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Clover: Measuring Gravitational-Waves from Inflation

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Clover: Measuring Gravitational-Waves from Inflation ClOVER: Measuring gravitational-waves from Inflation Executive Summary The existence of primordial gravitational waves in the Universe is a fundamental prediction of the inflationary cosmological paradigm, and determination of the level of this tensor contribution to primordial fluctuations is a uniquely powerful test of inflationary models. We propose an experiment called ClOVER (ClObserVER) to measure this tensor contribution via its effect on the geometric properties (the so-called B-mode) of the polarization of the Cosmic Microwave Background (CMB) down to a sensitivity limited by the foreground contamination due to lensing. In order to achieve this sensitivity ClOVER is designed with an unprecedented degree of systematic control, and will be deployed in Antarctica. The experiment will consist of three independent telescopes, operating at 90, 150 or 220 GHz respectively, and each of which consists of four separate optical assemblies feeding feedhorn arrays arrays of superconducting detectors with phase as well as intensity modulation allowing the measurement of all three Stokes parameters I, Q and U in every pixel. This project is a combination of the extensive technical expertise and experience of CMB measurements in the Cardiff Instrumentation Group (Gear) and Cavendish Astrophysics Group (Lasenby) in UK, the Rome “La Sapienza” (de Bernardis and Masi) and Milan “Bicocca” (Sironi) CMB groups in Italy, and the Paris College de France Cosmology group (Giraud-Heraud) in France. This document is based on the proposal submitted to PPARC by the UK groups (and funded with 4.6ML), integrated with additional information on the Dome-C site selected for the operations. This document has been prepared to obtain an endorsement from the INAF (Istituto Nazionale di Astrofisica) on the scientific quality of the proposed experiment to be operated in the Italian-French base of Dome-C, and to be submitted to the Commissione Scientifica Nazionale Antartica and to the French INSU and IPEV. 1 Index 1: Scientific Objective 3 1.1: Cosmic microwave background polarization observables 1.2: Current status of cosmic microwave background polarimetry 1.3 The scientific case for CLOVER 1.4: Foregrounds 1.5: Other Polarization Experiments 2: Technical specification 21 2.1: Overview of the experiment 2.2: Detailed description of key areas 2.2.1: Telescope and mount 2.2.2: Cryogenics 2.2.3: Array and polarimeter 2.3.3: Horns 2.3.4: Orthomode transducers (OMTs) 2.3.5: Waveguide hybrids 2.3.6: Phase shifters 2.3.7: Waveguide twists, transitions and bends 2.3.8: Superconducting detector array 2.4: Systematics 2.5: Scanning strategy 2.6: Site and Logistics 3: New technologies 41 4: Implementation of ClOVER/BRAIN at Dome-C 41 4.1: Introduction 4.2: Buidings/Construction 4.3: Electrical 4.4: Data Storage 4.5: Calibration 4.6: Timing and Personnel 5: Costs to UK 44 6: Costs to Italy and France 44 7: Draft Memorandum of Understanding between UK, France, Italy 45 2 1: Scientific objective 1.1: Cosmic microwave background polarization observables Thomson scattering of anisotropic radiation at last scattering gives rise to linear polarization in the cosmic microwave background (Rees 1968). The polarization signal is expected to have an r.m.s. ~5 mK, peaking at multipoles l~1000, corresponding to the angle ( q~p/l ) subtended by the photon mean free path at last scattering. The polarization signal depends sensitively on the fluctuations on the last scattering surface, and thus encodes a wealth of cosmological information, complementary to that contained in the temperature anisotropies. In addition, large-angle polarization is generated by subsequent re-scattering as the universe reionizes, providing a unique probe of the ionization history at high redshift. Linear polarization, characterised by Stokes parameters Q and U, can be decomposed into a curl-free part (electric; denoted E) and a divergence-free part (magnetic or B; Seljak & Zaldarriaga 1997; Kamionkowski, Kosowsky & Stebbins 1997). The electric and magnetic parts can be represented by their spherical multipoles on the full sky, Elm and Blm, which are related to the Stokes parameters by a spin-2 spherical harmonic expansion: r m r (Q ± iU )(n) = å(Elm m Blm )m2 Yl (n) l,m Under the assumption of statistical isotropy, different l and m modes are uncorrelated, and the * E polarization power spectra are defined by e.g. Elm Elm = Cl . The cross-correlation between E and B vanishes if parity invariance holds in the mean. Rotationally- and parity-invariant T E B theories thus predict four non-vanishing power spectra, C l , C l , C l and the cross-correlation of TE E and T, the temperature anisotropies, Cl . Examples of pure electric and magnetic polarization patterns are given in Fig.1.1. The cosmological importance of the E-B decomposition stems from the result that linear, scalar (density) perturbations do not produce magnetic polarization (Seljak & Zaldarriaga 1997; Kamionkowski