PaperlayerView exact revision on arXiv

Source-linked AI summary

Exploring Cosmic Origins with CORE: Survey requirements and mission design

J. Delabrouille, P. de Bernardis, F. R. Bouchet, A. Achúcarro, P. A. R. Ade, R. Allison, F. Arroja, E. Artal, M. Ashdown, C. Baccigalupi, M. Ballardini, A. J. Banday, R. Banerji, D. Barbosa, J. Bartlett, N. Bartolo, S. Basak, J. J. A. Baselmans, K. Basu, E. S. Battistelli, R. Battye, D. Baumann, A. Benoît, M. Bersanelli, A. Bideaud, M. Biesiada, M. Bilicki, A. Bonaldi, M. Bonato, J. Borrill, F. Boulanger, T. Brinckmann, M. L. Brown, M. Bucher, C. Burigana, A. Buzzelli, G. Cabass, Z. -Y. Cai, M. Calvo, A. Caputo, C. -S. Carvalho, F. J. Casas, G. Castellano, A. Catalano, A. Challinor, I. Charles, J. Chluba, D. L. Clements, S. Clesse, S. Colafrancesco, I. Colantoni, D. Contreras, A. Coppolecchia, M. Crook, G. D'Alessandro, G. D'Amico, A. da Silva, M. de Avillez, G. de Gasperis, M. De Petris, G. de Zotti, L. Danese, F. -X. Désert, V. Desjacques, E. Di Valentino, C. Dickinson, J. M. Diego, S. Doyle, R. Durrer, C. Dvorkin, H. -K. Eriksen, J. Errard, S. Feeney, R. Fernández-Cobos, F. Finelli, F. Forastieri, C. Franceschet, U. Fuskeland, S. Galli, R. T. Génova-Santos, M. Gerbino, E. Giusarma, A. Gomez, J. González-Nuevo, S. Grandis, J. Greenslade, J. Goupy, S. Hagstotz, S. Hanany, W. Handley, S. Henrot-Versillé, C. Hernández-Monteagudo, C. Hervias-Caimapo, M. Hills, M. Hindmarsh, E. Hivon, D. T. Hoang, D. C. Hooper, B. Hu, E. Keihänen, R. Keskitalo, K. Kiiveri, T. Kisner, T. Kitching, M. Kunz, H. Kurki-Suonio, G. Lagache, L. Lamagna, A. Lapi, A. Lasenby, M. Lattanzi, A. M. C. Le Brun, J. Lesgourgues, M. Liguori, V. Lindholm, J. Lizarraga, G. Luzzi, J. F. Macìas-Pérez, B. Maffei, N. Mandolesi, S. Martin, E. Martinez-Gonzalez, C. J. A. P. Martins, S. Masi, M. Massardi, S. Matarrese, P. Mazzotta, D. McCarthy, A. Melchiorri, J. -B. Melin, A. Mennella, J. Mohr, D. Molinari, A. Monfardini, L. Montier, P. Natoli, M. Negrello, A. Notari, F. Noviello, F. Oppizzi, C. O'Sullivan, L. Pagano, A. Paiella, E. Pajer, D. Paoletti, S. Paradiso, R. B. Partridge, G. Patanchon, S. P. Patil, O. Perdereau, F. Piacentini, M. Piat, G. Pisano, L. Polastri, G. Polenta, A. Pollo, N. Ponthieu, V. Poulin, D. Prêle, M. Quartin, A. Ravenni, M. Remazeilles, A. Renzi, C. Ringeval, D. Roest, M. Roman, B. F. Roukema, J. -A. Rubino-Martin, L. Salvati, D. Scott, S. Serjeant, G. Signorelli, A. A. Starobinsky, R. Sunyaev, C. Y. Tan, A. Tartari, G. Tasinato, L. Toffolatti, M. Tomasi, J. Torrado, D. Tramonte, N. Trappe, S. Triqueneaux, M. Tristram, T. Trombetti, M. Tucci, C. Tucker, J. Urrestilla, J. Väliviita, R. Van de Weygaert, B. Van Tent, V. Vennin, L. Verde, G. Vermeulen, P. Vielva, N. Vittorio, F. Voisin, C. Wallis, B. Wandelt, I. Wehus, J. Weller, K. Young, M. Zannoni

arXiv:1706.04516v1astro-ph.IMastro-ph.CO

TL;DR

Detailed CMB polarisation observations remain limited across useful angular scales, leaving important cosmological information underexploited. The paper sets survey requirements and discusses CORE’s design, finding that it reaches the sensitivity and resolution requirements and that its lensing reconstruction can reduce lensing B-mode power by 70%.

  • Problem

    Only temperature anisotropies have been mapped with good signal-to-noise over most of the sky and useful angular scales, while detailed CMB polarisation remains largely unexploited.

    Source: §10 · p. 57

  • Method

    The paper lists future CMB polarisation survey requirements and discusses CORE’s mission design, sensitivity, angular resolution, and observing strategy.

    Source: §9 · p. 56, §10 · p. 58

  • Results

    CORE reaches the sensitivity and angular-resolution requirements; its lensing reconstruction can reduce lensing B-mode power by 70%, improving the primordial-gravitational-wave-amplitude error by a factor of 2.5.

    Source: §9 · p. 56, §10 · p. 58

  • Takeaways & Limitations

    Space observations offer unmatched precision and accuracy where foreground emission, cosmic variance, or systematic effects dominate, down to about 10 arcminutes; smaller scales are best observed from the ground.

    Source: §10 · p. 58

    If r ≪0.001, detecting primordial inflationary B modes is out of reach for the foreseeable future, although precise CMB polarisation remains scientifically necessary.

