CMB Observations with the Cosmic Background Imager
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CMB Observations with the Cosmic Background Imager. Tim Pearson for the CBI team. Tony Readhead (Principal Investigator), Steve Padin (Project Scientist until 2002). CBI Timeline. 1995-1999: design and construction 1998-1999: testing in Pasadena 1999: ship to Chile and commission

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CMB Observations with the Cosmic Background Imager

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CMB Observations with the Cosmic Background Imager

Tim Pearson

for the CBI team

Tony Readhead (Principal Investigator),

Steve Padin (Project Scientist until 2002).

2005 March 24


CBI Timeline

  • 1995-1999: design and construction

  • 1998-1999: testing in Pasadena

  • 1999: ship to Chile and commission

  • 2000-2001: CMB T and SZE observations (Stokes L)

    • 2-field differencing

  • 2002-2005: CMB polarization observations (Stokes L&R)

    • 6-field common mode

  • Jun 2005 - present: idle (unfunded)

  • May-Dec 2006: upgrade to larger antennas, T/SZE observations

  • 2007- : replace with QUIET receivers

2005 March 24


13 Cassegrain antennas

0.9 m diameter

26–36 GHz, 10 channels

HEMT amplifiers, Tsys ~ 27 K

Baselines 1 m – 5.5 m

Analog correlator

Alt-az mount with parallactic rotation

2005 March 24


An interferometer cross-correlates the signals received by two separated antennas: the response (“visibility”) is proportional to a Fourier component with spatial frequency u = d/λ.

The power spectrum Cl is the expectation of the square of the Fourier transform of the sky intensity distribution: i.e., closely related to the square of the visibilityVV*.

Multipole l = 2p u

Estimate spectrum by squaring visibility and subtracting noise bias.

The observed sky is multiplied by the primary beam, corresponding to convolution (smoothing) in the (u,v) plane: so the interferometer measures a smoothed version of the power spectrum.

Mosaicing several fields is equivalent to using a larger primary beam, thus improving resolution in l.

CMB interferometers

CAT, DASI, CBI, VSA, MINT, Amiba

The CBI – Interferometry of the CMB

2005 March 24


Interferometry Advantages

  • Insensitive to large-scale structure

  • Uncorrelated noise

  • Direct measurement of polarization Q ± iU

  • Beam uncertainty not very important

  • Very different systematics

2005 March 24


Chajnantor Observatory

Home of CBI, QUIET, and other experiments

2005 March 24


Total Intensity Observations

  • Observations made in 1999-2002

  • Problem 1: Ground spillover

    • Differencing of two fields observed at same AZ/EL

  • Problem 2: Foreground point sources

    • Measure with higher resolution instrument

    • “Project out” of dataset sources of known position

    • Statistical correction to power spectrum

2005 March 24


Mosaic images

  • Emission from ground (horizon) dominant on 1-meter baselines

  • Observe 2 fields separated by 8 min of RA, lead for 8 min followed by trail for 8 min; subtract corresponding visibilities. Ground emission cancels.

  • Images show lead field minus trail field

  • Also eliminates low-level spurious signals

2005 March 24


CBI Polarization

• Compact array

• switchable RCP or LCP

36 RR or LL baselines measure I

42 RL or LR baselines measure Q+iU or E+iB

• New ground strategy: strips of 6 fields, remove common mode (mean);(Lose 1 mode per strip to ground)

• CBI observes 4 patches of sky – 3 mosaics & 1 deep strip

Pointings in each area separated by 45’. Mosaic 6x6 pointings, for 4.5deg square, deep strip 6x1.

• 2.5 years of data, Aug 02 – Apr 05.

* Note bug in earlier analysis: omitted one antenna (12/78 baselines!)

2005 March 24


Raw Images

2005 March 24


“Ground subtracted” images

2005 March 24


Data Reduction

  • Editing and calibration

  • Noise estimation

  • Gridding of RR+LL, RL, LR or T, E, and B with full covariance matrix calculation

  • Project out common ground (downweight linear combination of data)

  • Project out point sources in T

  • Ignore point sources in polarization

  • Images of E and B (FT of gridded estimators)

  • Power spectrum estimation by max likelihood

2005 March 24


CBI Combined TT (2000-2005)


2.9σ above model

2005 March 24


Projecting Out Variable Sources

Marginalize over 1 parameter (flux) for each source,

Or 2 parameters (2000-01 and 2002-05 flux).

