Eliminating the optical depth nuisance from the CMB with 21 cm cosmology

Eliminating the optical depth nuisance from the CMB with 21 cm cosmology Adrian Liu, 1, 2, ∗ Jonathan R. Pritchard, 3 Rupert Allison, 4 Aaron R. Parsons, 1, 5 Uroˇs Seljak, 1, 2, 6 and Blake D. Sherwin 2, 6, 7 Department of Astronomy, UC Berkeley, Berkeley, CA 94720, USA Berkeley Center for Cosmological Physics, UC Berkeley, Berkeley, CA 94720, USA Imperial Center for Inference and Cosmology, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, United Kingdom Sub-department of Astrophysics, University of Oxford, Denys Wilkinson Building, Oxford, OX1 3RH, United Kingdom Radio Astronomy Laboratory, UC Berkeley, Berkeley, CA 94720, USA Department of Physics, UC Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Miller Institute for Basic Research in Science, University of California, Berkeley, CA, 94720, USA (Dated: September 30, 2015) arXiv:1509.08463v1 [astro-ph.CO] 28 Sep 2015 Amongst standard model parameters that are constrained by cosmic microwave background (CMB) observations, the optical depth τ stands out as a nuisance parameter. While τ provides some crude limits on reionization, it also degrades constraints on other cosmological parameters. Here we explore how 21 cm cosmology—as a direct probe of reionization—can be used to indepen- dently predict τ in an effort to improve CMB parameter constraints. We develop two complementary schemes for doing so. The first uses 21 cm power spectrum observations in conjunction with semi- analytic simulations to predict τ . The other uses global 21 cm measurements to directly constrain low redshift (post-reheating) contributions to τ in a relatively model-independent way. Forecasting the performance of the upcoming Hydrogen Epoch of Reionization Array, we find that the marginal- ized 68% confidence limit on τ can be reduced to ±0.0015 for a reionization scenario tuned to fit Planck’s TT+lowP dataset, and to ±0.00083 for Planck’s TT,TE,EE+lowP+lensing+ext dataset, assuming early 21 cm data confirm and refine astrophysical models of reionization. These results are particularly effective at breaking the CMB degeneracy between τ and the amplitude of the primordial fluctuation spectrum A s , with errors on ln(10 10 A s ) reduced by a factor of four for both datasets. Stage 4 CMB constraints on the neutrino mass sum are also improved, with errors reduced to 12 meV regardless of whether CMB experiments can precisely measure the reionization bump in polarization power spectra. Observations of the 21 cm line are therefore capable of improving not only our understanding of reionization astrophysics, but also of cosmology in general. PACS numbers: 95.75.-z,98.80.-k,95.75.Pq,98.80.Es I. INTRODUCTION Through a complementary blend of cosmological probes, the last decade has seen the emergence and strengthening of a concordance ΛCDM model of our Uni- verse. Using just a handful of parameters, the ΛCDM model provides an adequate fit to data from a wide range of epochs in our cosmic timeline, ranging from Big Bang Nucleosynthesis (BBN) to the Cosmic Microwave Back- ground (CMB) to galaxy surveys and supernovae mea- surements. Examined in more detail, however, tensions have emerged between various datasets. Consider the latest CMB results from the Planck satellite [1], for instance. Distance measures inferred from Planck are in mild ten- sion with Lyman-α baryon acoustic oscillation (BAO) constraints derived from quasar observations [2]. As an- other example, Planck data is best fit by a higher am- plitude of density fluctuations than is preferred by mea- surements of weak lensing and galaxy cluster counts [3]. acliu@berkeley.edu; Hubble Fellow While currently still tolerable, these tensions may be the result of experimental systematics, or may be the first sign of new physics. To make progress, it will be necessary to sharpen our cosmological constraints. In doing so, the hint of incon- sistencies between data sets will either vanish or become statistically significant. One way to accomplish this is to simply take more data. Galaxy surveys, for instance, are poised to significantly improve their reach with new ex- periments such as the Dark Energy Spectroscopic Instru- ment (DESI) [4]. With the CMB, on the other hand, it is likely that many improvements will come from exploit- ing qualitatively new probes, such as a measurement of the primordial B-mode signal, or better measurements of CMB lensing and secondary anisotropies. These have the ability to access previously unconstrained phenomena, as well as to break existing degeneracies between cosmolog- ical parameters. Better measurements will also pave the way for expanded cosmological models that constrain the neutrino mass or the time-evolution of dark energy. In this paper, we examine the role that the emerging field of 21 cm cosmology can play in sharpening CMB constraints. With 21 cm cosmology, one seeks to use the 21 cm hyperfine transition to map the large scale dis-