SPT-3G: A Next-Generation Cosmic Microwave Background Polarization Experiment on the South Pole Telescope

Abstract

We describe the design of a new polarization sensitive receiver, SPT-3G, for the 10-meter South Pole Telescope (SPT). The SPT-3G receiver will deliver a factor of ~20 improvement in mapping speed over the current receiver, SPTpol. The sensitivity of the SPT-3G receiver will enable the advance from statistical detection of B-mode polarization anisotropy power to high signal-to-noise measurements of the individual modes, i.e., maps. This will lead to precise (~0.06 eV) constraints on the sum of neutrino masses with the potential to directly address the neutrino mass hierarchy. It will allow a separation of the lensing and inflationary B-mode power spectra, improving constraints on the amplitude and shape of the primordial signal, either through SPT-3G data alone or in combination with BICEP-2/KECK, which is observing the same area of sky. The measurement of small-scale temperature anisotropy will provide new constraints on the epoch of reionization. Additional science from the SPT-3G survey will be significantly enhanced by the synergy with the ongoing optical Dark Energy Survey (DES), including: a 1% constraint on the bias of optical tracers of large-scale structure, a measurement of the differential Doppler signal from pairs of galaxy clusters that will test General Relativity on ~200 Mpc scales, and improved cosmological constraints from the abundance of clusters of galaxies.

Figures (9)

• Figure 1: A typical $\sim$30 $\mathrm{deg}^2$ field from the spt-pol survey. Top: wmap$W$-band ( Left) and Planck 143 GHz ( Right) data from the same region, high-pass filtered at $\ell$$\sim$50, for comparison. The large-scale CMB features are measured with high fidelity in both the wmap, Planck, and spt-pol maps. Bottom Left: Minimally filtered spt-pol data, showing degree-scale and larger structure in the primordial CMB as well as small-scale features such as emissive sources and SZ decrements from galaxy clusters. Bottom Right: Zoomed-in view of spatially filtered spt-pol data indicated by black square in the bottom left panel. In this single 10 $\mathrm{deg}^2$ region, we indicate three newly spt-discovered clusters at redshift $z > 0.75$.
• Figure 2: Left: Projected error bars (per logarithmic bin size of d(ln$\ell$=0.1)) on the CMB lensing power spectrum for spt-3g (red) and Planck (blue). Dashed vertical lines show the scale at which mapping of individual modes will be possible for Planck (blue) and spt-3g (red). Right: Projected 1$\sigma$ parameter constraints on $\Sigma m_\nu$, the sum of the neutrino masses and $N_\mathrm{eff}$, the effective number of neutrino species for spt-3g when combined with data from Planck and the BOSS spectroscopic survey. The addition of the extremely deep CMB polarization and lensing maps from spt-3g will substantially improve cosmological constraints on neutrino physics, including the sum of the masses and the number of relativistic species.
• Figure 3: Projected $EE$ (left) and $BB$ (right) constraints from four years of observing with the spt-3g camera (black points and error bars). Constraints are from simulated observations including realistic treatment of foregrounds, atmosphere, instrument $1/f$ noise, and $E$-$B$ separation. Overplotted are projected constraints from Planckplanck06 (cyan) and spt-pol (purple). The inset in the $EE$ plot shows the amplitude of the low-$\ell$$EE$ uncertainties from the three instruments (same color scheme), showing that spt-3g is competitive with Planck's low-$\ell$$EE$ constraints down to $\ell \sim 200$. Model curves in the $BB$ plot (solid lines) are for $\Sigma m_\nu=0$, with $r=0$ and $r=0.2$. The red points and dashed lines in the $BB$ plot show the added sensitivity to primordial gravitational-wave $B$ modes from delensing. The spt-3g error bars are recalculated for a $4$ reduction in lensed $BB$ power, and the model lines are shown with this reduction for the same models as the solid lines in the main plot.
• Figure 4: (Left) Mass versus redshift for three cluster samples: (1) SZ-selected clusters from 2500 $\mathrm{deg}^2$ of the spt-sz survey bleem14b, (2) SZ-selected clusters from the Planck survey, and (3) the projected spt-3g cluster sample. Also over-plotted is the expected selection threshold from the upcoming eRosita X-ray cluster survey pillepich12. (Right) Projected measurement by spt-sz and spt-3g of the relative velocities of pairs of des-selected clusters using the kinetic SZ effectkeisler13, a unique probe of gravity on scales of 100s of Mpc. spt-3g is expected to provide a $30\sigma$ detection significance using the des cluster sample with photometric redshifts.
• Figure 5: Left: Layout of the spt-3g optical design. The optics consist of an ellipsoidal secondary mirror and three cryogenically cooled alumina lenses. A cold Lyot stop defines the 8 m primary illumination and controls stray light. Right: Strehl ratio vs focal plane position at 95 (blue), 150 (red), and 220 (green) GHz. The lines corresponds to a cut across the focal plane's y-axis (solid) and x-axis (dashed). The 430 mm diameter focal plane covers a 1.9$^\circ$ FOV at with $f/1.83$ beams. The optical performance is excellent, with Strehl ratios $> 0.98, 0.96, 0.93$ at 95, 150, $220\,$GHz, respectively, across the FOV.