One Part in a Billion

The second gap does not look like a gap: nothing here is unaccounted for. The trouble is that the books balance almost exactly, and the small failure to balance completely is everything that exists.

Antimatter was a consequence before it was an observation. Dirac's 1928 equation for the electron admitted a mirror solution alongside the ordinary one, and four years later Carl Anderson found the particle in cosmic-ray cloud-chamber tracks at Caltech. Antiprotons and antineutrons followed, each identical to its counterpart in mass, spin, and half-life, and opposite in every additive quantum number. The symmetry is not a speculation; it is the best-confirmed thing in the story.

Carry it back to the first instants and it predicts a definite outcome: the hot plasma makes equal numbers of both, the cooling plasma annihilates itself, and what is left is radiation and nothing solid. That near-total annihilation is not hypothetical either — the microwave background and the light elements from Big Bang nucleosynthesis both record it. What they also record is the exception. For roughly every billion antimatter particles there were about a billion and one of matter, and that surplus is every galaxy and star since.

This is a different species of absence from the one in the last chapter. Dark matter is a thing not yet found, presumed to be here now, passing through the detector unnoticed. The asymmetry is not a thing. It is a process that ran once, at temperatures the universe has not reached since, leaving a residue. Nothing can be dug up. Whatever did it shows up as a bias in physics that still operates, or it does not show up.

In 1967 Andrei Sakharov set out what such a process must include: interactions that change the number of matter particles, violation of both C and CP symmetry, and conditions far from thermal equilibrium. Two had already been seen — parity violation in the weak force in 1957, then CP violation in neutral kaon decay in 1964, direct evidence that some processes run at different rates for matter and antimatter.

The Standard Model contains, in principle, all three ingredients. Sphaleron processes, unsuppressed at early-universe temperatures, change the count of matter particles; a complex phase in the way quarks mix supplies CP violation; and the electroweak transition, when the Higgs field switched on, offered a departure from equilibrium. The failure is arithmetic rather than conceptual. Quark-mixing CP violation falls about ten orders of magnitude short, and the electroweak transition is a crossover, smooth where the story needs a break — too gentle to lock in an excess before sphalerons erase it again.

Being off by a factor of ten billion while holding the correct ingredients is an unusual place to stand. It is not ignorance about the shape of the answer: the recipe is written down, one theory satisfies every line of it, and the quantity it delivers is short by a margin no known parameter can supply. What is missing is not a mechanism of some unfamiliar kind but more of a mechanism we already have.

So the search runs wherever another source of CP violation might sit. The LHCb collaboration has reported the first observation of it in a baryon rather than a meson, comparing a beauty baryon's decay rates against its antimatter counterpart's. ATLAS has put the same question to the Higgs sector, and the CP-mixing parameter it constrains sits in a narrow range consistent with zero. Experiments hunting for electric dipole moments — a minuscule, forbidden separation of charge inside a particle — have pushed their limits far below where new physics was predicted to sit, without finding one.

Read as a set, they say something the individual results do not. Every instrument built to catch the effect is by now finer than the effect. The searches are not failing to resolve something blurry; they are resolving it cleanly and finding balance where a tilt was expected, narrowing the space where new physics can hide without producing it.

The proposals meant to supply the missing size differ in how much new structure they add. Electroweak baryogenesis introduces scalar fields that sharpen the transition enough to trap an asymmetry. Asymmetric dark matter goes further, giving dark matter a conserved charge and a primordial imbalance of its own, linked to ours through what theorists call a neutron portal. At the far edge sits a mirror universe holding the missing antimatter, alive only because current instruments cannot yet rule it out. A neighboring conjecture did not survive: antimatter was supposed by some to fall upward, and at CERN the ALPHA collaboration watched antihydrogen fall downward, close to ordinary gravity.

This gap and the last are normally filed apart — one a missing substance, one a missing process — yet some of the models that take the asymmetry seriously reach for dark matter to close it, on the grounds that ordinary and dark matter turn out to be similarly abundant today. That may be a coincidence in the accounts. It may also be that two entries have been sitting in separate columns that belong on the same line.