2005 March 24 1

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 3

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 4

13 Cassegrain antennas0.9 m diameter26–36 GHz, 10 channelsHEMT amplifiers, Tsys ~ 27 KBaselines 1 m – 5.5 mAnalog correlatorAlt-az mount with parallactic rotation

2005 March 24 5

The CBI – Interferometry of the CMB

• 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 visibility VV*.

– Multipole l = 2 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

2005 March 24 6

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 7

Chajnantor Observatory

Home of CBI, QUIET, and other experiments

2005 March 24 8

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 9

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 10

• 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 4.5 deg square, deep strip 6x1.

• 2.5 years of data, Aug 02 – Apr 05.

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

CBI Polarization

2005 March 24 11

Raw Images

2005 March 24 12

“Ground subtracted” images

2005 March 24 13

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)

2005 March 24 15

2.9σ above model

2005 March 24 16

Projecting Out Variable Sources

Marginalize over 1 parameter (flux) for each source,

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

2005 March 24 17

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 18

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, 87 . Requires 8~1.0

2005 March 24 19

Varying 6 parameters plus amplitude of SZ template component

2005 March 24 20

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 21

CBI2

2005 March 24 22

NVSS Sources in CBI Field

2005 March 24 23

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 24

SZE SZE SecondarySecondaryCMB CMB

PrimaryPrimary

87

CBI2 Projection

2005 March 24 25

CBI Polarization Spectra

TT

BB

TE

EE

•TT consistent with earlier results•EE and TE consistent with predictions•BB consistent with zero

2005 March 24 26

Shaped Cl Fit• 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

Likelihood of EE Amplitude vs. “TT Prediction”

2005 March 24 27

Comparison of Experiments

2005 March 24 28

Comparison of Experiments

2005 March 24 29

Comparison of Experiments

2005 March 24 30

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 31

Isocurvature

Isocurvature puts peaks in differentplaces from adi-abatic. We use seed isocurvaturemodel and findboth EE and TEprefer adiabatic w/iso consistent withzero.

2005 March 24 32

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 33

Foregrounds

WMAP synchrotron component(WMAP Science Team)

DRAO 1.4 GHz polarized intensity(Wolleben et al. astro-ph/0510456)

WMAP Ka-band polarization

2005 March 24 34

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 35

WMAP3

WMAP3+CBIcombinedTT+CBIpol

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

2005 March 24 36

People

2005 March 24 37

• 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