European Cosmic Microwave Background Studies: Context and Roadmap

European Cosmic Microwave Background Studies: Context and Roadmap A list of contributors can be found in the appendix. September 17, 2018 Contents 1 Introductory Summary 1 2 Science 1 3 Experimental Landscape 2 4 Proposed Roadmap 5 1 Introductory Summary Figure 1: Constraints on parameters of the base-ΛCDM The Cosmic Microwave Background has been one of the model from the separate Planck EE, TE, and TT high- primary drivers behind the advent of so-called precision l spectra combined with low-` polarization (lowE), and, cosmology. Europe in general, and the Planck Satellite in the case of EE also with BAO, compared to the joint in particular, has recently played a large role in the field. result using Planck TT,TE,EE+lowE. Parameters on the But as with any endeavor, planning and investment must bottom axis are sampled MCMC parameters with flat pri- be maintained in order to ensure that the European CMB ors, and parameters on the left axis are derived parameters −1 −1 community continues to thrive. The European CMB Co- (with H0 in km s Mpc ). Contours contain 68 % and ordinators have taken it upon themselves to proselytize for 95 % of the probability. From Planck Collaboration et al. such planning. With this Florence Process, we attempt to (2018) inventory the community's priorities and to clearly present them in order to help with resource prioritization. There are at least three broad CMB science axes, which 2 Science here we'll call the Primordial Universe, Large-Scale Struc- ture, and Spectroscopy. While we can do all these different While a satellite such as Planck, with an approximately things with the CMB, to do each well does take some spe- 1.7 m mirror and an orbit at the second Sun-Earth La- cialization and differentiation. For example, he first two grange point (L2) could address a number of different sci- targets are (usually) done with imaging, and can be at- ence subjects (see figure 1), it is difficult to address the tempted from the ground. The third, is something that same range of topics from the ground. So here we sepa- may ultimately be necessary, but which can only be done rate the main science drivers for the CMB into three sepa- well from space. rate science topics, which while inter-related, also demand In this document we have underlined larger, ground- different technological trade-offs. based, European-wide projects. We recognize the impor- Below we discuss three \headline" science targets for tance of smaller projects but note that smaller projects the CMB { each of which would merit dedicated efforts, don't need European-wide coordination of efforts. Simi- even individually. In the interest of succinctness, we don't larly, considerable expertise exists in Europe in the con- mention here the myriad \ancillary" CMB science pos- ception and implementation of space-based CMB mis- sible, all of which merits significant investment as well; sions. In fact, the 2017-2026 APPEC roadmap endorsed we will often get these results \for free". Such topics in- a European-led satellite mission to map the CMB from clude cosmic birefringence, hot gas in the Universe probed space (APPEC 2017). However, there are a number of by the Sunyaev-Zeldovich (SZ) effect, spatial variations of agencies already intimately involved in the CMB, with deviations from CMB's Planck spectrum, tests of the so- well-defined programs and for which a proposal and de- called \anomaly" in temperature data with polarization, liberation process already exists. We hence accentuate Galactic science, and much more. sub-orbital CMB, briefly mentioning in space-based ini- tiatives that contributeDRAFT to the CMB landscape. 2.1 The Primordial Universe and Infla- We take as our long-term horizon roughly a decade from tion now, 2027, which is the timescale for new satellite missions being launched as well as first light for the ground-based Often called the Holy Grail of cosmology, detection of a CMB Stage 4 project. divergent-free \B-Mode" component of primordial CMB In section 2 we present the science that can be done polarization is often held up as a possible \smoking gun" with the CMB. In section 3 we present the state of the for the Inflationary model of the Universe. Were it to field. In section 4 we present the \Road Map". be detected, it would immediately transform our vision of 1 as it gives us a way to probe the difficult-to-observe Dark Matter, but it also means that complementary observations with larger telescopes, such as those described in the next section, will also be necessary. 