The Baryon Acoustic Ruler

Plate 72 speaks for itself: “The first sound, now a 147 Mpc ruler.”

The Physics

Sound waves in the early photon–baryon plasma travelled until the baryons were released at the drag epoch. That distance froze into galaxy clustering as a standard ruler.

c_s = c / √(3(1 + R))

• c_s: sound speed in the plasma, m s⁻¹
• c: speed of light, m s⁻¹
• R: 3ρ_b / 4ρ_γ, baryon-to-photon momentum ratio, dimensionless

They froze in at the drag epoch. Comoving r_drag = 147.09 ± 0.26 Mpc (Planck 2018).

Sources

• D. J. Eisenstein et al., Astrophys. J. 633, 560 (2005).
• Planck Collaboration, Astron. Astrophys. 641, A6 (2020).

On the poster

Plate 072, “The Baryon Acoustic Ruler”: the pixel-art plate with its slogan “The first sound, now a 147 Mpc ruler.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.