Sakhara

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

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.

η = n_B / n_γ = (6.104 ± 0.058) × 10⁻¹⁰

• η: baryon-to-photon ratio (dimensionless)
• n_B: number density of baryons (m⁻³)
• n_γ: number density of CMB photons (m⁻³)

Andrei Sakharov set out the three conditions in 1967; the value of η comes from Planck CMB data (Fields et al. 2020).

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

On the shirt

Front: Sakhara, the pixel-art character, with the title. Back: η, Sakharov's three conditions, the Standard Model shortfall and two sources. Printed on a unisex tee.