Cosmic Inflation Confronts Dueling Data
The Universe is thought to have expanded exponentially in the first fraction of a second after the big bang. Models compete to explain how such inflation transpired, with each one predicting a different value for the scalar spectral index ns. This dimensionless parameter characterizes how the amplitude of density fluctuations in the early Universe varies across different length scales. Last year, by combining different astrophysical datasets, two teams reported values of ns that exceeded previous estimates, disfavoring several popular inflation models [1, 2]. Now Elisa Ferreira at the University of Tokyo and her colleagues have shown that this discrepancy is mainly caused by differences in the underlying datasets [3]. They argue that the inflation models should remain in play until these differences are better understood.
If the amplitude of primordial density fluctuations is constant across different scales, then ns = 1. Values less than 1 indicate that the amplitude is higher on larger scales. The two 2025 studies combined measurements of the cosmic microwave background (CMB), the relic radiation from the big bang, with measurements of baryon acoustic oscillations (BAO), the imprint of primordial sound waves on the large-scale distribution of galaxies. Both analyses gave a value of ns of about 0.973, compared with 0.968 for CMB data alone.
Ferreira and her colleagues compared BAO information indirectly extracted from the CMB dataset with that directly extracted from the BAO dataset. They found a mismatch between the two and showed that this difference increases the inferred value of ns when the CMB and BAO datasets are combined. The researchers suggest that the origin of the mismatch will be clarified by future observations and could reflect unknown systematic errors, modeling issues, or even new physics.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- T. Louis et al., “The Atacama Cosmology Telescope: DR6 power spectra, likelihoods and ΛCDM parameters,” arXiv:2503.14452.
- E. Camphuis et al., “SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G main field,” arXiv:2506.20707.
- E. G. M. Ferreira et al., “BAO-CMB tension and implications for inflation,” Phys. Rev. D 113, 043524 (2026).