TO GRAPPLE WITH questions of time’s end, let’s first look the other way, to its beginning. Some 13.8 billion years ago, our universe, once thought to be eternal and unchanging, began. Space and time popped up spontaneously, out of nothing, in the big bang. Except we don’t know for sure that is what happened. It is an extrapolation based on the equations of general relativity – Albert Einstein’s theory of gravity – and our observations of a universe that, over billions of years, has been expanding and cooling from something denser and hotter.
Our observations take us back to some 380,000 years after a putative beginning, when the universe had cooled enough for the first atoms to form, sending radiation pinging through the cosmos. We hear that radiation today as the hiss of the cosmic microwave background.
With Einstein’s theories, we can go back further, to the first microsecond, when the entire observable cosmos was about the size of our solar system. Beyond that, we are all at sea, with neither theory nor observations to help us. Wind Einstein’s equations back enough, and you end up in a “singularity” of infinite temperature and density that seems to be a physical nonsense. Did time – did everything – really begin then?
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The answer to that might depend on where you think the universe is going. “There are a couple possibilities there,” says , author of The End of Everything (Astrophysically speaking). “The expansion could continue forever, or it could stop and reverse.”
That second, “big crunch”, scenario has generally been more favoured. On its own, the density of matter in the universe seems to suggest that the pull of gravity will eventually overwhelm space-time’s expansive momentum. The universe will reach a zenith of expansion and then begin contracting towards a kind of big bang in reverse.
That doesn’t necessarily mean the end of time. A sub-genre of big-crunch scenarios proposes that the universe bounces back. In such “cyclic” cosmologies, time has no obvious end, but is constantly recreated – a process that presumably accounts for our own big bang.
Recent discoveries have swung the pendulum in favour of eternal expansion, however. A few billion years ago, the expansion of the universe started accelerating, spurred on by a mysterious agent known only as dark energy that overwhelmed gravity’s contracting force. Wind this forwards, and the space between galaxies becomes so stretched that they gradually lose all causal contact with one another. The universe gets ever emptier and nothing useful can happen anymore – a scenario known as the heat death of the universe, or the “big freeze”.
“It’s not really clear what happens to time at that point,” says Mack. “In principle, time still exists, it still keeps passing, but there’s no future direction anymore.” With nothing able to cause anything else, any definition of time rooted in causality ceases to have any meaning.
There are more exotic alternatives. Perhaps the accelerating strength of dark energy will continue to increase and tear everything – galaxies, stars, atoms – apart before we reach a heat death, a scenario known as the “big rip”. Even more dramatically, quantum field theory might allow for vacuum decay, in which a bubble of a different kind of space pops up in our universe and devours it.
Whether any of these things truly mean the end, however, depends on the beginning. Most cosmologists believe that our universe underwent a period of faster-than-light expansion in its first microsecond, which smoothed out its wrinkles, like an inflating balloon. This cosmic inflation is very hard to stop. In this scenario, universes keep budding off from one another to form a “multiverse” of causally disconnected universes. If this is the reality, it seems unlikely that ours was the “original” universe, and implausible to say that time began – or will end – with it. “If we get a heat death, then the arrow of time in our observable universe becomes meaningless,” says Mack. “But that doesn’t necessarily mean that there’s not something happening much further away.”
Of course, all of this depends on time having any true cosmic meaning. If, as some physicists speculate, time isn’t real or fundamental to the universe, but is just of relevance to us to make sense of everything, then time will be over long before the universe ever is.
Read more from our time special
What is time? The mysterious essence of the fourth dimension
Could we ever go back in time? Relativity does not rule it out
How do we sense time? The brain cells that order our memories
Why does time fly or drag? How emotions warp our temporal perceptions
Jun Ye interview: What use is the world’s most accurate clock?
How long does evolution take? It happens on two different timescales
Can we live without time? Not if we value a solid sense of self
How do we make the most of our time? The power of confronting death
Will we ever unite physics? Clocks in superposition could offer clues
