FATE OF THE UNIVERSE
FATE OF THE UNIVERSE
GLOSSARY
absolute zero Zero-point of the temperature units Kelvin, absolute zero (-273°C or -460°F) is the temperature at which there is no thermal energy, and as such is the theoretical lowest possible temperature. While temperatures very close to absolute zero have been reached in labs, it is thought to not be possible to cool material to absolute zero.
Anthropocene Also known as the ‘human epoch’, this is the period of Earth’s history during which humans have left a noticeable impact on the planet. During this period, the climate has warmed, and there has been a mass extinction of species other than humans (the ‘Anthropocene Extinction’).
cosmic background temperature The temperature of the cosmic background radiation in the universe. The early universe was very hot, but has cooled constantly ever since so that the current background temperature is just 2.7 Kelvin.
electron and anti-electron An electron is one of the elementary particles (one of the leptons) in the universe, and has a negative charge, and can usually be found in the outer parts of atoms. An anti-electron (or positron) is identical to an electron except for its positive charge. If an electron and a positron meet in the universe they will annihilate into high-energy photons.
entropy Entropy is often explained as a measure of the disorder in a system. In physics, this concept describes the number of possible microscopic states of a macroscopic system. Without an external influence, the entropy of a system will only ever increase until it reaches its maximum entropy state.
event horizon Line beyond which no information can reach an observer. In a black hole, this describes the line of last return for anything (even light) which is falling in. In cosmology, this described the maximum distance from which light has had time to reach us since the beginning of the universe.
half-life In radioactive decay, the half-life describes the time it takes for half of the nuclei of a given type to have fallen apart.
Hawking radiation Caused by particle–anti-particle pairs spontaneously appearing at the event horizon of a black hole, Hawking radiation slowly causes black holes to lose mass (or evaporate). The rate of evaporation increases as the mass of the black hole decreases and it is only expected to be important for (theorized) microscopic black holes.
mass extinction An event which (on geological timescales) quickly reduces the number of different species of life on a planet.
neutron stars An end state of the evolution of massive stars, created in supernova explosions, neutron stars are giant city-sized, spherical atomic nuclei, made entirely of neutrons, and held up by neutron degeneracy pressure (basically the fact that in quantum mechanics, the uncertainty principle says that there is a minimum space a neutron can occupy).
planetary nebula The remnant of a lower mass star, a planetary nebula is created when its outer atmosphere is blown off at the end of its life, leaving behind a white dwarf. The confusing name resulted from early observations and was a reflection on how they appeared extended, like planets do through telescopes.
quantum tunnelling A phenomenon whereby a particle is able to escape a potential barrier it could not classically scale.
red giant A type of evolved star, which, having run out of hydrogen to burn its core, has heated up to begin burning helium, and because of this its outer layers have expanded and cooled, making it much larger in size, and appearing red. When the Sun becomes a red giant, in about four billion years’ time, it will expand to swallow the inner planets in the solar system.
second law of thermodynamics This physical law states that without an external influence, the entropy of a system will only ever increase until it reaches its maximum entropy state (for example, maximally mixed sets of different gases in a room).
supernova Observationally this is a new extremely bright star in the sky. Physically it has two sources – the first is the explosion of a massive star at the end of its life, which may leave behind either a neutron star or a black hole. The second type of supernova is created when material falls onto a white dwarf, pushing it past the mass it can sustain, and causing it to explode.
white dwarf The end star of lower mass stars, a white dwarf begins life as the core of a giant star, and when the outer atmosphere blows off as a planetary nebula, the exposed core remains. These Earth-sized stellar remnants start out very hot, but have no energy source, and so they simply cool (and fade) over time. They are made of highly compressed atoms, and held up by electron degeneracy pressure (a quantum mechanical effect that means there is a minimum space each electron must occupy).
