Using Big Bang Nucleosynthesis to Extend CMB Probes of Neutrino Physics

TL;DR

The paper addresses tightening neutrino mass and degeneracy constraints from the CMB using a self-consistent BBN+CMB pipeline. It extends CAMB to treat flavor-dependent degeneracy parameters $\xi_e,\xi_{\mu},\xi_{\tau}$ and computes $Y_p$ via a full BBN network, then infers parameters with MCMC while incorporating CMB lensing. Forecasts show PLANCK, POLARBEAR, and EPIC can constrain $M_{\nu}$ to roughly $0.29$, $0.75$, and $0.20$ eV (2σ) and bound $\xi_e$ and $|\xi_{\mu,\tau}|$ to sub-unity levels, with EPIC providing the strongest gains and competitive limits relative to BBN. These results demonstrate that next-generation CMB experiments, especially with lensing extraction, can probe neutrino properties and primordial helium more tightly than current CMB analyses and in ways complementary to BBN.

Abstract

We present calculations showing that upcoming Cosmic Microwave Background (CMB) experiments will have the power to improve on current constraints on neutrino masses and provide new limits on neutrino degeneracy parameters. The latter could surpass those derived from Big Bang Nucleosynthesis (BBN) and the observationally-inferred primordial helium abundance. These conclusions derive from our Monte Carlo Markov Chain (MCMC) simulations which incorporate a full BBN nuclear reaction network. This provides a self-consistent treatment of the helium abundance, the baryon number, the three individual neutrino degeneracy parameters and other cosmological parameters. Our analysis focuses on the effects of gravitational lensing on CMB constraints on neutrino rest mass and degeneracy parameter. We find for the PLANCK experiment that total (summed) neutrino mass $M_ν > 0.29$ eV could be ruled out at $2σ$ or better. Likewise neutrino degeneracy parameters $ξ_{ν_{e}} > 0.11$ and $| ξ_{ν_{μ/τ}} | > 0.49$ could be detected or ruled out at $2σ$ confidence, or better. For POLARBEAR we find that the corresponding detectable values are $M_ν> 0.75 {\rm eV}$, $ξ_{ν_{e}} > 0.62$, and $| ξ_{ν_{μ/τ}}| > 1.1$, while for EPIC we obtain $M_ν> 0.20 {\rm eV}$, $ξ_{ν_{e}} > 0.045$, and $|ξ_{ν_{μ/τ}}| > 0.29$. Our forcast for EPIC demonstrates that CMB observations have the potential to set constraints on neutrino degeneracy parameters which are better than BBN-derived limits and an order of magnitude better than current WMAP-derived limits.

Figures (11)

• Figure 1: The calculated CMB temperature anisotropy power spectrum for $\xi_\nu = 0$ (fiducial model), $0.5$, $1.0$, and $2.0$. The WMAP5 data points are included for reference.
• Figure 2: Neutrino free streaming scale: The figure on the left is the neutrino free streaming scale as a function of neutrino mass with $\xi_\nu = 0$. The figure on the right is a contour plot of constant free streaming scale in the $m_{\nu}$-$\xi$ plane; the contours from right to left correspond to $\lambda_{\rm FS} = 0.8,$$1.2$, $1.6$, and $2.0~{\rm Gpc/h}$.
• Figure 3: Susceptibility of the transfer function to $M_{\nu}$ (left) and $\xi_{\nu}$ (right). The values used for $\xi$ are 0.1 (blue), 0.5 (cyan) and 1.0 (yellow).
• Figure 4: CMB power spectra response to changing $\xi_\nu$: $C_{l}^{TT}$ (top-left), $C_{l}^{EE}$ (top-right), $C_{l}^{dd}$ (bottom). In all three plots the black curves correspond to the fiducial model ($\xi_{\nu_{e}}=\xi_{\nu_{\mu}}=\xi_{\nu_{\tau}}=0$), the red curves correspond to a non-zero $\xi_{\nu_e}$ model ($\xi_{\nu_{e}}=3$, $\xi_{\nu_{\mu}}=\xi_{\nu_{\tau}}=0$), and the blue curves correspond to a non-zero $\xi_{\nu_{\mu,\tau}}$ model ($\xi_{\nu_{e}}=0$, $\xi_{\nu_{\mu}}=\xi_{\nu_{\tau}}=3$).
• Figure 5: The $M_{\nu}$-$w$ degeneracy: Shown are the results from PLANCK $\xi_\nu = 0$ (top left), $\xi_\nu \neq 0$ (top right), POLARBEAR $\xi_\nu \neq 0$ (bottom left) and EPIC $\xi_\nu \neq 0$ (bottom right) simulations. In this plot and each successive plot, the contours correspond to the 1- and 2-$\sigma$ regions. . The fiducial cosmological model is WMAP best-fit data and the neutrino masses $m_{2}$ and $m_{3}$ subject to neutrino oscillation results with $m_{1}$ assumed $0.01$eV.