Empty space is not quite empty. In physics, the vacuum is the lowest-energy state of the fields that fill the universe. But a state can be stable without being the lowest possible one, like a ball resting in a dip halfway down a hill. A vacuum like that is called a false vacuum. If ours is one, it could one day drop to a lower state, and the change would spread through the universe.
How it would happen
In 1977 the American physicist Sidney Coleman worked out how a false vacuum would decay. Somewhere, through quantum tunnelling, a tiny bubble of true vacuum would appear. If it were large enough, it would grow. Its wall would accelerate almost at once to nearly the speed of light, and everything it reached would be converted.
Inside, the laws of physics would be different. In 1980 Coleman and Frank De Luccia wrote that in such a bubble even “chemistry as we know it” would be impossible. Because the wall moves at nearly the speed of light, there would be no warning: light from the bubble would arrive at almost the same moment as the bubble itself.
What the Higgs boson says
Whether our vacuum is stable depends on the masses of the known particles, above all the Higgs boson and the top quark. When the Higgs was discovered at CERN in 2012 with a mass of about 125 GeV, it turned out to sit in a curious place. Calculations suggest our vacuum is probably metastable: not truly stable, but very long-lived. The answer hinges on the exact mass of the top quark, which is still being refined.
In 2018 Anders Andreassen, William Frost and Matthew Schwartz calculated the expected lifetime of our vacuum within the Standard Model of particle physics: about 10¹³⁹ years, and at least 10⁵⁸ years with 95% confidence. The universe is about 1.4 × 10¹⁰ years old. Physics beyond the Standard Model could change the answer either way.
Could we cause it?
The worry that a particle collider might set off a bubble was examined long ago. In 1983 Piet Hut and Martin Rees pointed out that cosmic rays have been colliding with the Earth, the Moon and other bodies at far higher energies than any accelerator for billions of years, and the vacuum has survived. In 2014 Stephen Hawking noted that the Higgs field could become unstable at energies far beyond anything humans could reach, and that “we wouldn’t see it coming”.
Vacuum decay may never happen, and if it does, it is very unlikely to happen anywhere near us for an almost unimaginable length of time. It remains one of the strangest possible endings for the universe.