END OF LIFE ON EARTH
the 30-second blast
Earth has undergone numerous mass extinctions over the aeons – for every species that currently lives, more than a thousand others have already gone extinct. Even now, scientists generally agree that an ongoing mass extinction (the Holocene, or Anthropocene, Extinction) is being caused primarily by human activity. In the future, any one of a number of unlikely but not impossible cosmic events may end life on Earth – a devastating collision with a comet or asteroid, perhaps. The Sun, however, may have the final say on Earth’s ability to sustain life. Our life-giving star is increasing its luminosity at the rate of just under 1 per cent every 100 million years. When it is 10 per cent more luminous than it is now – about a billion years hence – Earth’s average surface temperature will exceed 46°C (115°F). The oceans will boil away, and almost all liquid water – the most crucial ingredient to living things on our planet – will be gone, except perhaps for deep underground deposits. Increased ultraviolet radiation levels will break apart DNA and sterilize the planet. Life, however, has been resilient so far. A billion years from now, could life have evolved enough to adapt to these lethal conditions?
3-SECOND FLASH
In roughly one billion years conditions on Earth will have changed to the point where life as we currently know it will not be able to survive.
3-MINUTE EXPANSION
Human activity has increased the current rate of species extinction by between 100 and 1,000 times above the level typical over the past billion years. Humans also affect the evolution of species by dramatically changing the process of natural selection – in selectively breeding plants and animals, for example. Conversely, human technology and intention can preserve populations and save species from extinction and perhaps may eventually allow species to spread beyond our world’s confines via interplanetary colonization.
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3-SECOND BIOGRAPHIES
DONALD BROWNLEE
1943–
PETER WARD
1949–
American astrobiologists who have applied astronomy and palaeontology to examine the past, present and future of life on Earth
If an asteroid just a few kilometres across collided with Earth, it would threaten humanity with extinction.
FATE OF OUR PLANET
the 30-second blast
Roughly five billion years from now, long after the Sun’s increasing temperature has already sterilized the surface of our planet, our home star will undergo a series of changes in how it produces energy at its core. As a result, the Sun’s luminosity and size will increase drastically, swallowing the two inner planets, Mercury and Venus. Earth may be spared from consumption, but, even if it is, it will be so flayed by solar radiation that its crust and much of its mantle will likely be vaporized. In the Sun’s final throes it will hurl a hundred thousand or more Earth masses-worth of itself billions of kilometres in every direction – and if what is left of our planet is in the path of even a small fraction of that Sun-stuff, complete destruction is more than likely. However, Earth’s existence may be prematurely ended before the Sun does its work if an interloper enters the solar system in just the right (or, perhaps, just the wrong) way. A passing rogue planet, star or black hole passing too close could disrupt Earth’s orbit and either plunge our planet into the Sun or push it way out into deep space.
3-SECOND FLASH
Planet Earth will survive only for as long as our Sun will allow; if it hasn’t already been destroyed by some cosmic calamity, then the Sun will ultimately seal its fate.
3-MINUTE EXPANSION
Computational models suggest that planetary systems can routinely lose planets over the course of their history. Our own solar system could well have had numerous planets perturbed from their stable orbits and thrown out to interstellar space. It is impossible to predict whether such a strong gravitational encounter will happen to Earth – and, fortunately, such an event would be highly improbable even over multimillion-year timescales.
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3-SECOND BIOGRAPHIES
ICKO IBEN JR
1931–
JAMES LIEBERT
1946–
American astronomers who have conducted significant research that increased the scientific understanding of the ageing process of stars, including the kinds of processes in the Sun that would lead to Earth’s destruction
The physical body of our planet will be destroyed by the ageing process of the Sun within approximately five billion years.