    Source: §3.1 · p. 13

Abstract

from arXiv · show

Future observations of cosmic microwave background (CMB) polarisation have the potential to answer some of the most fundamental questions of modern physics and cosmology. In this paper, we list the requirements for a future CMB polarisation survey addressing these scientific objectives, and discuss the design drivers of the CORE space mission proposed to ESA in answer to the "M5" call for a medium-sized mission. The rationale and options, and the methodologies used to assess the mission's performance, are of interest to other future CMB mission design studies. CORE is designed as a near-ultimate CMB polarisation mission which, for optimal complementarity with ground-based observations, will perform the observations that are known to be essential to CMB polarisation scienceand cannot be obtained by any other means than a dedicated space mission.

1 Introduction

CMB observations have tightly constrained ΛCDM, yet fundamental questions remain, and CMB polarisation could provide additional information. The paper sets requirements and discusses design options and expected performance for CORE, proposed to ESA as an M5 medium-size mission.

1 Introduction

  • CMB anisotropies have driven precise ΛCDM constraints, but questions about inflation, dark matter and energy, large-scale anomalies, and physics beyond the standard model remain.The introduction also asks whether the overall ΛCDM scenario is correct or an essential piece is missing.Source: §1 · p. 5
  • CMB photons last scattered about 380,000 years after the Big Bang and encode information about the early Universe and their subsequent interactions on the way to us.Planck extracted most information in the primordial CMB temperature anisotropy power spectrum.Source: §1 · pp. 5–6
  • 50 µK.arcmin was Planck's polarisation sensitivity, whereas near-optimal exploitation requires a few µK.arcmin or better—at least an order of magnitude better.The passage gives sensitivity in CMB thermodynamic temperature units.Source: §1 · p. 6
  • Suborbital experiments target primordial inflationary B modes, lensing B modes, or both, but fully exploiting CMB polarisation is widely accepted to require a space mission.Reviews cited for these targets are Kamionkowski and Kovetz 2016 and Lewis and Challinor 2006.Source: §1 · p. 6
  • Proposed CMB space missions differ in sensitivity, resolution, frequency coverage, detector and band counts, and orbit, reflecting science aims, foreground assumptions, and budgetary constraints.The introduction notes that none of the proposals discussed had yet been selected.Source: §1 · p. 6
  • The paper presents performance requirements, design drivers, options, expected performance, and scientific impact for CORE, proposed to ESA under M5 as a medium-size mission for launch before 2030.It also compares CORE's design with other proposals and discusses the pros and cons of the options.Source: §1 · p. 7

2 Overview of CORE

CORE is a space-based polarimetric imager designed to measure CMB E and B modes, especially primordial and lensing B modes. Its broad frequency coverage supports foreground separation, which is essential to exploiting polarization signals over large sky areas.

2 Overview of CORE

  • CORE targets detection and characterization of primordial and lensing B modes, while foreground emission over large sky fractions makes accurate component separation essential.Over 70% of the sky, 130 GHz Galactic foreground emission dominates noise down to about 12′; on the cleanest 5%, its amplitude is reduced by an order of magnitude, but E- and B-mode cosmic variance increases.Source: §2 · p. 7
  • The 2100-detector array on a 1.2-m telescope observes 19 bands from 60 to 600 GHz and delivers about 1.7 µK.arcmin aggregate CMB polarization sensitivity.Six channels from 130 to 220 GHz primarily observe the CMB; lower-frequency channels monitor synchrotron and other foregrounds, while higher-frequency channels monitor dust and CIB anisotropies.Source: §2 · pp. 7–9
  • Figure 1 illustrates primordial B-mode sensitivity for r = 0.01 and r = 0.001, assuming perfect component separation over 70% of the sky and lensing reduction using CORE-measured small-scale E and B modes.For large sky fractions, foregrounds dominate lensing B modes; restricting observations to the cleanest 5% reduces foreground amplitude by an order of magnitude but increases E- and B-mode cosmic variance.Source: §2 · pp. 7–8, Figure 1 · p. 8
  • Passive cooling keeps the payload near 40 K, reducing detector background and improving sensitivity, especially above 220 GHz, although temperatures as high as 90 K or more have acceptable performance impact.The payload is shaded by the service module and thermally decoupled using reflective V-grooves; its conical screen radiates heat to free space.Source: §2 · p. 9
  • At L2, CORE combines a roughly 2-minute spacecraft spin, 4-day precession, and annual revolution; its scan gives each detector many viewing orientations to support polarization measurement and systematic control.The baseline uses no active polarization modulator; instrument requirements, calibration, scanning redundancies, and data pipelines control systematics, which are characterized and corrected a posteriori by marginalizing over nuisance parameters.Source: §2 · pp. 9–10, Figure 3 · p. 11

3 Scientific objectives

CORE’s scientific objectives use precise CMB polarisation and related survey analyses to investigate the early Universe, test cosmological and particle-physics models, and support additional studies of cosmic structures and astrophysical processes, including delensing, primordial B-mode detection, and recovery of CMB spectral-distortion parameters.

3 Scientific objectives

  • CORE’s primary objectives are to probe primordial inhomogeneities, test ΛCDM and particle-physics extensions; its data also target lensing structures, foregrounds, star formation, cosmic dipoles, and reionisation.The additional objectives include testing isotropy and homogeneity on the largest scales and constraining energy dissipation through dipole spectrum distortions.Source: §3 · pp. 11–12

3.1 Inflation

  • CORE aims to detect or rule out primordial B modes at r ≃0.001 and improve constraints on other inflationary observables, including ns, its scale variation, and non-Gaussian signatures.A non-detection at this sensitivity would rule out large-field models; a detection could distinguish among favoured inflationary models. Even if r ≪0.001, precise polarisation measurements can rule out large-field models.Source: §3.1 · p. 13

3.2 The cosmological model

  • CMB-constrained parameter errors could fall by factors reaching an order of magnitude or more; depending on extensions, the cosmological figure of merit could improve by as much as 10^7.CORE’s polarisation and lensing measurements are intended to clarify whether existing cosmological tensions reflect new physics, statistical fluctuations, or measurement systematics; dipole analyses could approach ideal cosmic-variance-limited signal-to-noise up to l ≃2000.Source: §3.2 · p. 14

