The next time you are eating a piece of Swiss cheese, think about the fate of the Universe. The cheese may just hold a clue as to how the Universe will end; not the cheese itself exactly, but the characteristic pattern of holes within it. Matter in the Universe appears to be distributed in a rather similar way—there are titanic voids that contain hardly anything, surrounded by large tracts of dust and gas where the galaxies form. Understanding this distribution is crucial to determining the Universe’s ultimate fate.
Almost as soon as astronomers came to accept in the late 1920s that the expansion of the Universe was a reality, it led them backward to a creation moment, the now familiar “Big Bang.” With that in mind, they began looking forward and wondering how the Universe might end. The basic mechanics of the problem seemed straightforward: the Big Bang sent everything bursting outward but the gravity of the celestial objects would attempt to pull it all back together again. So what might happen seemed to rest on how much gravity-generating matter there is in the Universe, and how it is distributed. Two scenarios are possible: if the overall density of matter is greater than a certain critical value then gravity will overcome the expansion and the Universe will contract, sending all celestial objects crashing back into each other; if the density is lower than the critical amount then the Universe will expand for the rest of time, its galaxies becoming ever farther apart. Astronomers refer to the first scenario as the “closed” Universe and the second as the “open” Universe.
THE FATE OF THE UNIVERSE: THE DENSITY OF THE UNIVERSE DETERMINES WHETHER THE UNIVERSE WILL EXPAND FOREVER (OPEN) OR COLLAPSE ON ITSELF (CLOSED)
General relativity tells us that the density of matter governs the overall curvature of space (see How Did the Universe Form?). Imagine for a moment that the Universe has just two dimensions: length and breadth. The closed Universe scenario can be represented by the surface of a ball and is mathematically described as negatively curved. This is the kind of Universe known as unbounded but finite; you could travel all the way around it eventually arriving back at your starting point (see Are There Alternative Universes?). Such a Universe could be extraordinarily big but not infinite in size. If it were infinite it would not curve back on itself—it would extend in all directions forever.
So let us assume we live in a closed Universe. Since it cannot expand to infinite size, at some point in the future it will stop expanding and begin to contract. At this point the redshift of distant galaxies (see How Big is the Universe?), caused by the expansion of space, will reverse. As the galaxies begin to fall back toward us, their light will be squashed to shorter and shorter wavelengths, making the stars in those galaxies appear to blaze with blue brilliance, as if they were shining at a much higher temperature than normal. Astronomers term this phenomenon “blueshift”; eventually, as the galaxies fall toward us faster and faster, the blueshift will become more and more pronounced, squashing the starlight into ultraviolet rays and then X-rays.
Galaxies will be pulled closer and closer together, heading for the “big crunch.” A billion years or so before this supercollision, clusters of galaxies will merge together. Around 100 million years before the end individual galaxies will begin merging. For the last million years of the Universe’s existence, there will be no such thing as an individual galaxy—the entire Universe will be one great ocean of stars. By this time, the blueshift will be having a highly noticeable effect on the cosmic microwave background radiation; it will transform the microwaves first into infrared and then into visible light, making the whole night sky light up around 100,000 years before the now inevitable big crunch. Finally, the blueshifted background radiation will become so intense that it will exceed the temperature of the stars themselves. The stars will dissolve into space and the Universe will resemble the fireball of the Big Bang.
These spectacular death throes are, in some ways, a rewind of the Big Bang. Some cosmologists have speculated that something may eventually prevent the Universe from collapsing completely and make it rebound, turning the big crunch into another Big Bang and starting the whole process of cosmic evolution off all over again. If so, it would be like rebooting the Universe.
The alternative scenario is that of the open Universe, where the density of matter is less than the critical value. The shape of space in such a Universe is more complex; the best two-dimensional analogy is rather like the upper surface of a saddle, with one dimension curving upward and the other curving downward. Unlike a saddle, however, it extends infinitely in all directions. The open Universe will expand forever, but this does not mean that it will remain more or less the same forever.
Stars will continue to live and die much as they have done over the last 13 billion years, but space itself will change around them. As space continues to expand, galaxy clusters will be driven ever farther away from each other. Most of the galaxies that draw our attention today will eventually be lost from sight; only the stars in the Milky Way and in the 30 or so nearest galaxies in our own cluster will remain visible to us. For all the rest, their light will be redshifted away from visible light, into the infrared and then into the weak radio region of the spectrum. Perhaps the only clue future beings will have of the existence of other galaxies will be a faint radio hiss coming at them from all around, similar to the cosmic microwave background of today. The already weak background radiation itself will have been redshifted too, rendering it unobservably feeble. Future civilizations will thus have no evidence that the Big Bang ever took place (see How Did the Universe Form?).
Within each galaxy cluster, where gravity can resist the cosmic expansion of space, the individual galaxies will all eventually coalesce. As the galaxies collide, some stars will be flung out into intergalactic space to wander the Universe alone. Others will be catapulted into the supermassive black hole at the center of their galaxy, temporarily reigniting the black hole and making the galaxy active again with a blazing core. This could well be the ultimate fate of our own Sun, as the Milky Way Galaxy and the Andromeda Galaxy collide. Every galaxy, it is believed, has a supermassive black hole and as these vortexes of space–time interact, releasing a blaze of cosmic energy, an even more massive, voracious cosmic dustbin will be forged. After perhaps 100,000 billion years, all the cosmic gas will have been either pulled into existing stars or sucked into gigantic black holes.