2.2 Large-Scale Structure and Neutrinos As the Universe evolved from its initial hot, dense, homo- geneous state just after the Big Bang, the structure we see in the Universe today { the galaxies, the clusters of galaxies, and the pattern of the Cosmic Web in general { began to form under the effects of gravity. The result is sensitive to a number of things, including Dark Energy, Dark Mat- ter, and the numbers and masses of light particles, such as neutrinos, present in the Universe at early times. We therefore use this structure to test for the presence of and to characterize these phenomena. This structure appears at small angular scales { on the order of arc-minutes { and thus requires larger telescopes Figure 2: Theoretical predictions for the temperature to measure. Calculations indicate that 6 m telescopes (black), E-mode (red), and tensor B-mode (blue) power will suffice given the wavelengths of interest for the CMB spectra. Primordial B-mode spectra are shown for two (CMB-S4 CDT 2017). In addition, in order to get the representative values of the tensor-to-scalar ratio: r=0.001 statistics required { that is, in order to measure enough and r=0.05. The contribution to tensor B modes from examples of structure formation { we will need to do these scattering at recombination peaks at l∼80 and from reion- measurements over a large fraction of the sky. It is, it ization at ` < 10. Also shown are expected values for should be noted, essentially impossible to observe the full the contribution to B modes from gravitationally lensed sky from a single location on the Earth. E modes (green). Current measurements of the B-mode spectrum are shown for BICEP2/Keck Array (light or- 2.3 Spectroscopy ange), POLARBEAR (orange), and SPTPol (dark orange). The lensing contribution to the B-mode spectrum The confirmation of the Blackbody nature of the CMB can be partially removed by measuring the E and ex- was one of the keys to general acceptance of the Standard ploiting the non-Gaussian statistics of the lensing. From Model of Cosmology. A number of processes that occur at Abazajian et al. (2016). different points along the history of the Universe do change this spectrum, and there is therefore a wealth of informa- tion which can be gleaned from precise measurements of the creation of the Universe and launch new investigations the CMB's spectrum. From the physical processes that into what caused the Big Bang and how it transpired. occur during the so-called phase of recombination, where The most unambiguous signal indicating that Inflation the CMB is last scattered, to relatively more recent pro- might have occurred would be a polarized B-Mode sig- cesses such as the reionization of the Universe and the nal with correlations of order a degree or larger (the blue Sunyaev-Zeldovich effect in clusters and in general, the curves at the lower/left of figure 2). While ancillary data myriad processes that inject any sort of energy into the will be necessary to address astrophysical contaminants, photons of the CMB can be probed with its spectrum. the relatively large angular scales at which these features Tantalizingly, the decades-old COBE/FIRAS measure- appear mean that one does not need a large telescope to ment is still our most sensitive spectroscopic measurement measure these signals { relatively small instruments can of the CMB. But we are now capable of making much more suffice. Additionally, because we are simply looking for sensitive instruments than this venerable experiment. a detection in the first instance, rather than a statistical characterization of the signal on multiple scales, the opti- mal strategy seems to be to integrate deeply on a limited 3 Experimental Landscape sky area, and then to \open" the sky coverage after detection, in order to verify foreground removal, assure oneself Successive generations of experiments have recently been that the statistics, are understood, and so on. categorized in four stages based on the number of detectors they host in their focal planes. Recently completed experiments have been Stage 1 (∼102 detectors) or Stage 2.1.1 Foregrounds 2 (∼103 detectors), while some coming on line now are While raw sensitivityDRAFT is still important to the CMB, in re- Stage 3 (∼104 detectors). As an example, the BICEP2 cent years it has become clear that understanding the so- and Keck Array are Stage 2, while BICEP3 and BICEP called \foregrounds" will also be crucial for digging out the Array are Stage 3. The ultimate plan is to reach a Stage 4 low-laying primordial signals we search (BICEP2/Keck (and thus the name \CMB-Stage 4"), with focal planes of Collaboration et al. 2015). Observations at \neighboring" order (∼105 detectors). Figure 8 shows the corresponding frequencies will be important for future results. In addi- sensitivity progression, assuming no impact from system- tion, nearby structure will \scramble" primordial signals atic effects and no significant penalty from foregrounds or to some extent via lensing.

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