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The QUEST for CMB Polarization

• Walter K. Gear
• Cardiff University

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Talk Structure

• CMB Review
• Why Polarization ?
• The QUEST Experiment
• (Future Plans)

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Curtesy Wayne Hu htp\\background.uchicago.edu
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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 BigBang/inflation should also be imprinted
  there...

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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 rT/S
• and the slope n of the assumed power-law
  spectrum P(k)

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• 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

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CMB The Golden age.
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Temperature power spectra
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• CMB The Golden Age ..

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• CMB The Golden Age ..

Flat, n1 ?b 0.021, ?c 0.196, Ho 47 ?b
0.022, ?c 0.132, Ho 68, ? 2/3
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The MAP Temperature results..
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• 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)

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The power of polarization

• Fundamental prediction of standard theory, if not
  detected at mK 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 DT 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

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• 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 !!
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E and B modes

• The scalar function E represents pure density
  fluctuations
• The tensor function B represents metric
  fluctuations - gravity waves

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Polarisation of the CMB
Temperature
Q
U

• Generated by Thompson scattering off electrons in
  quadrupolar motion.

Polarisation Matrix
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E/B Decomposition
Cold Spot Hot Spot

• Can decompose Q,U into
• E-modes (even-parity)
• B-modes (odd-parity)
• E-modes generated by scalar tensor
  perturbations.
• B-modes generated by tensors grav. lensing.

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Pure E(left) B(right)
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CMB polarisation spectra

• Have 4 possible spectra TT, TE, EE, BB.
• TB EB 0 by parity.

Sachs-Wolfe
Silk Damping
Acoustic Oscillations
Gravitational Lensing
Reionisation
Gravitational Waves
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• 19/9/2002 DASI announces E-mode detection !!

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WMAP Results

• Temp-Polzn Cross-Power spectra (l1)ClTE/2p

High low-l modes. Adiabatic acausal
perturbations. Line based on T-data only. (no
free parameters.)
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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 gt mass
  clumping history from LSS to now gt dark energy?
• Eventually measure primordial B-modesgt constrain
  inflation

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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 gt Bolometers
• Need careful foreground identification and
  subtraction gt multi-frequency

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Planck Surveyor

• Planck-HFI will conduct all-sky survey to 5 in
  2007-2009

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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 DT ground-based experiments
• Can upgrade and repair instrument, more
  flexibility and (a lot!) less cost

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THE QUEST Project

• There is a need for a deep (mK), small area (10s
  to 100s 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, SuZie, BOOMERANG
  and Herschel/Planck, amongst many.

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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
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Flexibility of QUEST

• A real experiment has a sensitivity of

• (Knox 1995)
• ?T sensitivity/pixel/Stokes parameter
• ?pix pixel size
• Optimum

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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
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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
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Choice of Filter Bands

• Motivated by science avoid and remove
  foregrounds
• Only two frequencies simplifies the design of the
  refracting reimaging optics

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Predicted sensitivities
For 1mm PWV NETs - 100 GHz0.3 150 GHz0.4mK
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The QUEST Focal Plane Design

• Each channel will use a PSB

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QUEST OPTICAL DESIGN

• Wide-field (1.5 degrees), good optical quality
  (strehl gt0.9), broadband (90-220 GHz)
• On-axis and symmetric
• Cold pupil-stop, small in order to fit waveplate

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QUEST OPTICAL DESIGN
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Cold Optics Overview

• The lenses and waveplate are cooled to 4K
• All components have a broad-band anti-reflection
  coating
• sapphire waveplate is located close to the cold
  stop
• The cold stop is located at an image of the
  primary mirror

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QUEST TELESCOPE

• QUEST telescope is 2.6m 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

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QUEST Site and schedule

• Officially begin operations in Chile spring 2004
• But .

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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.

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QUEST Science Goals

• To map CMB polarization on angular scales gt 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
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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..

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E-Modes

• Maximum (S/N)EE 100.
• 1000 sq degs, 2000hrs.

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TE-Correlation

• 1000 sq degs
• Cross-correlate QUEST WMAP.

TT
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B-Modes

• Maximum (S/N)BB gt 5, detection of B-modes.
• 2 x 30 sq degs, 2000hrs.

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Comparing QUEST with other experiments
EE
BB, GW
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Cosmological Parameter Forecasts

• Fisher Information Matrix analysis of
  cosmological parameters.
• Use a 7 parameter set

Wmh2 - Matter density Wbh2 - Baryon
density h - Hubble parameter t
- Reionisation optical depth ns -
Scalar spectral index A - Scalar
amplitude ( s8) r - Ratio of scalar
to tensors
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Cosmological Parameter Forecasts

• DWbh2 Dh Dt Dns DA
  Dr

DWmh2 DWbh2 Dh Dt Dns DA
4 yr WMAP
2 yr QUEST 4 yr WMAP
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Cosmological Parameter Forecasts

• Factor 3 improvement in r.
• Factor 2 improvement in ns.

4 yr WMAP
2 yrs QUEST 4 yrs WMAP
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Science Summary

• QUEST will measure EE-Power with s/n100 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.

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Future Plans .

• QUEST and other plannned experiments will only
  measure T/S0.05-0.1
• To go deeper requires more sensitivity and
  systematic rejection
• Lensing contamination probably means a limit
  gt0.001
• NASA already planning a dedicated B-mode
  satellite 2015

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Future Plans

• These 4th generation experiments will require
  x100 improvement in sensitivity and systematic
  rejection
• We (CardiffCambridge) planning a UK programme of
  ground-based and possibly balloon-borne 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 !