# The Neutrino Puzzle: Anomalies, Interactions, and Cosmological Tensions

**Authors:** Christina D. Kreisch, Francis-Yan Cyr-Racine, Olivier Doré  
**arXiv:** [1902.00534](https://arxiv.org/abs/1902.00534)  
**Date:** 2019-02-01  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1902.00534v1/

## Key points

- Authors highlight the need for new physics in the neutrino sector to reconcile discrepancies in neutrino oscillation experiments
- Proposal of a novel approach involving self-interacting neutrinos and non-zero sum of neutrino masses to address conflicting measurements of key cosmological parameters like $H_0$ and $\sigma_8$
- Adjustment allows for a larger value of $H_0$ ($72.3 \pm 1.4$ km/s/Mpc) and a lower value of $\sigma_8$ ($0.786\pm 0.020), while maintaining compatibility with CMB data
- Strongly interacting neutrino cosmology favors an effective number of neutrinos ($N_{\rm eff}$) at $4.02 \pm 0.29, impacting particle model-building and addressing neutrino oscillation anomalies
- Importance of considering multiple factors in cosmological analyses due to compensatory adjustments when absence of phase shift in CMB from neutrino free-streaming is observed
- Distinct imprints on matter power spectrum due to impact of self-interacting neutrinos on early gravitational potential evolution, including unique localized feature on specific scales entering the horizon during free-streaming onset
- Interacting neutrino cosmology provides superior overall fit to current cosmological data but may be penalized by traditional Bayesian analyses compared to standard models, emphasizing exploration of alternative scenarios

## Summary

In their paper titled "The Neutrino Puzzle: Anomalies, Interactions, and Cosmological Tensions," authors Christina D. Kreisch, Francis-Yan Cyr-Racine, and Olivier Doré delve into the intriguing realm of neutrino physics and its potential implications for cosmology. They highlight the need for new physics in the neutrino sector to reconcile discrepancies observed in various neutrino oscillation experiments. This necessity opens up a fascinating avenue that could also offer a solution to the conflicting measurements of key cosmological parameters such as $H_0$ (the Hubble constant) and $\sigma_8$ (matter fluctuations). The authors propose a novel approach where they delay the onset of neutrino free-streaming until close to the epoch of matter-radiation equality by introducing self-interacting neutrinos alongside a non-zero sum of neutrino masses. This adjustment can naturally accommodate a larger value for $H_0$ ($72.3 \pm 1.4$ km/s/Mpc) and a lower value for $\sigma_8$ ($0.786\pm 0.020), while still maintaining a good fit with cosmic microwave background (CMB) data. Their strongly interacting neutrino cosmology favors an effective number of neutrinos ($N_{\rm eff}$) at $4.02 \pm 0.29$, which has significant implications for particle model-building and addressing neutrino oscillation anomalies. The absence of a phase shift in the CMB due to neutrino free-streaming is compensated by adjusting other cosmological parameters, underscoring the importance of considering multiple factors in cosmological analyses. Furthermore, the impact of self-interacting neutrinos on early gravitational potential evolution leaves distinct imprints on the matter power spectrum, including a unique localized feature on scales entering the horizon during neutrino free-streaming onset. While this interacting neutrino cosmology provides a superior overall fit to current cosmological data, traditional Bayesian analyses tend to penalize it compared to standard models. This study emphasizes the possibility of radically different yet equally valid cosmological models that challenge existing paradigms and encourage thorough exploration of alternative scenarios in understanding the universe's complex dynamics and evolution over time.