3.3 Fundamental particles and interactions

  • With Euclid and DESI, CORE is projected to yield σ(Mν) = 16 meV and a 4 σ neutrino-mass-sum detection; σ(Neff) is 0.041 alone and 0.039 with future BAO data.The survey can also constrain dark-matter annihilation and decay, varying fundamental constants, topological defects, and stringy-physics signatures.Source: §3.3 · p. 15

3.4 Structures

  • CORE traces structure growth through CMB lensing, galaxy clusters, and the cosmic infrared background; its baseline survey is expected to detect tens of thousands of clusters, including several hundred at z > 1.5.CMB lensing could calibrate the SZ signal-to-mass relation to percent level at z < 1 and better than 10% approaching z = 2. Combining CORE with CMB-S4 is projected to increase cluster yield fourfold over either experiment alone and push the detection mass limit towards 3 × 10^13 M⊙; the survey also maps gas and dust and traces star formation around z ≃2.Source: §3.4 · pp. 15–17

3.5 Legacy

  • CORE’s high-sensitivity, high-resolution dust-polarisation maps would advance studies of Galactic magnetic fields, turbulence, and dynamo action, with a few hundred times Planck’s measured modes.Its survey could also find thousands of strongly lensed high-redshift galaxies and detect high-redshift protoclusters and polarised radio sources and dusty galaxies. The frequency-dependent CIB dipole could reduce uncertainty in the absolute CIB spectrum by at least one order of magnitude.Source: §3.4 · p. 17, §3.5 · pp. 18–19
  • Spectral-distortion recovery could improve over COBE-FIRAS by a factor of several to about 50, or much better with ideal CORE, depending on foreground removal and relative calibration.The analyses could detect energy release associated with cosmological reionisation and probe both early and late dissipation processes.Source: §3.5 · p. 19
  • The unresolved high-redshift dusty-galaxy background that forms the CIB can support CMB B-mode delensing and low-level primordial B-mode detection, and enable study of cosmological star formation (Wu and Doré 2016).Source: §3.5 · p. 19

4 Survey requirements

This section sets out CORE’s space-survey requirements for controlling foregrounds and systematic errors over a large sky area while detecting and characterizing primordial and lensing B modes. It covers sky coverage, sensitivity, angular resolution, frequency coverage, delensing, redundancy, and high-frequency foreground maps for component separation and validation.

4.1 The need for a space mission

  • Ground surveys may have greater raw sensitivity, but foreground residuals and limited clean sky restrict precision cosmology, making space better suited to large-area polarisation measurements.Residual foregrounds must be reduced by 3, 2, and 1 orders of magnitude at ℓ≃10, 100, and 1000, respectively; the text estimates ground surveys can safely use at most about 25% of the sky.Source: §4.1 · p. 20

4.1.1 Atmosphere

  • Atmospheric loading makes space mapping speed at least 100 times higher than ground mapping speed for the same detector count, while space avoids atmospheric emission and fluctuations.At 90 and 150 GHz, atmospheric fluctuations can leave residuals three orders of magnitude above target sensitivity on 2° scales, even after polarisation modulation.Source: §4.1.1 · p. 21

4.1.2 Astrophysical foregrounds

  • Foreground separation requires at least ten well-spread frequency channels, while ground-based atmospheric windows cannot accommodate that coverage and thermal dust must be mapped from space.The channel requirement accounts for modelling synchrotron and dust, plus multiple CMB bands for cross-checking residual foregrounds.Source: §4.1.2 · pp. 22–23

4.1.3 Systematic effects

  • A stable space environment helps minimise instrumental systematics and enables the redundancy needed to assess and correct their effects on CMB measurements.The cited benefits include reduced response fluctuations, stable calibration conditions, and redundant analyses across large sky regions.Source: §4.1.3 · p. 23

4.1.4 Why space – summary

  • Ground and sub-orbital observations can advance technology and probe small scales, but the next polarisation space mission should deliver the reference CMB polarisation data sets for decades.The paper presents CORE as designed to fulfil that role.Source: §4.1.4 · p. 24

4.2 What survey?

  • CORE’s design prioritises scientific breakthrough and science per cost, focusing on ground-inaccessible observations and near-ultimate performance within ESA M-class constraints.The target is performance limited by fundamental and astrophysical constraints rather than the instrument, so another CMB polarisation mission will not be required after CORE.Source: §4.2 · p. 24

4.3 Sky coverage

  • CORE should observe the largest possible sky fraction to measure large-scale E and B modes, constrain optical depth, and test inflationary B modes; targeted deep patches remain an option if r≲10^-3.The text also proposes the capability to concentrate observing time on selected patches late in the mission if very low primordial B modes remain undetected.Source: §4.3 · p. 25

4.4 Sensitivity and angular resolution

  • E-mode measurements require noise below about 2 µK.arcmin and resolution of about 4′ or better, while a 16′ beam remains adequate for the primordial recombination bump.A 32′ beam is marginal at ℓ=200, and a 64′ beam substantially degrades sensitivity across the recombination peak.Source: §4.4 · pp. 25–27
  • Over more than 50% of the sky, the survey should deliver 1.7–2.5 µK.arcmin sensitivity, with CMB resolution better than 30′ for primordial modes, <∼8′ for most lensing modes, and <∼4′ for optimal E- and lensing B-mode science.The 1.7–2.5 µK.arcmin range supports a signal-dominated B-mode map with signal-to-noise ratio 2–3 on lensing B modes.Source: §4.4 · p. 28
  • Dust polarization maps at about 2′ resolution support small-scale component separation, while CIB intensity maps provide an alternative delensing route essential for detecting primordial B modes below r ≃0.01.High-frequency observations needed for these maps must be made from space.Source: §4.4 · p. 29

