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arXiv · 2507.12459

The BAO-CMB Tension and Implications for Inflation

Elisa G. M. Ferreira·Evan McDonough·Lennart Balkenhol·Renata Kallosh·Lloyd Knox·Andrei Linde
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Abstract

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. Given the crucial role of $n_s$ in discriminating between inflationary models, we urge caution in interpreting CMB+BAO constraints on $n_s$ until the BAO-CMB tension is resolved.

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Elisa G. M. Ferreira, Evan McDonough, Lennart Balkenhol, Renata Kallosh, Lloyd Knox, Andrei Linde. 2025-07-16. The BAO-CMB Tension and Implications for Inflation. https://arxiv.org/abs/2507.12459

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