2005 March 24


Cosmology Results

CBI has measured power spectrum to much higher l than previous experiments, well into damping tail

Flat universe with scale-invariant primordial fluctuation spectrum

Low matter density, baryon density consistent with BBN, non-zero cosmological constant

Agreement with Boomerang, DASI, VSA and Maxima at l < 1000 is excellent

2005 March 24


  • At 2000 < l <3500, CBI finds power ~ 3 sigma above the standard models

  • Not consistent with any likely model of discrete source contamination

  • Suggestive of secondary anisotropies, especially the SZ effect

  • Comparison with predictions from hydrodynamical calculations: strong

  • dependence on amplitude of density fluctuations, s87 . Requires s8~1.0

2005 March 24


Varying 6 parameters plus amplitude of SZ template component

2005 March 24


CBI Upgrade

  • Larger 1.4-m dishes (Oxford University)

    • Lower ground pickup, lower noise

  • Ground screen

  • Close-packed array

  • Concentrate on high-l excess and SZE in clusters

  • 9–12 months of observing before QUIET

2005 March 24


CBI2

2005 March 24


NVSS Sources in CBI Field

2005 March 24


GBT observations

  • Green Bank telescope 30 GHz measurements of NVSS sources in CBI fields

  • New Caltech Continuum Backend for switched observations

  • 1580 (of ~4000) sources observed so far under photometric conditions

  • 175 detected S > 2.5 mJy (5σ) at 32 GHz

  • Non-detections can be safely ignored in CBI!

  • Additional GBT observations to characterize faint source population

  • Brian Mason, Larry Weintraub, Martin Shepherd

2005 March 24


CBI2 Projection

SZE

Secondary

CMB

Primary

~ s87

2005 March 24


CBI Polarization Spectra

  • TT consistent with earlier results

  • EE and TE consistent with predictions

  • BB consistent with zero

TT

EE

TE

BB

2005 March 24


Shaped Cl Fit

Likelihood of EE Amplitude vs. “TT Prediction”

  • Use WMAP’03+CBI TT+ Acbar best-fit Cl as fiducial model

  • Results for CBI

    • EE qB = 0.97 ± 0.14 (68%)

    • EE likelihood vs. zero : significance 10.1 σ

    • TE qB = 0.85 ± 0.25

    • BB qB = 1.2 ± 1.8 μK2

2005 March 24


Comparison of Experiments

2005 March 24


Comparison of Experiments

2005 March 24


Comparison of Experiments

2005 March 24


θ/θ0

  • Angular size of sound horizon at LSS should be same for TT and EE.

  • CBI only has multiple solutions (shift spectrum by one peak).

  • DASI removes degeneracy, but less sensitive.

  • CBI EE + DASI EE give scale vs. TT of 0.98 +/- 0.03.

2005 March 24


Isocurvature

Isocurvature puts

peaks in different

places from adi-

abatic. We use

seed isocurvature

model and find

both EE and TE

prefer adiabatic w/

iso consistent with

zero.

2005 March 24


Isocurvature

  • Normalize seed iso spectrum to total power expected from TT adiabatic prediction

  • Fit shapes for both EE, TE

  • EE adi =1.00±0.24, iso=0.03±0.20

  • TE adi = 0.86±0.26, iso=0.04±0.25

2005 March 24


WMAP Ka-band polarization

Foregrounds

DRAO 1.4 GHz polarized intensity

(Wolleben et al. astro-ph/0510456)

WMAP synchrotron component

(WMAP Science Team)

2005 March 24


Foregrounds

  • TT: template comparisons

    • 2.5σ detection of correlation with 100 μm template

    • CHFT observations to provide SZ template

  • Polarization: No evidence (yet) for foreground contamination:

  • No B-mode detection

  • No indication of discrete sources (power ∝ l2)

  • Upper limit on synchroton component (DASI)

2005 March 24


WMAP3

WMAP3+CBIcombinedTT+CBIpol

CMBall = Boom03pol+DASIpol +VSA+Maxima+WMAP3+CBIcombinedTT+CBIpol

2005 March 24


People

2005 March 24


  • http://astro.caltech.edu/~tjp/CBI/

  • Readhead et al. 2004, ApJ, 609, 498

  • Readhead et al. 2004, Science 306, 836

  • Sievers et al. 2005, astro-ph/0509203

2005 March 24


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