in an accelerating universe. Astrophys. J., 596, 713.)
Within their clusters, galaxies of ten pass near each other and distort each other’s structure with their strong gravitational fields. Sometimes these interactions lead to galactic collisions and merging. A rather important example of such a collision is coming up: the nearby Andromeda galaxy is headed straight for our Milky Way. Although this date with our sister galaxy will not take place for another 6 billion years or more, our fate is sealed – the two galaxies are a bound pair and will eventually merge into one (Peebles, 1994).
When viewed from the outside, galactic collisions are dramatic and result in the destruction of the well-defined spiral structure that characterizes the original galaxies. When viewed from within the galaxy, however, galactic collisions are considerably less spectacular. The spaces between stars are so vast that few, if any, stellar collisions take place. One result is the gradual brightening of the night sky, by roughly a factor of 2. On the other hand, galactic collisions are frequently associated with powerful bursts of star formation. Large clouds of molecular gas within the galaxies merge during such collisions and produce new stars at prodigious rates. The multiple supernovae resulting from the deaths of the most massive stars can have catastrophic consequences and represent a significant risk to any nearby biosphere (see Chapter 12, this volume), provided that life continues to thrive in thin spherical layers on terrestrial planets.
With its current age of 14 billion years, the universe now lives in the midst of a Stelliferous Era, an epoch when stars are actively forming, living, and dying. Most of the energy generated in our universe today arises from nuclear fusion that takes place in the cores of ordinary stars. As the future unfolds, the most common stars in the universe – the low-mass stars known as red dwarfs – play an increasingly important role. Although red dwarf stars have less than half the mass of the Sun, they are so numerous that their combined mass easily dominates the stellar mass budget of the galaxy. These red dwarfs are parsimonious when it comes to fusing their hydrogen into helium. By hoarding their energy resources, they will still be shining trillions of years from now, long after their larger brethren have exhausted their fuel and evolved into white dwarfs or exploded as supernovae. It has been known for a long time that smaller stars live much longer than more massive ones owing to their much smaller luminosities. However, recent calculations show that red dwarfs live even longer than expected. In these small stars, convection currents cycle essentially all of the hydrogen fuel in the star through the stellar core, where it can be used as nuclear fuel. In contrast, our Sun has access to only about 10% of its hydrogen and will burn only 10% of its nuclear fuel while on the main sequence. A small star with 10% of the mass of the Sun thus has nearly the same fuel reserves and will shine for tens of trillions of years (Laughlin et al., 1997). Like all stars, red dwarfs get brighter as they age. Owing to their large population, the brightening of red dwarfs nearly compensates for the loss of larger stars, and the galaxy can maintain a nearly constant luminosity for approximately one trillion years (Adams et al., 2004).
Even small stars cannot live forever, and this bright stellar era comes to a close when the galaxies run out of hydrogen gas, star formation ceases, and the longest-lived red dwarfs slowly fade into oblivion. As mentioned earlier, the smallest stars will shine for trillions of years, so the era of stars would come to an end at a cosmic age of several trillion years if new stars were not being manufactured. In large spiral galaxies like the Milky Way, new stars are being made from hydrogen gas, which represents the basic raw material for the process. Galaxies will continue to make new stars as long as the gas supply holds out. If our Galaxy were to continue forming stars at its current rate, it would run out of gas in ‘only’ 10–20 billion years (Kennicutt et al., 1994), much shorter than the lifetime of the smallest stars. Through conservation practices-the star formation rate decreases as the gas supply grows smaller – galaxies can sustain normal star formation for almost the lifetime of the longest-lived stars (Adams and Laughlin, 1997; Kennicutt et al., 1994). Thus, both stellar evolution and star formation will come to an end at approximately the same time in our cosmic future. The universe will be about 100 trillion (1014) years old when the stars finally stop shining. Although our Sun will have long since burned out, this time marks an important turning point for any surviving biospheres – the power available is markedly reduced after the stars turn off.
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 afew 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., Binney and Tremaine, 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 1033 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.
After the protons decay, the universe grows even darker and more rarefied. At this late time, roughly when the universe is older than 1045 years, the only stellar-like objects remaining are black holes. They are unaffected by proton decay and slide unscathed through the end of the previous era. These objects are often defined to be regions of space-time with such strong gravitational fields that even light cannot escape from their surfaces. But at this late epoch, black holes will be the brightest objects in the sky. Thus, even black holes cannot last forever. They shine ever so faintly by emitting a nearly thermal spectrum of photons, gravitons, and other particles (Hawking, 1974). Through this quantum mechanical process – known as Hawking radiation – black holes convert their mass into radiation and evaporate at a glacial pace (Fig. 2.2). In the far future, black holes will provide the universe with its primary source of power.
Although their energy production via Hawking radiation will not become important for a long time, the production of black holes, and hence the black hole inventory of the future, is set by present-day (and past) astrophysical processes. Every large galaxy can produce millions of stellar black holes, which result from the death of the most massive stars. Once formed, these black holes will endure for up to 1070 years. In addition, almost every galaxy harbours a super-massive black hole anchoring its centre; these monsters were produced during the process of galaxy formation, when the universe was only a billion years old, or perhaps even younger. They gain additional mass with time and provide the present-day universe with accretion power. As these large black holes evaporate through the Hawking process, they can last up to 10100 years. But even the largest black holes must ultimately evaporate. This Black Hole Era will be over when the largest black holes have made their explosive exits from our universe.
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 1045 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 non-zero 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.
The discussion in this chapter has focused on physical processes that can take place in the far future. But what about life? How far into the future can living organisms survive? Although this question is of fundamental importance and holds enormous interest, our current understanding of biology is not sufficiently well developed to provide a clear answer. To further complicate matters, protons must eventually decay, as outlined above, so that carbon-based life will come to a definitive end. Nonetheless, some basic principles can be discussed if we are willing to take a generalized view of life, where we consider life to be essentially a matter of information processing. This point of view has been pioneered by Freeman Dyson (1979), who argued that the rate of metabolism or information processing in a generalized life form should be proportional to its operating temperature.
If our universe is accelerating, as current observations indicate, then the amount of matter and hence energy accessible to a given universe will be finite. If the operating temperature of life remains constant, then this finite free energy would eventually be used up and life would come to an end. The only chance for continued survival is to make the operating temperature of life decrease. More specifically, the temperature must decrease fast enough to allow for an infinite amount of information processing with a finite amount of free energy.
According to the Dyson scaling hypothesis, as the temperature decreases, the rate of information processing decreases, and the quality of life decreases accordingly. Various strategies to deal with this problem have been discussed, including the issue of digital versus analogous life, maintaining long-term survival by long dormant periods (hibernation), and the question of classical versus quantum mechanical information processing (e.g., Dyson, 1979; Krauss and Starkman, 2000). Although a definitive conclusion has not been reached, the prospects are rather bleak for the continued (infinite) survival of life. The largest hurdle seems to be continued cosmic acceleration, which acts to limit the supply of free energy. If the current acceleration comes to an end, so that the future universe expands more slowly, then life will have a better chance for long-term survival.
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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