4.5 Frequency channels

  • CMB bands should be centered mainly between about 130 and 200 GHz, avoiding the 115 and 230 GHz CO lines, with foreground-monitoring bands spanning about 65–400 GHz.At least 10 channels, preferably more, are required across this range; logarithmic spacing yields 15 channels with a safety margin.Source: §4.5 · p. 30
  • With at least five channels above 300 GHz, generalized internal linear combination can separate CIB emission from thermal dust for delensing and cross-checking CMB-based delensing.CORE’s eight channels above 200 GHz also support CIB science; increasing channel counts above 200 GHz from 5 to 10 and then 25 yields successive factor-of-30 gains in the cited SFR figure of merit.Source: §4.5 · p. 31

4.6 Systematic effects

  • A mismatch between the assumed and actual detector-response matrix can mix I, Q, and U, especially leaking intensity into polarization and E modes into B modes; inversion must keep final uncertainty detector-noise dominated at 2 µK.arcmin.An unbiased Stokes estimate requires a correct response matrix and enough measurements at different scan angles for the relevant matrix to be invertible.Source: §4.6 · p. 32

4.7 Flexibility, safety margins, and redundancy

  • Although exact sensitivity, sky coverage, and resolution targets have flexibility, safety margins and redundant frequency channels are needed to test B-mode signals across mission halves, sky regions, Galactic cuts, and component-separation methods.Such data splits reduce sensitivity by a factor ranging from 2 to a few.Source: §4.7 · pp. 32–33

4.8 Survey requirements and goals – summary

  • The survey must map at least 50% of the sky in at least two 100–200 GHz channels and map polarized synchrotron and dust in at least four channels, at 30′ resolution or better.Foreground emission must be characterized well enough for cleaning below the noise sample variance over ℓ = 2–200.Source: §4.8 · p. 33
  • For r = 0.001, a foreground-cleaned, delensed B-mode map needs noise <∼3 µK.arcmin at 30′ or better, with both the reionisation and recombination bumps detected.The survey must also detect the turnover above ℓ = 80 to confirm an inflationary origin.Source: §4.8 · p. 33

5 Mission design

CORE’s design balances M-class cost, spacecraft limits and sensitivity goals with an L2 orbit and a redundant, full-sky scanning strategy. The baseline avoids an onboard half-wave plate and plans a four-year survey, with an optional extension for deeper observations.

5.1 Practical constraints

  • CORE must fit an ESA M-class project budget below about EUR 700 million and remain near Planck’s scale while boosting detector and frequency-channel counts and controlling systematics.The spacecraft is constrained to about 2 tonnes, a 1.5-m-class telescope and about 2 kW of power; the telescope’s diffraction limit at 150 GHz is about 5.6′.Source: §5.1 · p. 34

5.2 Orbit

  • L2 is the baseline orbit because it keeps the Sun, Earth and Moon away from the main beam; LEO would require sidelobe rejection of about 4 × 10^9 for the Moon and 8 × 10^10 for the Sun.The target polarisation sensitivity on the relevant angular scale is about 0.07 µK, and Earth’s large apparent sky coverage makes its temperature and emissivity variations a concern for sidelobe pickup.Source: §5.2 · pp. 34–35

5.3 Observing strategy

  • The survey requires each pixel to receive varied polarisation angles over short and mission-long timescales, with distant pixels revisited before long-term instabilities drift and most pixels revisited near the inverse knee-noise frequency.The goals are to control temperature-to-polarisation leakage and polarisation-angle errors while suppressing low-frequency noise.Source: §5.3 · pp. 35–36
  • The baseline uses scanning without an onboard HWP, with complete-sky coverage and roughly even integration time; the satellite spins every ≃2 min and precesses every ≃4 days.The spin axis is kept roughly anti-solar, offset ≃30° from the precession axis, while the optical axis is offset ≃65° from the spin axis.Source: §5.3 · pp. 36, 38

5.4 Mission phases and operations

  • The planned operations include about 2 months of calibration and performance verification, about 4 years of nominal sky scanning, and a possible extension for deeper integration on selected patches.The extension depends on whether the nominal-survey results justify it.Source: §5.4 · p. 39

5.5 Telemetry

  • With about 30 times more detectors than Planck, CORE’s baseline requires continuous telemetry of about 1.15 Mbit s−1, supported by a steerable Ka-band antenna of roughly 30 cm.The antenna would communicate with large ground-based antennas already in use.Source: §5.5 · p. 39

6 Payload

The payload design addresses the configuration of CORE’s spacecraft, detectors, telescope, and shielding, including the baseline choice to omit a half-wave plate. It balances cooling, mass and power budgets, and scanning constraints, including trade-offs in scan geometry.

6 Payload

  • The payload module stays opposite the Sun, Earth, and Moon across a main sun screen; its spin-axis geometry defines a shadow cone that keeps payload elements out of direct sunlight.The telescope focuses radiation onto a large focal plane, while the main payload shield reduces sidelobe stray light and active coolers reach 100 mK, with intermediate stages at 1.7 K, 4 K, and 15–20 K.Source: §6 · p. 39, Figure 8 · p. 40

6.1 Instrument design

  • The baseline instrument observes 60–600 GHz with 2100 background-limited detectors across 19 bands of approximately 30% fractional bandwidth, cooled to 0.1 K.Metal-mesh filters define the bands; polarization sensitivity uses grids above 115 GHz and ortho-mode transducers at or below 115 GHz.Source: §6.1 · p. 41

6.2 Telescope

  • The crossed-Dragone baseline provides a larger, flat, near-telecentric focal plane than a compact Gregorian, but requires a tertiary mirror and F ≃2.5 to fit and shield the focal plane.The design addresses the crossed-Dragone’s fairing and direct-sky-illumination drawbacks by relocating the focal surface.Source: §6.2 · pp. 41–42