With no gas clouds left to collapse into new stars, stellar activity will be coming to an end. The lights will be going out across the cosmos, leaving space to contain nothing but highly isolated collections of dead stars: white dwarfs, neutron stars, and black holes both large and small. These stellar cadavers will occasionally collide, releasing a sudden flash of brilliance, but, by and large, no light will be shining through the Universe.
“Eventually all black holes will simply disappear in a puff of radiation.”
PAUL DAVIS CONTEMPORARY COSMOLOGIST
But this might not be quite the end. There are hints that protons, which make up a vital constituent of each atomic nucleus, may not be permanently stable. If they eventually decay, all the atoms in the Universe will disintegrate, leaving a sea of subatomic particles. Chemical reactions and nuclear reactions will become impossible, and the only structures able to form will be black holes as the particles clump together.
Even the black holes may not last forever. They may gradually evaporate, radiating their matter content back into the Universe in the form of more subatomic particles. Such a phenomenon, known as “Hawking radiation” after its proponent (see What is a Black Hole?), would take place over an inconceivably long time period, perhaps a googol years (1 followed by 100 zeros). So, the long-term fate of an open Universe is to become a dilute sea of particles, all at approximately the same low temperature and unable to react with one another. Such a state is known as the “heat death” of the Universe.
We have talked of two scenarios, but between the open Universe and the closed Universe eventualities is the unique possibility of a “flat” Universe. Its two-dimensional analogy is simply a flat sheet that extends forever in all directions. While there are myriad types of both open and closed Universes, depending upon the density of matter, there is only one flat Universe. It corresponds to the Universe containing exactly the critical density of matter. Its ultimate fate would be identical to the open Universe scenario—the dilute sea of cold particles—but it is a highly improbable case, given how finely tuned the Universe would need to have been during the Big Bang to create such a precise amount of matter. Nevertheless, it is type of Universe that cosmologists believe that we live in. Astronomers have been able to analyze ripples in the microwave background to discover the geometry of the Universe, and have found it to be almost perfectly flat. But this result did not balance with their inventories of matter until they discovered the acceleration of space and decided that some form of “dark energy” must also permeate the cosmos, making up the deficit (see What is Dark Energy?).
DARK ENERGY VARIATIONS: ASTRONOMERS DO NOT KNOW HOW DARK ENERGY WILL BEHAVE IN THE FUTURE
Until we know exactly what dark energy is, we have no way of being certain how it behaves and so the straightforward picture of a flat Universe expanding forever may not be true after all. Dark energy may turn off, gain strength or even reverse its effects. Astronomers need to be able to determine which behavior pattern dark energy will follow; until then they cannot rule out any possibility for the fate of the Universe. If the dark energy effect remains constant, the expansion of space will continue to accelerate, shortening the time it will take for the galaxies to disappear from view. But if the dark energy were to reverse its behavior, it would increase the strength of gravity in the Universe and could pull everything back into a big crunch.
The most bizarre option would be if dark energy continually increased with time. If this were to happen, the Universe’s expansion would accelerate at an ever-increasing rate. After driving all of the galaxies so far away that we could no longer see them, dark energy would then go to work on the Milky Way. It would disrupt our Galaxy and even drive planets out of their orbits around stars. Ever-strengthening dark energy would then pull apart the stars, followed closely by the planets. Finally, it would explode the very particles that make up matter. Astronomers call this nightmare scenario “the big rip.”
This may all sound bleak, but string theory offers a number of even more bizarre scenarios—at least one of which, called “vacuum decay,” could result in the sudden death of the Universe at any time. If string theory is correct, there could well be a multitude of Universes (see Are There Alternative Universes?), each one separated from the others by energy barriers. According to quantum theory, it may be possible for a tiny region of our Universe to “tunnel” through to another Universe with a lower energy state. If this were to happen, our whole Universe would transform itself into this lower energy state (rather like a glass of water can turn itself into a glass of ice if the temperature drops low enough). The entire fabric of space would undergo the transformation, and this would destroy everything. The catastrophe would spill out from the epicenter, in all directions, at the speed of light. This could give us billions of light years of notice if it begins far away, but there will be nothing that we can do to stop it. Our fate will have been sealed.
Another idea, also relying on the “multiverse” proposed by string theory, emerges from attempt to understand what created the Big Bang. It envisages a collision between a parallel Universe and our own, which would excite both Universes into high-energy states. The theory, known as the “ekpyrotic” Universe after the Greek word meaning “out of fire,” suggests that the two Universes are joined by a piece of cosmic elastic and continually bounce into each other, like a pair of clapping hands. When they touch, everything is erased and the Universes are then reborn in a new Big Bang. The theory does away with the need for inflation after the Big Bang, and explains the ripples in the microwave background as the slight differences in time at which various parts of the two Universes collided. There would be no warning of when the two Universes would collide. One moment all would seem normal, the next: oblivion. But it may be comforting to know that a new Universe would be created from the ashes of our own.
It seems somewhat depressing that there are currently no hypotheses that preserve the Universe in its present state. The “steady state theory,” popular in the mid-20th century, relied on the continuous creation of matter to fill the gaps created by the expanding Universe. But the theory was disproved by the discovery of the cosmic microwave background radiation and its interpretation as the residual energy of the Big Bang. The majority of astronomers now believe that the most likely fate for the Universe is to expand forever and suffer a heat death. But none of the scenarios presented in this chapter can yet be ruled out.