New Interpretations of the Cosmological Preference for a Negative Neutrino Mass

Abstract

Recent observations of the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) show some tension with a $Λ$CDM cosmology. For one, the cosmological parameters determined by the CMB are at odds with the expansion history determined by latest BAO measurements. In addition, the combined data has placed uncomfortably strong constraints on neutrino mass. Both effects can be interpreted as negative neutrino mass, one describing the change to the expansion history and the other one describing enhanced lensing. In this paper, we show the current tensions can be solved with a single change either to the lensing of the CMB or the expansion of the universe. We show additional lensing could arise from a variety of models with new light fields. However, these models rarely give the same signal in temperature and polarization, giving a concrete test of the scenario. Alternatively, dark sector models can explain the changes to the expansion by changing the evolution of the matter density. These models introduce new forces, giving rise to long range signals in the three-point statistics of galaxies. We discuss a range of other examples which all illustrate the pattern that additional signals should appear if these tensions are explained by beyond the Standard Model physics.

Figures (12)

• Figure 1: Constraints on $\sum m_\nu^\mathrm{BAO}$ and $\sum m_\nu^\mathrm{clustering}$ in $\Lambda$CDM+$\sum m_\nu^\mathrm{BAO}$+$\sum m_\nu^\mathrm{clustering}$ cosmology using data from Planck+ACT+DESI. The posterior peaks at negative values for both $\sum m_\nu^\mathrm{BAO}$ and $\sum m_\nu^\mathrm{clustering}$, indicating that data prefers both modified distances and enhanced clustering compared to a universe with only massless neutrinos, opposite from the expected impact of positive neutrino mass for both parameters. The direction of degeneracy between the two neutrino mass parameters shows that physics altering either the expansion history or the clustering could bring the behavior of the other in line with the expectation from positive neutrino mass. For example, new physics causing enhanced clustering ($\sum m_\nu^\mathrm{clustering}<0$) would shift parameter inferences such that the observed expansion history is consistent with expectations of a model containing the minimal positive neutrino mass inferred from flavor oscillation experiments ($\sum m_\nu^\mathrm{BAO}>58$ meV) at less that $1\sigma$.
• Figure 2: Effect of the parameter $\sum m_\nu^\mathrm{BAO}$ on the distances measured by BAO surveys compared to the observational errors of DESI DR1 and DESI DR2. Distances are shown relative to those predicted from the Planck best fit $\Lambda$CDM cosmology.
• Figure 3: Same as Figure \ref{['fig:mnu_BAO_clustering']}, including also dynamical dark energy models. One can see that allowing for dynamical dark energy primarily affects constraints on the parameter $\sum m_\nu^\mathrm{BAO}$ with a relatively small change to $\sum m_\nu^\mathrm{clustering}$. The case of non-phantom dynamical dark energy (NPDDE) shows a stronger preference for negative neutrino mass, since the behavior of both clustering and expansion due to dark energy obeying the null energy condition is opposite that expected from positive neutrino mass.
• Figure 4: Constraints on neutrino mass parameters in $\Lambda$CDM+$\sum m_\nu^\mathrm{BAO}$+$\sum m_\nu^\mathrm{clustering}$ cosmology overlaid with the value of the Hubble constant for various samples in the parameter space. One can see that samples with positive $\sum m_\nu^\mathrm{BAO}$ and negative $\sum m_\nu^\mathrm{clustering}$ satisfy the constraints and favor larger values of $H_0$.
• Figure 5: Forecast for future constraints on $\sum m_\nu^\mathrm{clustering}$ and $\sum m_\nu^\mathrm{BAO}$ expected from a CMB survey like Simons Observatory SimonsObservatory:2018koc (modeled here with white noise level $\Delta_T=6~\mu$K-arcmin at $\ell\geq30$, a 1.4 arcmin beam, covering $f_\mathrm{sky}=0.5$), BAO from the full DESI survey Font-Ribera:2013rwa, and a cosmic variance limited measurement of the optical depth $\sigma(\tau)=0.002$, compared to current constraints from Planck+ACT+DESI.