6.3 Shielding against sidelobe stray light

  • At least 1 order of magnitude of additional sidelobe rejection is expected from CORE’s baffling, while stray-light pickup could otherwise exceed patch-averaged noise by 100–1000 times.Absorptive screens also create internal emission whose temperature fluctuations can add spurious signals; these are uncorrelated with sky pointing and can be removed during map-making, but should be kept small by design.Source: §6.3 · pp. 42–44, Figure 10 · p. 43

6.4.1 Technical complexity

  • An HWP can mitigate beam-asymmetry and low-frequency-noise effects, but rotating and stepped designs pose thermalisation, magnetic-field, orientation-knowledge, and modulation tradeoffs.In space, these challenges add instrument complexity, cost, and risk; a stepped HWP also does not shift the signal away from low-frequency noise.Source: §6.4 · p. 44, §6.4.1 · p. 44

6.4.2 Impact on science performance

  • 6.8 µK.arcmin is the estimated full-array sensitivity with a 30-cm transmissive HWP, versus 1.7 µK.arcmin for baseline CORE.The small aperture also degrades angular resolution, including to about 75′ at 60 GHz, compromising component separation across the full frequency set.Source: §6.4.2 · p. 46

6.4.3 Mitigation or generation of systematics?

  • A 2.5 µK spurious signal from HWP thermal-emission inhomogeneities would be 3 orders of magnitude above targeted CMB B modes; CORE therefore baselines no HWP.The design decision weighs possible systematic mitigation against HWP-generated effects and the greater complexity of instrument calibration and data analysis.Source: §6.4.3 · p. 47

6.5 Cooling chain

  • CORE uses passive space cooling plus active cooler stages at 15–20 K, 4 K, 1.7 K, and 100 mK; the cooling chain is a single-point-failure system, making redundancy a design question.Operating a large array of sub-kelvin detectors in space remains a challenge and a mission-performance risk.Source: §6.5 · p. 47
  • CORE assumes redundant cooling down to 1.7 K, with only one sub-kelvin cooler.The redundancy trade-off balances integration complexity, cost and schedule risk against in-flight failure risk.Source: §6.5 · p. 48

6.6 Mass and power budgets

  • CORE’s estimated dry mass is 1540 kg, propellant mass 150 kg, and wet mass 1690 kg; required onboard power is about 1700 W, or 2100 W with margins.The cooling chain accounts for 1290 W, while end-of-life solar-panel power is slightly above 2300 W in the worst case.Source: §6.6 · p. 48

6.7 Scanning strategy and payload design

  • At 0.5 RPM, spacecraft angular momentum is ≃105 Nms, requiring two 70–100 Nms reaction wheels and limiting spin rate to 0.66–0.95 RPM.Reaction-wheel sizing and star-sensor pointing reconstruction together limit the typical allowed spin rate to <∼2 RPM.Source: §6.7 · p. 49
  • The scan and payload design compromise balances polarization-angle coverage, spacecraft subsystem constraints, and cold-payload geometry.Increasing precession angle α and decreasing scanning angle β lowers temperature-to-polarisation leakage by improving detector polarization-angle distribution; full orientation coverage requires α ≥ β. Solar power constrains α ≤ 37.8°, while increasing α reduces the shadow cone available for the cold payload.Source: §6.7 · pp. 48–49

7 Controlling systematic effects

CORE plans to control systematic effects through in-flight calibration and map-level deprojection, since ground-based instrument models alone cannot achieve the accuracy needed for sensitivity-limited B-mode measurements. The section describes correction strategies for angular-response and bandpass mismatch, with simulations showing they can meet the mission sensitivity under stated conditions.

7 Controlling systematic effects

  • Because ground-based measurements cannot provide the accuracy needed for sensitivity-limited B-mode measurements, CORE combines theoretical modeling and ground calibration with further in-flight instrument knowledge.In-flight knowledge may come from dedicated payload calibration or the scientific data themselves.Source: §7 · p. 49

7.1 Systematic-correction mapmaking

  • Beam ellipticity leaks intensity into polarization, while pointing errors can leak intensity gradients into E and B and E gradients into B.Small polarimeter misalignment and photometric calibration or cross-polarisation leakage errors also affect the signal.Source: §7.1 · p. 50
  • Assuming detector effects remain fixed over mapmaking, the method first maps intensity, subtracts modeled beam-asymmetry leakage from timelines, then remakes I, E, and B maps.This approach performed well in Planck-preparation simulations; CORE’s implementation reduced beam-asymmetry effects below its sensitivity target (de Bernardis et al. 2017; Natoli et al. 2017).Source: §7.1 · pp. 50–51
  • The CORE beam-asymmetry correction reduced its impact well below the mission’s sensitivity target in demonstrations.The method was developed specifically for CORE (de Bernardis et al. 2017; Natoli et al. 2017).Source: §7.1 · p. 51
  • With imperfect calibration or beam knowledge, mapmaking can jointly fit beam-asymmetry and pointing parameters, then iterate to fit terms governing E-to-B leakage.The fit linearises using intensity-map gradients and curvature from an initial reconstruction.Source: §7.1 · p. 51

7.2 Bandpass leakage correction

  • When detector bands differ, sky components have different frequency responses, so calibration to one component can leave unequal mixing coefficients and leak intensity into polarization.The mismatch can be a few percent for synchrotron, dust, and CMB, and can vary by factors of a few for CO spectral lines.Source: §7.2 · p. 52
  • One iteration reduced bandpass-mismatch effects to a level compatible with required mission sensitivity, provided detector-band mismatch was no worse than for Planck (Natoli et al. 2017).The simulations also discuss a second correction method.Source: §7.2 · p. 52

8 Options

Section 8 presents MiniCORE as a minimal descoped mission that retains key CMB polarisation capabilities, then considers detector, frequency, and aperture upgrades and their scientific tradeoffs.

