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The “QUEST” for CMB Polarization Walter K. Gear Cardiff University
Talk Structure • CMB Review • Why Polarization ? • The QUEST Experiment • (Future Plans)
Curtesy Wayne Hu htp: \background. uchicago. edu
CMB: Cosmic Rosetta Stone. . . • CMB arises from last-scattering surface ~300, 000 years after the Big Bang • This is the earliest direct image of the Universe we can ever obtain (EM anyway…) • The imprints of structure of the Universe today AND Big. Bang/inflation should also be imprinted there. . .
Constraining Inflation • Accurate measurement of the CMB can constrain the nature of the inflationary potential • in particular the ratio of scalar to tensor fluctuation amplitude r=T/S • and the slope n of the assumed powerlaw spectrum P(k):
Scalars and Tensors Inflation predicts a mixture of scalar (pure density) and Tensor (gravity wave)fluctuations • • The precise ratio is a function of the type of field which causes inflation • Scalar fluctuations couple to matter and provide the “seeds” for structure formation • Tensor perturbation causes a ‘background’ of gravity waves
CMB: The Golden age….
Temperature power spectra
CMB: The Golden Age …. .
CMB: The Golden Age …. . Flat, n=1; b = 0. 021, c = 0. 196, Ho = 47; b = 0. 022, c = 0. 132, Ho = 68, = 2/3
The MAP Temperature results…. .
The anisotropy measurements have been a triumph, BUT …. • “With temperature data alone, r of less than ~0. 1 cannot be detected, no matter how accurate the measurement” (Kinney 1999 astroph/9806259) • With polarization data however we can break this degeneracy (amongst others)
The power of polarization… • Fundamental prediction of standard theory, if not detected at m. K then there would be real problem • The extra information provided by polarization allows much better constraints on some vital cosmological parameters - 4 power spectra rather than 1. • Combination of P and T improves some parameter constraints by factors 2 -3 in most models • Break degeneracy between intrinsic fluctuation amplitude and re-ionization • Separate scalar and tensor modes in the initial fluctuation spectra, if B as well as E modes can be detected
Temperature is a scalar but Polarization is a second-rank Tensor It is convenient to write this is as the sum of the gradient and curl of a scalar and vector field E and B [but has nothing to do with E and B EM fields !!]
E and B modes • The scalar function E represents pure density fluctuations • The tensor function B represents metric fluctuations - gravity waves
Polarisation of the CMB Temperature Q + U • Generated by Thompson scattering off electrons in quadrupolar motion. Polarisation Matrix: P = Q + U
E/B Decomposition Cold Spot Hot Spot – E-modes (even-parity): E E – B-modes (odd-parity): B B • Can decompose Q, U into: • E-modes generated by scalar & tensor perturbations. • B-modes generated by tensors & grav. lensing.
Pure E(left) & B(right)
CMB polarisation spectra • Have 4 possible spectra: TT, TE, EE, BB. • TB = EB = 0 by parity. Sachs-Wolfe Acoustic Oscillations Reionisation Silk Damping Gravitational Lensing Gravitational Waves
19/9/2002: DASI announces E-mode detection !!
WMAP Results • Temp-Polzn Cross-Power spectra: (l+1)Cl. TE/2 p High low-l modes. Adiabatic acausal perturbations. Line based on T-data only. (no free parameters. )
CMB Polzn exists! What now? • Detection only so far, need to first map out the E-mode spectrum into the peak region& damping tail & properly measure reionization peak. • Measure B-mode contamination from lensing => mass clumping history from LSS to now => dark energy? • Eventually measure primordial B-modes=> constrain inflation
How to measure polarization ? • Measuring such tiny signals inevitably involves differencing to minimize systematics and multiple levels of modulation • Broad bandwidths also generally required for sensitivity => Bolometers • Need careful foreground identification and subtraction => multi-frequency
Planck Surveyor • Planck-HFI will conduct all-sky survey to 5’ in 2007 -2009
Why do it from the ground ? • Can in principle obtain much smaller angular scales than from satellite • Can concentrate on smaller pieces of sky than MAP or Planck and go deeper quicker • Can concentrate on range of multi-poles that offer largest predicted amplitude and best parameter discrimination • Differencing means both polzns go through same column of atmosphere - not so sensitive to atm noise as T ground-based experiments • Can upgrade and repair instrument, more flexibility and (a lot!) less cost
THE QUEST Project • There is a need for a deep (~m. K), small area (10 s to 100 s sq. deg) polzn experiment which will report on a short timescale • The Q and U E xtragalactic S ubmm T elescope project aims to fill this gap. • It is a joint UK/US project capitalising on expertise and heritage of SCUBA, Su. Zie, BOOMERANG and Herschel/Planck, amongst many.