DEATH OF THE SUN
the 30-second blast
As our home star, the Sun represents 99.9 per cent of the matter in our solar system and completely dictates the workings of our small corner of the universe. Debris left over from the Sun’s birth is the material that formed Earth and all the other planets and most other bodies in the solar system – and five billion years from now the Sun’s death will spell the end of our planet and everything else in the solar system. By that time the nuclear processes deep inside the Sun that have maintained its equilibrium – neither growing nor shrinking, heating up nor cooling down – will start sputtering as the helium ‘ash’ chokes the fusion mechanisms there, much in the way that a car’s engine begins to fail if too many deposits build up inside. The internal heat and pressure will grow, triggering new and more powerful fusion reactions. The star’s size and brightness will swell in response, and it will become a red giant. Finally, it will eject its outer layers to create a planetary nebula, and that will be the Sun’s final act of life. Its corpse, a tiny white dwarf, will gradually cool and eventually fade into the darkness of space.
3-SECOND FLASH
At approximately 4.6 billion years old, the Sun is almost halfway through the most stable phase of its life – eventually, however, it will run out of fuel and die.
3-MINUTE EXPANSION
In rare instances stars can be ‘rejuvenated’ by other stars. If, for example, the Sun were to acquire a binary companion through gravitational interactions, and then that star were to expand as a red giant, its outer gas layers may fall into the Sun’s gravity and be pulled into the Sun. Recharged in this way with fresh fuel, the Sun could return to its youthful rates of nuclear fusion and live for many millions of years longer.
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3-SECOND BIOGRAPHIES
KARL-HEINZ BÖHM
1923–2014
ERIKA BÖHM-VITENSE
1923–2017
German-born husband-and-wife astronomer team who helped establish our modern scientific understanding of stellar ageing processes
The Sun will recycle about half its mass into space upon its death, perhaps seeding future generations of star-birth.
GALAXIES CRASH
the 30-second blast
Gravity is ever present, pulling clumps of matter together like drops of water coalescing as they run down a window in the rain. Distances between galaxies are vast, but so is the amount of time available. Galaxies that get too close become locked in a cosmic dance, and their delicate structures majestically and slowly ripped apart, spreading out tidal tails and debris for hundreds of thousands of light years. Galaxies do not crash into one another like solid objects – the space between the stars, the interstellar medium, comprises most of their volume, so they pass almost imperceptibly through one another, changing shape only with the forces of gravity. This all sounds terribly destructive, but, in fact, it is constructive, too. We now know that almost all galaxies in the universe form through the build-up and merger of many smaller galaxies. If enough galaxies collide and enough gas is used up in the merging process that star formation cannot continue, the result might be a boring, dead elliptical galaxy, a ‘bee swarm’ of stars. But collisions where interstellar gas is present are now thought to result in just bigger and more dramatic spiral galaxies – ones with polar rings, bars and some of the most beautiful structures we see in the universe – and may even cause the formation of relatively tiny ‘tidal-dwarf’ galaxies in their wake.
3-SECOND FLASH
Even in an expanding universe, gravity pulls galaxies together, and galaxy collisions are inevitable.
3-MINUTE EXPANSION
Our own galaxy, the Milky Way, will not escape a collision. We move 75 kilometres (45 miles) closer to the Andromeda galaxy every second – that’s two billion kilometres (1.25 billion miles) a year. But even at this rate it will take another four billion years before any collision will occur, as the distances are just so vast. Quite possibly our Sun will not survive long enough for Earth to be affected, but if by that time humanity has moved to the stars perhaps our descendants will witness this spectacular event.
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3-SECOND BIOGRAPHIES
ALAR AND JÜRI TOOMRE
1937–and 1940–
Estonian-American brothers, both astrophysicists, who performed the first computer simulations of galaxy collisions; following this they came up with the ‘Toomre sequence’ describing the stages of spiral-galaxy mergers
When galaxies collide they create some of the most beautiful sights in the universe, with tidal tails, bridges and rings often making galaxies resemble all manner of objects, from mice to penguins – even roses.