et al. 1997) since they give rise to a spatial pattern of the polarization field at last scattering that is curl-free. However, a cosmological background of gravitational waves (tensor modes), such as that generated in most models of inflation, produces both E-and 3 B-mode polarization with similar power in each mode. The power is concentrated around l~100 since the amplitude of gravitational waves damps once inside the horizon. The polarization from gravitational waves adds incoherently with that from density perturbations, but the E-B decomposition allows the two contributions to be cleanly disentangled. Vortical motion of the primordial plasma, due to vector modes at last scattering, would also produce B-mode polarization, but vector modes are only expected to be significant in models with active generation of perturbations (e.g. defect models). Such models are definitively ruled out as the main source of structure formation by current CMB temperature anisotropy data. A detection of large-angle, primordial magnetic polarization in the CMB would thus be very strong evidence for a cosmological background of gravitational waves. An additional source of magnetic polarization arises from weak gravitational lensing of CMB photons by large-scale structure. These deflections re-map the curl-free pattern of polarization from density perturbations, redistributing some E-mode power into B-mode, even in the absence of gravitational waves (Zaldarriaga & Seljak 1998). The various theoretical power spectra of the polarization and temperature anisotropies are shown in Figure 1.2 for the concordance flat, L-CDM model (Spergel et al. 2003) with optical depth through reionization t=0.148. The density fluctuations are adiabatic with spectral index 2 ns=0.96 , and the tensor-to-scalar ratio r =0.15 appropriate to chaotic (f ) inflation. Only the temperature anisotropy spectrum has been mapped accurately to date, but the E and B TE power spectra, and the lensing contribution to C l , can be predicted with reasonable accuracy from current data. However, the amplitude of the gravitational wave background is subject only to weak upper bounds by the anisotropy data. Although the amplitude of is not known, its shape is robust since it depends mainly on cosmological parameters that are already well constrained. 1.2: Current status of cosmic microwave background polarimetry A detection of the polarization of the CMB was first announced in 2002 by the Degree Angular Scale Interferometer team (DASI; Kovac et al. 2002). They report a five-sigma detection of electric polarization, but no evidence for magnetic polarization in their data. These conclusions 4 have been reiforced but not changed significantly by the recently published analysis of 3 years of data from DASI (Leitch et al. 2004). The CBI experiment has also detected E-modes polarization at smaller scales, and with a similar significance (Readhead et al. 2004). At the time of writing the only other published detection is of the TE cross-correlation from the one- year WMAP data (Kogut et al. 2003). The DASI, CBI and WMAP detections have already had considerable scientific impact: •The presence of acoustic oscillations in the TE and E power spectra with precisely the phase relation and amplitude to the temperature spectrum that is predicted for passive, adiabatic fluctuations, lends further support to this paradigm for structure formation, and allows strong upper limits to be placed on the amplitude of isocurvature fluctuations (Peiris et al. 2003; Gordon & Lewis 2003). •The detection of a correlation between polarization and temperature around l~150 is clear evidence for adiabatic, apparently super-horizon fluctuations at last scattering. Inflation is the simplest, causal mechanism for generating such fluctuations. •The excess power on large scales in the cross-correlation of T and E, seen by WMAP, points to early (and complex) reionization with optical depth t˜0.17 (based on a fit to the TE power spectrum alone). As already noted, B-mode polarization of the CMB is currently subject only to weak upper limits. The best limit on large scales, l~10, is from the POLAR experiment (Keating et al. 2001), while the best limits on smaller scales are from DASI . These (95-per cent) upper limits are shown in Figure 1.4. Although the current upper limit on r from B-mode polarization is much weaker than that inferred from the CMB temperature anisotropies, the former route has the potential to detect much lower levels of r with improvements in instrumentation, but the latter route is limited by the inescapable cosmic variance of the dominant density- perturbation signal in the temperature anisotropies. Indeed, a perfect temperature-anisotropy experiment can only detect r>0.07, even assuming all other parameters are known. Supplementing the temperature anisotropies with electric polarization makes only a modest further improvement. 5 1.3 The scientific case for CLOVER 1.3.1: Mapping the B-mode power spectrum The main science goal of CLOVER is to measure the power spectrum of B -mode polarization in the multipole range 20–1000.
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