Understanding acoustic scale observations: the one-sided fight against $\Lambda$

Understand

The cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) provide precise measurements of the cosmic expansion history through the comoving acoustic scale.

• The CMB angular scale measurement $\theta_*$ is particularly robust, constraining the ratio of the sound horizon to the angular diameter distance to last scattering independently of the late-time cosmological model.
• For models with standard early-universe physics, this measurement strongly constrains possible deviations from $\Lambda$CDM at late times.
• We show that the null energy condition imposes strict inequalities on the BAO observables $D_H(z)$, $D_M(z)$, $D_V(z)$ and $F_{\rm AP}(z)$ relative to $\Lambda$CDM predictions.