STARS INTO ASH
the 30-second blast
Astronomers generally consider a star to be ‘alive’ when nuclear fusion – the process of creating more massive and complex atomic nuclei from lighter and simpler ones, converting matter into energy along the way – is taking place at the star’s core. Fusion in stars is self-limiting, however; the resulting heavier nuclei collect in the core, clogging the nuclear engine more and more until the star ‘dies’, leaving a glowing stellar corpse that slowly cools to the cosmic background temperature of 3 Kelvin above absolute zero. The manner of a star’s death varies greatly depending on its mass at birth. The least massive stars continue to shine faintly until they run out of fuel and fade away as Earth-sized white dwarfs. Middling-mass stars – our Sun is one – grow dramatically into red giants but do not quite explode, then also end as white dwarfs. The most massive stars detonate in titanic supernova explosions, leaving super-dense black holes or neutron stars. In these cases a substantial proportion of the star’s initial mass – much of it transformed into carbon, oxygen, iron and other elements essential for biological life as we know it – is sent back out into interstellar space, and this will, in turn, seed the birth of new generations of stars, planets and, perhaps, ecosystems.
3-SECOND FLASH
In roughly one quadrillion (1015) years from now, all the stars in the universe will have burned out.
3-MINUTE EXPANSION
The less massive a star is when it begins nuclear fusion, the longer it lives. A star one hundred times the mass of our Sun survives at most a few million years; our Sun, about ten billion years; and a tiny dwarf star less than one-tenth the Sun’s mass will live more than one trillion years. Cosmologists estimate that the universe can sustain, at most, about a thousand generations of the longest-lived stars – roughly a quadrillion years in total.
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3-SECOND BIOGRAPHY
SUBRAHMANYAN CHANDRASEKHAR
1910–95
Nobel Prize-winning Indian-American theoretical astrophysicist who showed how massive stars would evolve beyond the white-dwarf stage to produce supernovae and neutron stars
Stars have long life cycles, from baby protostars to ageing giants to stellar corpses, but their lifetimes are all finite.
MATTER DISINTEGRATED
the 30-second blast
Currently our universe is about 13.8 billion years old. Over that time, vast amounts of matter in the form of complex atomic nuclei have broken down into simpler pieces through radioactive decay. Most often, alpha decays reduce nuclei by two protons and two neutrons, and beta decays transform neutrons into protons (or the other way round), ejecting electrons and neutrinos (or positrons/anti-electrons and antineutrinos). These common decays can occur within fractions of a second after a nucleus is formed, or they may take billions of years. The most fundamental disintegration of matter, however, would be that of protons decaying into simpler components. Although this hypothesis remains as yet unproven and unobserved scientifically, if it is the case, all atoms from hydrogen to iron to uranium and beyond would be subject to destruction. Current scientific theories posit that the half-life of a proton – the average amount of time that a proton lasts before it decays – is about 1034 years; if experimental evidence confirms that value, then after less than a thousand such half-lives – a total of 1037 years – essentially every proton in the observable universe will have decayed and all matter as we know and understand it will have dissociated into a chaotic scattering of subatomic particles.
3-SECOND FLASH
In 1037 years, if current theories of matter and energy are correct, every atom in the universe will be gone.
3-MINUTE EXPANSION
Although it has been more than half a century since the proton-decay hypothesis was first formulated by Dr Andrei Sakharov, there has, to date, been no actual proof that proton decay occurs, despite attempts to witness such an event. The largest experiment currently being conducted is at the Super-Kamiokande facility in Japan. After more than two decades of observation, not a single proton decay has yet been measured there.
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3-SECOND BIOGRAPHIES
SHELDON LEE GLASHOW
1932–
HOWARD GEORGI
1947–
American physicists who pioneered grand unified theory (GUT) models that predict the time it takes for protons to decay
ANDREI SAKHAROV
1921–89
Soviet physicist and high-profile dissident who, in 1967, first hypothesized that protons decay
For years, vast numbers of sensors have been aimed into giant underground tanks of fluid, seeking evidence of proton decay.
