(Eisenstein et al., ApJ, 2005) Surveys of large scale structure will probe Dark Energy by measuring the BAO scale vs. redshift. Planned: DESI, Euclid, WFIRST,...

2 Hydrogen Intensity Mapping To measure BAO scale there is no need to resolve individual galaxies. Map intensity of HI gas using redshifted 21 cm emission along the line of sight. For 15 Mpc spatial resolution at z~1-2, need ~15-25’ angular resolution. This requires observations with a ~100 m aperture and a fast mapping speed in the MHz band.

The Canadian Hydrogen Intensity Mapping Expt. (CHIME) Frequency: MHz (21 cm radiation, z = ) Coverage: 20,000 sq. deg. sky; drift scanning (no moving parts); 5% of the co-moving volume of the observable universe Resolution: 1 MHz bandwidth; FWHM beam Receivers: 1024 dual polarization feeds, cosmic variance limited in ~2 years Four 20 m x 100 m N-S cylindrical reflectors with receivers along each 100 m focal line. Drift scan telescope with ~2° x 90° FOV.

4 CHIME Site: DRAO near Penticton BC Site of Full CHIME CHIME Pathfinder

Construction of Full CHIME well underway... CHIME Pathfinder is in operation...

CHIME Pathfinder Data Sample 8 x 8 correlation matrix from Tau-A transit in one 1 MHz frequency band.

“No mode left behind” Towards a program to measure all fluctuation modes within the last-scattering surface. CMB-oriented approach to experiment and analysis. Observational challenges: Calibration (phase, pass-band, angular response,...) Foreground filtering (!) Dark energy sensitivity comparable to Euclid/BAO, with a complementary observable. Cross correlation with other probes: galaxy surveys, CMB lensing, Sunyaev-Zeldovich effect, etc.

An Incomplete Selection of Yesterday’s Quotes Pertaining to the Future for CMB, Paraphrased Devlin, Pryke, Dunkley : Lots of new CMB detectors will be on the sky in 2 yrs. Bersanelli : will need to control systematics and foregrounds at the nK level Dunkley : You need 3N+2 bands to remove N foregrounds Richards : Ground-based r-instruments are in their infancy; we will learn a lot from them before the next space mission. Dunkley : What’s left to do with TT? Fully exploit the kSZ effect. Gorski : (Make up for the) under-measured low frequency side of foregrounds. Papitashvili : Plan on years of site testing (for any new site). Partridge : Theorists will find new ways to use observations (if we make them).

kSZ (momentum field) Clusters Lensing B-modes Inflationary B-modes Reionization (first stars) Fainter (higher z) clusters More lensing modes Lensing Field 30’1’ SCIENCE REACH # frequency bands needed ACT SPT

See Snowmass: CF5 Neutrinos Document arxiv: Figure by Clem Pryke Today A Moore’s Law of CMB sensitivity future Satellite?

ACTPol BICEP/KECK Spider Polarbear SPTpol Stage-2 Now O(1000) detectors Stage-4 ~ O(500,000) detectors Stage O(10,000) detectors CMB-S4: A coordinated community wide program to put O(500,000) detectors spanning GHz on multiple telescopes and map over ≳ 70% of sky at 1 μk arcm. Maintaining Moore’s Law for ground based CMB: focal planes are saturated so move to parallel processing and multiple telescopes. ACTPol, BICEP/KECK, Polarbear, Spider, SPTpol

Multiple telescopes exploiting superb, established sites at Chile and South Pole and possibly northern site(s) in Greenland, Tibet… Slide from Jeff McMahon Tibet, Greenland

Inflationary B-Modes: conservative goal σ( r ) < using only recombination bump with foreground cleaning and de-lensing. Neutrinos: σ( N eff ) = 0.020, σ( Σm ν ) = 16 meV SZ clusters: N clust ~ 100,000 with < 1% CMB lensing mass scaling Tests of gravity: e.g., pairwise kinematic SZ measurements Reionization: kinematic SZ CMB lensing cross correlation studies … “Science Book” is focus of next CMB-S4 workshop University of Michigan, tentative dates Aug 31-Sep 1 examples of CMB-S4 science

CHALLENGES BEAMS : in situ measurement of beams, esp. sidelobes (  & polzn dependence, stability) BANDPASSES: in situ characterization, matching, polzn dependence, avoiding CO etc GROUND PICKUP : shielding, sufficient suppression of scan synchronous pickup, stability I  Q/U LEAKAGE :  dependence, polarization dependence, stability, spatial dependence SENSITIVITY : low loading, high optical throughput CALIBRATION : stability, dynamic range,  dependence, pointing jitter POLARIZATION ANGLES : in situ measurement,  dependence STRIPING : minimize 1/f with fast modulation FOREGROUNDS SYSTEMATICS SENSITIVITY

-Spectral coverage -Duration of stable conditions over 900 days -Sky coverage [Xcorr, fnl…] (value of northern hemisphere modes?) litebird Inflation probe pixie Core+ … Should space aim at r/μ or be definitive? Complementarity of probes

Data challenges Extract the most from this expensive data flow – Moore’ s law on cpus unlikely to be enough – Simulations will become more challenging (and so will be the size of the analysis groups?), but needed for precision science (and also for accurate science). Sharing the data efficiently? – at TOI level? (e.g. to surround pixelization issues) – X-correlation needs a lot of detailed knowledge in both sides …

Questions for all of us When shall we be done (with the CMB) in T, E, B? Is there a Moore’s law on residual CMB systematics? Can ground-based instruments measure the low-l reionization peak? Can ground-based instruments measure large scale lensing potential? Can big telescopes get r? Do you need ground, balloons AND space for future CMB? Is a space mission without an FTS/spectral measurement worth it? Or would two smaller missions be better? Is a space mission with only ~1 degree resolution worth it? Will ground-based instruments have enough frequency channels to detect r if 0.01 <r < 0.1? if <r < 0.01? What about for other science goals? How best to combine space, balloon, ground efforts? Does each have a clear niche for the ultimate final measurements? How to address the nature of the anomalies? How to get to the guaranteed f nl <~ 1 level? And all other lottery tickets…