The BAO-CMB Tension and Implications for Inflation

Understand

The scalar spectral index $n_s$ is a powerful test of inflationary models.

• The tightest constraint on $n_s$ to date derives from the combination of cosmic microwave background (CMB) data with baryon acoustic oscillation (BAO) data.
• The resulting $n_s$ constraint is shifted significantly upward relative to the constraint from CMB alone, with the consequence that previously preferred inflationary models are seemingly disfavored by $\gtrsim 2 \sigma$.
• Here we show that this shift in $n_s$ is the combined effect of a degeneracy between $n_s$ and BAO parameters exhibited by CMB data and the tension between CMB datasets and DESI BAO data under the assumption of the standard cosmological model.