In every bit of nothing, there is something. If you zoom in on empty space and take out all the planets and stars and galaxies, you might expect a pure vacuum, but you'd be wrong. Instead you would find a dynamic scene, with particles sparking to life and disappearing almost immediately.
Quantum mechanics, the theory governing the infinitesimal world, doesn't allow for nothingness. At any given moment in time and space, energy can never be perfectly zero—there is always some wiggle room. Out of that wiggle room, “virtual” particles can arise—specifically, a pair made of a particle and its antiparticle, which annihilate each other and are gone as quickly as they came. As bizarre as this may seem, experiments have observed the real-world effects of virtual particles. When particle accelerators first measured the mass of the Z boson, it was slightly off from its pure mass because it was sometimes turning into a virtual top quark—one of many observations proving that virtual particles exist.
The effect of all these particles wiggling into and out of being is a thrumming “vacuum energy” that fills the cosmos and pushes outward on space itself. This activity is the most likely explanation for dark energy—the reason the universe, rather than staying static or even expanding at a steady rate, is accelerating outward faster and faster every moment.
The problem with vacuum energy is that there's not enough of it. When scientists first started thinking about the concept, they calculated that this energy should be huge—it should have expanded the universe so forcefully and quickly that no stars and galaxies ever formed. Because that is clearly not the case, the vacuum energy in the universe must be very small—about 120 orders of magnitude smaller than what quantum theory predicts. That's like saying that something weighing five pounds should really weigh five-with-120-extra-zeros-after-it pounds. The discrepancy has prompted some scientists to call vacuum energy “the worst theoretical prediction in the history of physics.”
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