8.1 Descoping options

  • MiniCORE must provide clean, multi-frequency full-sky CMB maps at scales larger than about 10–15′, where foregrounds and cosmic variance dominate below 5 µK.arcmin noise.These scales support E-mode and lensing B-mode measurements.Source: §8.1 · p. 53
  • It must also map high-frequency foregrounds down to a few arcminutes for ground-based complementarity and provide CIB maps for delensing independently of CMB-polarisation methods.Source: §8.1 · p. 53
  • A proposed MiniCORE uses an 80 cm aperture, 900 rather than 2100 detectors, a 3- rather than 4-year mission, and a reduced frequency range; Table 4 outlines a possible channel distribution.Table 4 gives 3.6 µK.arcmin polarisation sensitivity for six CMB channels and 3.2 µK.arcmin for the full array.Source: §8.1 · p. 53, Table 4 · p. 53
  • MiniCORE's 3.2 µK.arcmin full-array sensitivity maps lensing B modes at S/N ≃1.5, but leaves the recombination bump below noise for r <∼0.006 and lacks clear margins and redundancy.The authors describe it as the minimal next-generation CMB polarisation space mission; further descoping would not adequately address the science goals or guarantee breakthroughs, including primordial B-mode detection.Source: §8.1 · pp. 53–54, Table 4 · p. 53

8.2 Upgrades

  • Making all detectors dual-polarisation would halve noise in channels centred at ν ≥115 GHz, yielding 1.5 µK.arcmin from 130–220 GHz or 1.3 µK.arcmin for the full array.Source: §8.2 · p. 54
  • Adding 96 photon-noise-limited detectors at 1200 GHz could improve dust and infrared-source sensitivity and resolution to 1′, with little added focal-plane area or telemetry.The added resolution could also improve pointing reconstruction using science data.Source: §8.2 · p. 54
  • Multichroic detectors could increase detector count by 2–3 and enable 1 µK.arcmin or better map sensitivity, provided foreground residuals fall by a matching amount.Source: §8.2 · p. 54
  • Aperture growth could improve resolution, lensing, and astrophysics; a 1.8 m design needs redesign and likely international collaboration, while added resolution and sensitivity chiefly benefit cluster, extragalactic, and CIB science.The upgrades do not drastically improve inflationary science or cosmological-model investigations; a larger optical system would also allow more detectors and sensitivity.Source: §8.2 · p. 54

9 Discussion

The discussion presents CORE as the ESA M-class choice among proposed CMB space-mission designs, emphasizing its inflation, foreground, and broader cosmology capabilities. It also argues for complementarity between a space mission and sub-orbital observations.

9 Discussion

  • CORE’s resolution and low noise target σr = 0.0004 to distinguish r ≪ 0.001 from r = 0.003; 70% lensing B-mode reduction improves gravitational-wave amplitude error by a factor of 2.5 (Challinor et al. 2017).The authors argue that delensing is essential to reach this sensitivity and avoid an ambiguous r = 0.002 ± 0.001 result.Source: §9 · pp. 55–56
  • CORE’s frequency bands and per-band sensitivity are intended to measure galactic sources despite foregrounds at ℓ<20 being orders of magnitude stronger than primordial B modes.The discussion notes that some early polarization missions lack sufficient foreground-determination capability, while foreground complexities remain unknown.Source: §9 · p. 56
  • CORE targets cosmic-variance-limited E modes to ℓ≃2500 and lensing B modes to ℓ≃1000, with allowed parameter space reduced by up to ∼107 versus Planck 2015 and 105 versus Planck 2015 + future BAO (Di Valentino et al. 2016).These measurements support constraints on ns, its scale-dependent running, non-Gaussianity, and extensions involving curvature, neutrinos, dark energy, and other physics.Source: §9 · p. 56
  • CORE’s scan strategy is designed to discriminate polarimetric systematics, making a continuously rotating half-wave plate an unnecessary technical risk that would increase costs.A half-wave plate compatible with broad frequency coverage and an entrance aperture larger than 0.5 m is described as a technical challenge with cost and schedule consequences.Source: §9 · p. 56

9.1 Complementarity with sub-orbital experiments

  • Comprehensive, precise polarization cosmology requires a space mission, while high-resolution ground observatories complement CORE through overlapping measurements at 500 ≤ ℓ ≤ 1000.The discussion also identifies a spectrometer such as PIXIE, an imager such as CORE, and a high-resolution ground observatory as a powerful combination for CMB science.Source: §9.1 · p. 57

10 Conclusion

The paper frames CMB polarization as a way to investigate unresolved questions within the concordance inflationary ΛCDM picture. It presents CORE as a space survey designed to complement ground observations while controlling foreground and instrumental errors.

10 Conclusion

  • CMB polarization can probe cosmology through underused E modes, map dark-matter structure via lensing B modes, and test inflation through primordial B modes on scales larger than 30′.These probes address questions including whether inflation occurred and the nature of dark matter and dark energy.Source: §10 · p. 57
  • Foreground contamination and systematics are major challenges; CORE uses stable scanning, shielding from the Sun, Earth, and Moon, and joint sky and instrument-response fitting.Its stationary optical path is designed to stabilize the response and enable in-flight calibration at the required accuracy.Source: §10 · pp. 57–59
  • Space should map more than 10 frequency channels across scales >∼10′ and high-frequency dust/CIB above 300 GHz, while ground surveys extend CMB resolution to 2′.Dust mapping supports subtraction from CMB E and B modes; the CIB traces cosmic structure and helps disentangle primordial B modes from lensing.Source: §10 · p. 58
  • CORE maps 19 frequency channels from 60 to 600 GHz at 2–18′ resolution, reaching 1.7 µK.arcmin polarization sensitivity after four years with 2,100 detectors on a 1.2-m telescope.It maps 45% of the sky every 4 days; six independent channels between 130 and 220 GHz, each with sensitivity of order 5 µK.arcmin, provide foreground cross-checks.Source: §10 · p. 58
  • If descoping is required, an 80-cm aperture, half the detectors, and removal of channels below 100 GHz would preserve most essential polarization science but increase reliance on ground measurements.Survey quality could alternatively improve through dual-polarization or multichroic detectors, or a somewhat larger aperture.Source: §10 · p. 59
  • Beyond primary CMB polarization, CORE can probe Galactic magnetic fields, constrain dark matter, dark energy, or modified gravity with clusters, and study dusty galaxies and protoclusters.The survey is expected to detect tens of thousands of galaxy clusters, or hundreds of thousands when combined with a high-resolution ground-based CMB survey; its 19 intensity and polarization maps will also provide a lasting scientific legacy.Source: §10 · p. 59

