dark energy

Throw a ball straight up and it slows down, because gravity pulls it back. For most of the twentieth century, physicists expected the whole universe to behave like that ball: it has been expanding since the Big Bang, and the gravity of all its matter should be gently slowing that expansion down. Then in 1998 two teams measuring distant exploding stars found the opposite. The expansion of the universe is not slowing — it is speeding up. Something is pushing space apart, working against gravity, and it is becoming more dominant as the universe grows. Physicists named this unknown push dark energy.

Dark energy is, frankly, a placeholder name for whatever is causing the acceleration. The simplest description is that empty space itself carries a constant tiny amount of energy — an idea called the cosmological constant — that acts like a kind of pressure pushing outward everywhere, all the time. Unlike matter, which thins out as space expands, this energy of empty space stays at the same density, so as the universe gets bigger there is more and more of it, and it increasingly wins out over the diluting gravity of matter. By the present day, dark energy is reckoned to make up roughly 70 percent of the total energy of the universe.

Dark energy belongs in a particle-physics glossary for an uncomfortable reason. Quantum field theory says empty space should not be truly empty — it should seethe with vacuum energy from quantum fluctuations — which sounds like a natural source for a cosmological constant. But when physicists estimate how much, they get a number larger than the observed dark energy by a staggering factor, often quoted as something like 10 to the 120th. This mismatch, the cosmological constant problem, is one of the deepest unsolved puzzles in physics, a glaring sign that something fundamental about how quantum mechanics and gravity fit together is still missing.