The Simons Observatory: Science goals and forecasts

TL;DR

The Simons Observatory aims to dramatically advance CMB science by deploying a large-aperture 6-m LAT and multiple small-aperture SATs to map the sky across six microwave bands with unprecedented detector count. The paper presents a comprehensive forecasting framework—covering instrument noise, atmospheric/foreground modeling, and component separation—and projects tight constraints on primordial tensor modes (r ~ 0.003), N_eff (~0.05–0.07), Σm_ν (~0.03–0.04 eV with external data), and high-ell damping-tail physics, including neutrino mass, dark matter properties, and BBN consistency. It also outlines powerful cross-correlations with LSST/DESI to probe growth of structure, local non-Gaussianity, curvature, and galaxy-halo physics, as well as a rich set of legacy catalogs (SZ clusters, AGN/DSFGs) for broader astrophysical use. The work highlights the need for meticulous control of instrumental systematics and foregrounds, and it frames SO as a key stepping stone toward CMB-S4 and precision cosmology. Overall, SO is forecast to deliver substantial gains in fundamental physics and provide high-value data products for the wider astronomical community.

Abstract

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The SATs will target the largest angular scales observable from Chile, mapping ~10% of the sky to a white noise level of 2 $μ$K-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, $r$, at a target level of $σ(r)=0.003$. The LAT will map ~40% of the sky at arcminute angular resolution to an expected white noise level of 6 $μ$K-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the LSST sky region and partially with DESI. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel'dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

Figures (42)

• Figure 1: The normalized uncertainties on the $C_\ell^{BB}$ power spectrum achieved by QUIET 2011ApJ...741..111Q2012ApJ...760..145Q, BICEP2 and Keck Array 2016PhRvL.116c1302B, and ABS 2018arXiv180101218K. The yellow data points are $\Delta C_\ell^{BB} / \sqrt{2/[(2\ell+1)\Delta\ell]} \propto N_\ell^{BB}$; the blue points have the beam divided out and are normalized to unity at high $\ell$. Solid lines show the modeled curves with Eq. \ref{['eq:actual_Nell']}. Dashed horizontal lines indicate the location of $\ell_{\rm knee}$ and are at $\ell \approx 50$ or below.
• Figure 2: Per-frequency, beam-corrected noise power spectra as in Sec. \ref{['subsec:sensitivity']} for the LAT temperature (top) and polarization (middle), and the SATs in polarization for the optimistic $\ell_{\rm knee}$ case of Table \ref{['tab:oneoverf']} (bottom). Baseline (goal) sensitivity levels are shown with solid (dashed) lines, as well as the $\Lambda$CDM signal power spectra (assuming $r=0$). The noise curves include instrumental and atmospheric contributions. Atmospheric noise correlated between frequency channels in the same optics tube is not shown for clarity, but is included in calculations.
• Figure 3: Anticipated coverage (lighter region) of the SATs (left) and LAT (right) in Equatorial coordinates, overlaid on a map of Galactic dust emission. For the SATs we consider a non-uniform coverage shown in Sec. \ref{['sec:bmodes']}. For the LAT, we currently assume uniform coverage over $40\%$ of the sky, avoiding observations where the Galactic emission is high (red), and maximally overlapping with LSST and the available DESI region. This coverage will be refined with future scanning simulations following, e.g., debernardis/etal:2016. The survey regions of other experiments are also indicated. The LSST coverage shown here represents the maximal possible overlap with the proposed SO LAT area; while this requires LSST to observe significantly further to the North than originally planned, such modifications to the LSST survey design are under active consideration 2018arXiv181200515L2018arXiv181202204O.
• Figure 4: Frequency dependence, in RJ brightness temperature, of the synchrotron and thermal dust emission at degree scales within the proposed footprint for the SATs, compared to the CMB lensing $B$-mode signal. The turnover of the modified blackbody law for the dust lies above this frequency range.
• Figure 5: Post-component-separation noise curves for the combination of six SO LAT (27--280 GHz) and seven Planck (30--353 GHz) frequency channels, assuming a wide SO survey with $f_{\rm sky} = 0.4$, compared to the expected signal (black). The left (right) panel shows CMB temperature ($E$-mode polarization). Foregrounds and component separation are implemented as in Sec. \ref{['subsec:fg_model']} and Sec. \ref{['sec:LAT_ILC']}, considering multipoles up to $\ell_{\rm max}=8000$. The blue (orange) curves show the component-separated noise for the SO baseline (goal) noise levels, assuming standard ILC cleaning. The dashed and dash-dotted curves show various ILC foreground deprojection options, described in Sec. \ref{['sec:LAT_ILC']}. The tSZ deprojection penalty is larger than that for CIB deprojection because of (i) the relatively high noise at 225 GHz compared to 93 and 145 GHz and (ii) the lack of a steep frequency lever arm for the tSZ signal as compared to the CIB. The dotted orange curves show the no-foreground goal noise, i.e., when SO LAT and Planck channels are combined via inverse-noise weighting. This is the minimal possible noise that could be achieved. The temperature noise curves fluctuate at low-$\ell$ due to the use of actual sky map realizations, as opposed to the analytic power-spectrum models in polarization.