A Impact of atmosphere on ground-based CMB observations · A.1 Atmosphere and detector sensitivity

Atmospheric emission adds substantial detector loading from the ground, reducing sensitivity relative to space. Within atmospheric windows, one space-borne detector can match 100–200 ground-based detectors; outside them, space is needed for sensitive, high-resolution observations of large sky areas.

A.1 Atmosphere and detector sensitivity

  • Ground-based atmospheric windows have emissivity of a few percent, adding 10–20 K of background from the 230−290 K atmosphere, with loading rapidly increasing above 200 GHz.BICEP2 measured 22 KRJ total loading, partly atmospheric and partly from its forebaffle, window, and filters (BICEP2 Collaboration et al. 2014).Source: §A.1 · p. 60

A Impact of atmosphere on ground-based CMB observations

  • Within atmospheric windows, one space-borne detector reaches sensitivity equivalent to 100–200 ground-based detectors, ignoring atmospheric fluctuations, ground pickup, and other systematics.Those effects further degrade ground-based performance relative to space.Source: Figure 11 · p. 60, §A.1 · p. 61

A.1 Atmosphere and detector sensitivity

  • The Figure 11 comparison assumes 0.5 mm precipitable water at 60◦ elevation in Atacama, 25% bands, 60% optical efficiency, and detector throughput λ2.It compares ground-based 3% emissive optics with Planck-like space-telescope emissivity and payload temperatures of 40, 60, or 100 K; total noise is twice photon noise.Source: Figure 11 · p. 60, §A.1 · p. 61
  • The mapping-speed comparison assumes 25% ground observing efficiency versus nearly 100% in space, and notes that such space efficiency requires a continuous sub-kelvin cooler.BICEP2's total observing efficiency was about 30% in February 2010–November 2012, while other ground experiments were typically lower; Planck-HFI data loss was 6%–20% (Planck Collaboration et al. 2014b).Source: Figure 11 · p. 60, §A.1 · p. 61
  • Outside atmospheric windows—near the main O2 and H2O lines and above 300 GHz—ground observations are extremely challenging, making space the only option for sensitive, few-arcminute surveys of large sky patches.The space-based option avoids resorting to massively multi-moded observations.Source: §A.1 · p. 61
  • At 150 GHz, modeled space-detector noise is 49, 43, and 40 µKCMB.√s for 40-K, 60-K, and 100-K payloads, respectively, slightly better than Planck's typical 50–53 µKCMB.√s.One Planck detector is reported at 59 µKCMB.√s (Planck HFI Core Team et al. 2011).Source: §A.1 · p. 61

A.2 Required observing time and focal-plane area

For a full-sky 5 µK.arcmin polarization map, space requires 500–1000 detector-years versus about 10^5 on the ground. The required focal-plane size and most efficient frequency also differ by platform.

A.2 Required observing time and focal-plane area

  • At 5 µK.arcmin over the full sky, the required observing time is 500–1000 detector-years from space and about 10^5 detector-years on the ground.The requirement scales proportionally to 1/σ^2 and fsky.Source: §A.2 · p. 61, Figure 12 · p. 62
  • For four years of observation, space requires 200–300 detectors at 100–150 GHz, while ground experiments need about two orders of magnitude more detectors.These counts are for the same 5 µK.arcmin full-sky-equivalent sensitivity.Source: Figure 12 · p. 62
  • At the same sensitivity, the focal-plane dimension is about 10 cm for space and around 1 m on the ground.For a single-mode pixel, required focal-plane area is roughly proportional to λ^2; the figure assumes 9λ^2 per detector.Source: §A.2 · pp. 61–62, Figure 12 · p. 62
  • Multi-frequency, dual-polarisation detectors can reduce focal-plane size by a factor of 6 for dual-polarisation, trichroic detectors, provided optical efficiency is not worse than for single-frequency, single-polarisation detectors.Source: §A.2 · p. 62
  • The most efficient observing frequency by focal-plane area is 170–200 GHz from space, depending somewhat on payload temperature, and 150 GHz from the ground.The ground-based optimum is stated in terms of sensitivity per focal-plane area.Source: §A.2 · p. 62, Figure 12 · p. 62

A.3 Atmospheric emission fluctuations · B Scan strategy optimisation

Atmospheric fluctuations can dominate low-frequency ground-based timestream noise and remain relevant to large-scale CMB B-mode measurements despite modulation. At sub-millimetre wavelengths, the atmosphere makes broad-band ground observations above 300 GHz very challenging, leaving space as the viable option for low-noise, few-arcminute, large-area surveys.

