1. CMB Observations with the Cosmic Background Imager
Tim Pearson
for the CBI team
The CBI site is located at an altitude of 5000 m in the Andes near Cerro Chajnantor, 40 km east of San Pedro de Atacama. We chose this site to minimize atmospheric noise at 30 GHz, but were unable to do competitive site testing. Alternatives we considered included Mauna Kea (Hawaii) and the South Pole.
Tony Readhead (Principal Investigator),
Steve Padin (Project Scientist until 2002).
2005 March 24

2. CBI Timeline 1995-1999: design and construction
: testing in Pasadena
1999: ship to Chile and commission
: CMB T and SZE observations (Stokes L)
2-field differencing
: CMB polarization observations (Stokes L&R)
6-field common mode
Jun present: idle (unfunded)
May-Dec 2006: upgrade to larger antennas, T/SZE observations
2007- : replace with QUIET receivers
2005 March 24

3. HEMT amplifiers, Tsys ~ 27 K Baselines 1 m – 5.5 m Analog correlator
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

4. 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 = 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
2005 March 24

5. 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

6. Chajnantor Observatory
Home of CBI, QUIET, and other experiments
2005 March 24

7. Total Intensity Observations
Observations made in
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

8. 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

9. 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 4.5 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!)
2005 March 24

10. Raw Images
2005 March 24

11. “Ground subtracted” images
2005 March 24

12. 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

13. CBI Combined TT ( )

14. 2.9σ above model
2005 March 24

15. Projecting Out Variable Sources
Marginalize over 1 parameter (flux) for each source,
Or 2 parameters ( and flux).
2005 March 24

16. 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

17. Not consistent with any likely model of discrete source contamination
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

18. Varying 6 parameters plus amplitude of SZ template component
2005 March 24

19. 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

20. CBI2
2005 March 24

21. NVSS Sources in CBI Field
2005 March 24

22. 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

23. CBI2 Projection
SZE
Secondary
CMB
Primary
~ s87
2005 March 24

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

25. Shaped Cl Fit EE qB = 0.97 ± 0.14 (68%)
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

26. Comparison of Experiments
2005 March 24

27. Comparison of Experiments
CBI defines high l at present, and would defne it better if we had been able to observe over the last year!
QUaD?
2005 March 24

28. Comparison of Experiments
2005 March 24

29. 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 /
2005 March 24

30. 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

31. 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

32. Foregrounds 2005 March 24 DRAO 1.4 GHz polarized intensity
(Wolleben et al. astro-ph/ )
WMAP Ka-band polarization
WMAP synchrotron component
(WMAP Science Team)
2005 March 24

33. 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

34. WMAP3+CBIcombinedTT+CBIpol
CMBall = Boom03pol+DASIpol +VSA+Maxima+WMAP3+CBIcombinedTT+CBIpol
2005 March 24

35. People
2005 March 24

36. Sievers et al. 2005, astro-ph/0509203
Readhead et al. 2004, ApJ, 609, 498
Readhead et al. 2004, Science 306, 836
Sievers et al. 2005, astro-ph/
2005 March 24