Testing for New Physics: Neutrinos and the Primordial Power Spectrum

TL;DR

This study assesses how assumptions about the primordial power spectrum affect neutrino constraints from cosmological data, finding no evidence for new neutrino physics but detecting features in the spectrum likely due to systematic biases.

Contribution

It demonstrates the impact of primordial power spectrum modeling on neutrino parameter constraints and highlights potential systematic biases in current cosmological measurements.

Findings

No evidence for massive neutrinos or extra relativistic species.

Strong PPS features suggested by certain data combinations.

Systematic biases likely explain apparent PPS features, not new physics.

Abstract

We test the sensitivity of neutrino parameter constraints from combinations of CMB and LSS data sets to the assumed form of the primordial power spectrum (PPS) using Bayesian model selection. Significantly, none of the tested combinations, including recent high-precision local measurements of $\mathrm{H}_0$ and cluster abundances, indicate a signal for massive neutrinos or extra relativistic degrees of freedom. For PPS models with a large, but fixed number of degrees of freedom, neutrino parameter constraints do not change significantly if the location of any features in the PPS are allowed to vary, although neutrino constraints are more sensitive to PPS features if they are known a priori to exist at fixed intervals in $\log k$. Although there is no support for a non-standard neutrino sector from constraints on both neutrino mass and relativistic energy density, we see surprisingly strong evidence for features in the PPS when it is constrained with data from Planck 2015, SZ cluster counts, and recent high-precision local measurements of $\mathrm{H}_0$. Conversely combining Planck with matter power spectrum and BAO measurements yields a much weaker constraint. Given that this result is sensitive to the choice of data this tension between SZ cluster counts, Planck and $\mathrm{H}_0$ measurements is likely an indication of unmodeled systematic bias that mimics PPS features, rather than new physics in the PPS or neutrino sector.