BAO Scale Mismatch (DESI vs Planck)

Baryon Acoustic Oscillations (BAO) are frozen sound waves from the early universe imprinted as a characteristic scale (~150 Mpc) in the distribution of galaxies. This scale serves as a 'standard ruler' for measuring cosmic distances at different epochs. The DESI (Dark Energy Spectroscopic Instrument) survey released its first-year BAO results in 2024, mapping the universe's expansion history across 11 million galaxies from z = 0.1 to 4.2. When the DESI BAO data is used to constrain dark energy, it finds a preference for w > −1 at low redshifts and w < −1 at higher redshifts — a behavior inconsistent with a pure cosmological constant.

ΛCDM predicts that the BAO scale, when divided by the sound horizon from Planck CMB parameters, should give a consistent expansion history with w = −1 at all redshifts. DESI's data deviates from this at approximately 2.5–3σ when dark energy is allowed to vary. If confirmed, this would mean the cosmological constant is not constant — a direct falsification of the simplest ΛCDM assumption. The mismatch also exacerbates the Hubble tension: the BAO ruler length is calibrated using the sound horizon from CMB physics, and if either changes, the entire distance ladder must be recalibrated.