BLACK HOLES EVAPORATED
the 30-second blast
After all the stars in the universe have burned out and all the atomic matter in the universe has decayed away, black holes will still remain. These inscrutable cosmic objects might seem impregnable to disintegration or decay – after all, if a black hole’s surface gravity is so powerful that not even light can leave it, how can it lose mass? And yet, theoretical studies pioneered by British physicist Stephen Hawking show that black holes can shrink. The key lies in the properties of a black hole’s event horizon – the gravitational boundary of no return. Rather than a solid, mathematically perfect surface, Hawking proposed that a black hole’s event horizon is subject to quantum-mechanical fluctuations. Energy within the event horizon, meanwhile, could spontaneously spawn matter–antimatter pairs of subatomic particles that fly apart in different directions; and if conditions were just right, one particle in the pair could dart through the barely shimmering horizon and escape an otherwise inescapable prison. As you might imagine, everything rarely aligns just right for this ‘Hawking radiation’ to escape, which suggests black holes would lose mass very slowly: supermassive black holes, many millions of times the mass of the Sun, found at the centres of galaxies would not lose all their mass for at least a googol (10100) years.
3-SECOND FLASH
It is possible that black holes can leak their mass out into the universe as subatomic particles very, very slowly; if they do, then after about 10100 years they will all be gone.
3-MINUTE EXPANSION
Hawking radiation, the process by which black holes could lose mass and ultimately evaporate, can be considered an extreme example of quantum tunnelling – an astonishing physical process that allows subatomic particles to go through seemingly solid or impenetrable barriers. Quantum tunnelling enables a wide variety of phenomena, from the modern microelectronics in our mobile phones to the nuclear fusion reactions that power the stars.
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3-SECOND BIOGRAPHY
KIP THORNE
1940–
American theoretical physicist who, with Charles Misner, literally co-wrote the book on gravitation – the definitive text on the subject; he earned a Nobel Prize in Physics for his life’s work, which has included research into black holes and gravitational waves
If Stephen Hawking was right, even black holes will all ultimately disappear.
FINALE DARK & COLD
the 30-second blast
Life – at least as we understand it – must, by definition, be finite: a thing isn’t alive unless it will eventually die. The cosmos, however, has all the time in the universe. After Earth, the Sun, all the stars, all the atoms and all the black holes are gone, more than one googol (10100) years from now, space will be left with a population of disorganized subatomic particles spreading ever further apart, their collective gravity too weak to resist cosmic expansion, which, because of dark energy, pushes outwards ever faster. With nothing to arrest the expansion there will be no recovery; the cosmic background temperature will creep down towards absolute zero and the mass density of the universe will drop to almost nothing. The result is a death that will take infinitely long to complete – a universe cold, dark and empty. Given how spectacular the Big Bang and subsequent life of the universe has been, this final scenario may in some ways feel anticlimactic or unimpressive. On the other hand, in a zero-density universe there are no asymmetric wrinkles or dimples in space-time caused by gravity, so, in one way at least, the universe will meet its end approaching a kind of ultimate perfection.
3-SECOND FLASH
Unless new laws of physics are discovered, the universe will end by expanding forever, losing all its order and structure while growing ever darker and colder.
3-MINUTE EXPANSION
According to the second law of thermodynamics, the total entropy – disorderly, chaotic thermal energy that cannot be harnessed to do work – of the universe can only increase with time. In some cosmological models, however, the universe ‘bounces’ from expanding to contracting over extremely long periods. If the cosmos is cyclic in that way, then someday in the very far future our universe will shrink back to a tiny point and be reborn with a new Big Bang.
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DAVID SPERGEL
1961–
American astronomer whose work on the cosmic microwave background has helped confirm the final fate of the cosmos
PAUL STEINHARDT
1952–
American physicist who has helped lead the development of theories of inflationary and cyclic cosmological models of the universe
Barring the effects of physics yet unknown, it appears the universe will end not in fire, but in ice.