Q and U Extragalactic Submm Telescope QUEST Collaboration: Cardiff: W. Gear, P. Ade, L. Piccirillo- telescope, cryogenics, filters Stanford: Sarah Church - Focal plane & electronics JPL/Caltech: Jamie Bock & Andrew Lange - detectors + K. Ganga (JPL), A. Taylor (Edin) + associates
Flexibility of QUEST • A real experiment has a sensitivity of: (Knox 1995) • T – sensitivity/pixel/Stokes parameter • pix – pixel size • Optimum
Normally in a ground-based CMB experiment one has to chop to remove atmosphere. However there is always a residual uncancelled emission which often dominates the noise
For a polarization experiment however we difference two polarizations which travel through the same column of atmosphere - no need to chop - and also makes dish simpler and cheaper
Choice of Filter Bands • Motivated by science – avoid and remove foregrounds • Only two frequencies simplifies the design of the refracting reimaging optics Center (GHz) Band 1 93 Band 2 147 Lower edge Upper edge Bandwidth (GHz) (%) 81 105 25. 0 128 165 25. 0
Predicted sensitivities For 1 mm PWV NETs : 100 GHz~0. 3 150 GHz~0. 4 m. K
The QUEST Focal Plane Design Frequency (GHz) Beam size (arcmin) 100 143 n Number of Feeds 12 19 6 4 Each channel will use a PSB
QUEST OPTICAL DESIGN • Wide-field (1. 5 degrees), good optical quality (strehl >0. 9), broadband (90 -220 GHz) • On-axis and symmetric • Cold pupil-stop, small in order to fit waveplate
QUEST OPTICAL DESIGN
Cold Optics Overview Lens 1 Lens 2 Sapphire achromatic waveplate • The lenses and waveplate are cooled to 4 K • All components have a broad-band antireflection coating • sapphire waveplate is located close to the cold stop • The cold stop is located at an image of the primary mirror
QUEST TELESCOPE • QUEST telescope is 2. 6 m Cassegrain with foam-cone supporting secondary • Designed to rotate around 3 axes - Az, El and also centreline of primary (‘Z’) • Will point and track +/- 45 deg from Zenith with 0. 3 arcmin rms • Because of novel optical design, cryostat is mounted through centre of primary
QUEST Site and schedule • Officially begin operations in Chile spring 2004 • But …….
QUEST on DASI • We have been approached by and are in detailed discussion with the DASI team • Which is likely to result to a late switch to the South Pole….
QUEST Science Goals n n To map CMB polarization on angular scales > 3 Optimized to map E-modes, and B-modes produced by gravitational lensing and gravity waves the largest scales will be determined by scan strategy and the exact science goals Planned l-space coverage of QUEST Hu et al. 2002
Survey Strategy • Two major surveys for separate goals • ~1000 sq. deg survey for detailed E-mode measurement (~6 months) • ~30 sq. deg survey for detailed B-mode measurement (~18 months) to detect lensing signal and possibly primordial gravity waves…. .
E-Modes • Maximum (S/N)EE ~100. • 1000 sq degs, 2000 hrs EE BB, GL BB, GW
TE-Correlation • 1000 sq degs • Cross-correlate QUEST & WMAP. TT
B-Modes • Maximum (S/N)BB > 5, detection of B-modes. • 2 x 30 sq degs, 2000 hrs. EE BB, GL BB, GW
Comparing QUEST with other experiments EE BB, GW
Cosmological Parameter Forecasts • Fisher Information Matrix analysis of cosmological parameters. • Use a 7 parameter set: Wmh 2 - Matter density Wbh 2 - Baryon density h - Hubble parameter t - Reionisation optical depth ns - Scalar spectral index A - Scalar amplitude (~ s 8) r - Ratio of scalar to tensors
Cosmological Parameter Forecasts Wbh 2 h t ns A r Wmh 2 Wbh 2 h t 4 yr WMAP ns 2 yr QUEST + 4 yr WMAP A
Cosmological Parameter Forecasts • Factor 3 improvement in r. • Factor 2 improvement in ns. 4 yr WMAP 2 yrs QUEST + 4 yrs WMAP
Science Summary n n n n QUEST will measure EE-Power with s/n=100 over very large l-range – reionisation? Neutrinos? Should detect and measure BB-Power Spectrum. Cosmological Parameters: 2 yr QUEST will improve 4 yr WMAP by factors 2 -3. Main improvement on ns & r, so stronger constraints on inflation. Test isocurvature modes from Inflation. Test for non-Gaussianity. Direct measure of P(k) from grav lensing. Due to start early 2005 & run for 2 yrs.
Future Plans …. • QUEST and other plannned experiments will only measure T/S~0. 05 -0. 1 • To go deeper requires more sensitivity and systematic rejection • Lensing contamination probably means a limit >0. 001 • NASA already planning a dedicated Bmode satellite ~2015
Future Plans • These ‘ 4 th generation’ experiments will require ~x 100 improvement in sensitivity and systematic rejection • We (Cardiff&Cambridge) planning a UK programme of ground-based and possibly balloonborne B-mode experiments • We believe a combination of the existing bolometer technology with interferometric imaging is the way to achieve this……… but that is another seminar entirely !