Primordial Helium Abundance from CMB: a constraint from recent observations and a forecast

TL;DR

The paper addresses whether the primordial helium abundance $Y_p$ can be constrained from CMB observations and how such constraints propagate to other cosmological parameters. It employs Markov chain Monte Carlo analyses of current CMB data and conducts Planck-era forecasts, exploring different priors on $Y_p$ and incorporating recombination uncertainties. The results show that current data yield $Y_p le 0.44$ (95% CL) with WMAP5 alone and $Y_p  0.25^{+0.10}_{-0.07}$ (68% CL) when including high-$$ data; Planck is expected to measure $Y_p$ with $    0.01$ (68% CL), though priors on $Y_p$ can affect $ _b$, $n_s$, and $H_0$ via parameter degeneracies. The study demonstrates that CMB-derived $Y_p$ constraints provide an independent cross-check of BBN and the recombination physics captured by the damping tail, underscoring the need to model recombination accurately in precision cosmology.

Abstract

We studied a constraint on the primordial helium abundance Y_p from current and future observations of CMB. Using the currently available data from WMAP, ACBAR, CBI and BOOMERANG, we obtained the constraint as Y_p = 0.25^{+0.10}_{-0.07} at 68% C.L. We also provide a forecast for the Planck experiment using the Markov chain Monte Carlo approach. In addition to forecasting the constraint on Y_p, we investigate how assumptions for Y_p affect constraints on the other cosmological parameters.

Figures (6)

• Figure 1: CMB $TT$ power spectra for several values of $Y_p$. In this figure, we take $Y_p = 0.1$ (blue dotted line), $0.24$ (red solid line) and $0.4$ (green dashed line). Other cosmological parameters are assumed to be the mean value of WMAP5 for a power-law $\Lambda$CDM model. For reference, in the left panel, data from WMAP5, ACBAR, BOOMERANG and CBI are also depicted. In the right panel, the expected data from the Planck experiment are also shown.
• Figure 2: One-dimensional marginalized distribution of $Y_p$ for the cases with WMAP5 alone (red solid line) and all CMB data combined (green dashed line).
• Figure 3: One-dimensional marginalized distributions of $Y_p$. Shown are the distributions from the Planck experiment for the cases with no priors on $Y_p$ (red solid line) and assuming the BBN relation (green dashed line). For reference, the distribution for the case with no priors on $Y_p$ using current CMB data is also shown (dash-dotted magenta line).
• Figure 4: One-dimensional marginalized distributions of $\omega_b, \omega_c, n_s, A_s, \tau, H_0$, using same data as in Fig. \ref{['fig:Yp_planck']}. Additionally, the distributions from Planck for the case with fixing $Y_p=0.24$ are also shown (dotted blue line).
• Figure 5: Two-dimensional marginalized constraints in several combinations of cosmological parameters. Shown are a Planck forecast on the constraints for the cases with no prior on $Y_p$ (orange and yellow shaded region), assuming the BBN relation (solid red line) and fixing $Y_p=0.24$ (blue dotted line). For reference, the constraint for the case with no prior on $Y_p$ using current CMB data is also shown (dash-dotted magenta line).