Sakhara: Baryon Asymmetry

Cosmology & the Sky arc, page 144 of 193. Back to The Pixelated Physics Universe

Who they are

Sakhara weighs matter against antimatter. The pans almost balance. Almost: that tiny tilt is why there is something.

Sakhara is a character of the Cosmology & the Sky arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.

The physics

The observable universe holds matter but almost no antimatter, and that excess needs explaining. It is measured by the ratio of baryons to photons, about 6 baryons for every 10 billion photons of the cosmic microwave background. Any mechanism must violate baryon number, violate C and CP symmetry, and act out of thermal equilibrium. Andrei Sakharov set out the three conditions in 1967; the value of η comes from Planck CMB data (Fields et al. 2020).

$$ \begin{gathered} \eta = \frac{n_B}{n_\gamma} = (6.104 \pm 0.058)\times10^{-10} \end{gathered} $$

Symbol Meaning SI unit
$\eta$ baryon-to-photon ratio 1
$n_B$ number density of baryons 1/m³
$n_\gamma$ number density of cosmic microwave background photons 1/m³

Honest caveat: The Standard Model seems to fall far short. The origin of the asymmetry is unknown.

Status: part of this topic is unsolved, unconfirmed or debated. The text above says which part; treat it as an open question, not settled fact.

Sources

Crossovers

Sakhara meets Marina Dirac (Dirac Sea). Marina Dirac's theory predicted antimatter. Sakhara asks why the universe kept so little of it. The Standard Model seems to fall far short of explaining the excess, and its origin is unknown.

Sakhara meets Theta Warden (Strong CP Problem). Theta Warden guards the strong force's CP-violating angle, which experiments find to be tiny. Sakhara needs CP violation to explain why matter won. One problem has too little, the other needs more, and both are unsolved.

Wear the lore

Concept keywords: Baryon Asymmetry, Sakhara, Cosmology & the Sky, physics lore, Pixelated Physics