Read more: “Existence special: Cosmic mysteries, human questions“
IT IS three weeks after the end of time, and at the Post-Universe Conference of Cosmology and Other Loose Ends, Professor Adams is standing in front of a restless audience telling them in smug tones what they already know. The universe ended in precisely the way that his own theory predicted, in a rather uncomfortable event known as the “Big Slurp”.
Of course, by definition there can be no such meeting and no way to prove or disprove a theory about the end of all things. But this untestable question tugs at our morbid curiosity. In recent years physicists have been peering deep into the tea leaves of time to try to foretell our ultimate fate. Will the universe be finished off by a big freeze, a big rip, a big crunch… or a big something else?
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To make a first attempt at this long-range forecast, we can just extrapolate current trends. Today’s universe is expanding, and the expansion is accelerating as the repulsive agent called dark energy takes hold. Projecting our ballooning universe into the future, we seem to be doomed to a dingy end. Most of known space will fly off into the darkness, isolating our local group of galaxies in its own lonely pocket universe. The stars will fade and eventually matter itself may fall apart as protons decay, leaving behind nothing but a wispy gas of fundamental particles, ever-more tenuous and ever colder.
Or it could be worse. We don’t know what dark energy is, so we don’t know whether it will remain constant into the distant future. The repulsion might get stronger as space expands. If this growing “phantom energy” really gets going, the eventual end will come in a split second of cosmic violence called the big rip, as planets, molecules and finally subatomic particles are shredded. Then again, some form of attractive cosmic force could arise to overpower today’s repulsion and pull the galaxies back together again, crushing everything to a point of infinite density – a big crunch.
Fortunately, neither of these violent ends will happen any time soon. Observations show that dark energy is changing slowly if at all, implying that a big rip or big crunch is probably tens of billions of years away at least.
An even more disquieting possibility could be just around the corner, however. The very nature of space-time may be unstable. According to string theory, for example, the vacuum of space seems to be free to adopt any of a bewildering variety of different states, which would support different kinds of forces and particles, even different numbers of dimensions. Our apparently firm reality might suddenly decay into a state with lower energy. The foundations of our existence would suddenly be yanked from under us and we, along with any familiar forms of matter, would cease to exist.
“The foundations would be yanked from under us, we would cease to exist”
Transmogrification
If the vacuum does decay, it will happen at some point in space first, and then race outwards in a spherical shock-front of grisly transmogrification travelling at just a tiny fraction less than the speed of light. In theory we would get some warning of approaching doom, but not a lot. “Much less than a microsecond,” says cosmologist of Tufts University in Medford, Massachusetts. At this very moment a wave of ultimate weirdness might be turning the moon into ectoplasm and bearing down on Earth.
Vilenkin thinks that such an end is almost inevitable; that unless a big rip gets us first, the vacuum will eventually drop into a negative energy state. After the transformation, space would then exert strong gravity of its own, pulling what’s left of the universe into a big crunch.
That, however, need not be the end of everything. If our own universe is merely one within an ever-branching and growing multiverse, as some theories predict, then the cosmos as a whole will endure even if each of its branches has a limited lifespan. And for our local universe there remains the hope of resurrection. Today’s physical theories break down at a big crunch or a big rip, allowing the possibility that a new universe could rise from the ashes (in a big bounce, or some other big as-yet-unnamed thing). And in the case of a big freeze, there will be so much time to play with that a random quantum fluctuation might spark a whole new big bang. Perhaps that impossible cosmology conference could happen after all. Perhaps existence will never end.
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