A.3 Atmospheric emission fluctuations

  • At 300 K and 2% emissivity, atmospheric emission is about 6 KRJ; 0.1% temperature inhomogeneities produce about 6 mKRJ fluctuations, over an order of magnitude above typical 150-GHz detector white noise on 1-s timescales.The fluctuations, mainly caused by precipitable-water-vapour and temperature inhomogeneities, dominate low-frequency timestream noise and are correlated across focal-plane detectors, so they do not average down as √Ndet.Source: §A.3 · p. 62
  • A rotating half-wave plate can reduce atmospheric fluctuations by around 10^3, but they cannot be ignored for large-scale B-mode measurements because B modes are more than 3 orders of magnitude below temperature anisotropies.Atmospheric emission is only weakly polarised, so residual fluctuations affect temperature anisotropy measurements more than polarisation.Source: §A.3 · p. 62
  • Even at excellent ground sites, broad-band observations above 300 GHz are very challenging, while balloons avoid atmospheric noise but offer only days to weeks of observing time.The passage identifies space as the only viable option for low-noise, few-arcminute-resolution, large-sky-area observations at sub-millimetre wavelengths.Source: §A.3 · p. 63

B.1 Main requirements and design drivers

The section links scan geometry to map quality and sets CORE’s baseline precession and line-of-sight offset angles to ensure full-sky coverage by all focal-plane detectors.

B.1 Main requirements and design drivers

  • In the assumed scanning strategy, the satellite spins about its symmetry axis while that axis precesses around the anti-solar direction; the LOS offset β sets the near-circle FOV-center scan radius β.Source: §B.1 · p. 63
  • CORE’s M5 baseline uses α = 30° and β = 65°, rather than COrE+’s α = β = 45°, with the angles chosen so α + β = 95°.COrE+’s angles could desirably be increased slightly to let all focal-plane detectors observe the complete sky.Source: §B.1 · p. 63
  • The CORE baseline ensures full-sky coverage for every detector in a focal plane with angular radius below 5°.The scan geometry is defined by precession angle α and line-of-sight offset angle β.Source: §B.1 · p. 63

B.2 Practical constraints

CORE’s practical design constraints link anti-solar scanning to payload thermal stability, shielding and full-sky coverage, while downlink requirements also constrain the orbit. The scan geometry trades angle-crossing coverage against rapid connections between distant sky pixels.

B.2 Practical constraints

  • For one precession, α > β gives small rings crossing at large angles and good angle coverage over timescales of order β days, whereas α < β gives large rings crossing at small angles but quickly connecting distant pixels.Source: Figure 13 · p. 64
  • With the precession axis maintained anti-solar and the payload symmetrical, integrated solar illumination on the solar panels and outer V-groove screen remains constant, supporting payload thermal stability.The payload temperature depends on the incidence angle of solar radiation on the spacecraft.Source: §B.2 · pp. 63–64
  • The Sun must not illuminate the inner V-groove screens or payload, limiting precession angle α; the Earth and Moon must also remain screened from the detector array.Source: §B.2 · p. 64
  • CORE’s field of view is <∼5° in radius, making α + β = 95° appropriate for full-sky coverage with all detectors; larger α + β brings the Sun, Earth, and Moon closer to the line of sight.Detector offset βd depends on its focal-plane location and the optical setup.Source: §B.2 · p. 64
  • Downloading data to Earth requires a steerable antenna pointed toward Earth to cancel spacecraft spin and precession; phased-array and mechanical pointing systems are options.Possible Earth aspect angle and scanning speed depend on antenna specifications, including maximum boresight angle and tracking speed.Source: §B.2 · pp. 64–65

B.3 Sampling

The section quantifies co-scan sampling cadence and cross-scan spacing for the CORE baseline, and reports the sky fraction covered during each precession.

B.3 Sampling

  • The baseline yields 199 samples per second, or about 5 ms per sample, for a 4′ beam with Ns = 4, β = 65◦, and Tspin = 120 s.Source: §B.3 · p. 65
  • The baseline cross-scan step is 3.75′, insufficient for the highest-frequency channels; increasing the spin rate to 1 RPM would improve it.For most of the sky covered during one precession, the cross-scan sampling is smaller than this maximum value.Source: §B.3 · p. 65
  • With baseline α = 30◦ and β = 65◦, each precession period covers 45% of the sky.Source: §B.3 · p. 65

B.4 Optimisation

CORE scanning design balances cross-scan sampling, sky coverage, scan-angle distribution, and spacecraft constraints. Although strategy B offers an attractive scan-angle distribution, matching CORE sampling with its geometry requires a much shorter spin period or a much longer precession period.

B.4 Optimisation

  • Both strategies cover the same sky area per precession, but strategy A meets the cross-scan sampling requirement whereas strategy B does not; B is nearly ideal for single-detector scan-angle distribution (Wallis et al. 2017).The examples are A: α = 30◦, β = 65◦, Tspin = 120 s, Tprec = 4 days; and B: α = 65◦, β = 30◦, Tspin = 600 s, Tprec = 93 minutes.Source: §B.4 · p. 67
  • After 4 days, strategy B leaves large scan gaps and a very inhomogeneous observing-time distribution, although accumulated precessions gradually fill the gaps.The coverage illustration is for one detector in a HEALPix Nside = 512 Galactic map, measured in √s per pixel.Source: §B.4 · p. 67
  • Tprec/Tspin ≃5220 matches CORE’s 3.75′ cross-scan sampling for α = 65◦ and β = 30◦; with Tspin = 10 minutes, Tprec must be 36 days, or a 4-day precession requires Tspin ≃1.1 minutes.Source: §B.4 · p. 67
  • Even optimally filling gaps takes at least 36 days with Tspin = 10 minutes, hindering cross-time map comparison; the moving precession axis leaves gaps, so shorter spins are needed, while LiteBIRD’s coarser resolution relaxes sampling.Over 36 days, the precession axis moves about 35.5◦ to follow the Sun’s yearly motion.Source: §B.4 · p. 67
  • CORE’s adopted scanning parameters are a compromise: strategy B’s angles are attractive, but α = 65◦ requires additional solar panels and V-groove redesign, while Tspin = 1.1 min challenges attitude control.The study notes that strategy B’s angles could be retained if Tspin and Tprec were modified.Source: §B.4 · p. 67
Loading 1706.04516v1…