CBI: The Cosmic Background Imager

Link to CBI Web site

I. General project/facility description

1. Overview of the facility/project
   The Cosmic Background Imager (CBI) is an instrument designed to make images of the cosmic microwave background radiation and to measure its statistical properties on angular scales from 4 arc minutes to one degree (spherical harmonic scales from l = 3500 down to l = 300). The CBI is a 13-element interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency bands from 26 GHz to 36 GHz. The instantaneous field of view of the instrument is 44 arcmin and its resolution ranges from 4.5 to 10 arcmin. The spectral capabilities of the CBI can be used to look for and separate diffuse foreground synchrotron, free-free, and dust emission from the interstellar medium in our Galaxy. The CBI is also a powerful instrument for observing the Sunyaev-Zel'dovich scattering of background radiation photons by the hot electrons in clusters of galaxies. Measurements of this effect can be used to study the properties of the hot cluster gas and the evolution of clusters, and to measure the Hubble constant directly. The CBI is located at an altitude of 5080 meters near San Pedro de Atacama, in northern Chile. A high, dry site is essential in order to reach the required sensitivity levels in a reasonable observing time. Following extensive tests on the campus of the California Institute of Technology in Pasadena, California, the CBI was shipped to Chile in August 1999 and has been making observations of the microwave background since November 1999. In 2002, the CBI was upgraded to enable sensitive polarization measurements of the CMB, the results of which were reported in September 2004. The CBI shared many design elements with DASI, an interferometer (located at the Amundsen-Scott South Pole station) that probed larger angular scales.

2. Managing institution and organization
   Primary managing institution: California Institute of Technology
3. Active participating institutions: National Radio Astronomy Observatory, Canadian Institute of Theoretical Astrophysics, Universidad de Chile, Universidad de Conception
4. Other participating institutions: University of Chicago, University of California Berkeley, Marshall Space Flight Center, University of Alberta
   See http://www.astro.caltech.edu/~tjp/CBI/personnel/ for full list.

5. Funding source(s)
   
   • NSF ($7.25M total as of Nov 2004, see below) (plus NRAO support of Myers, Mason and HEMTs supplied by Pospieszalski, total unknown)
   • Caltech ($5.57M total as of Nov 2004)
   • Maxine and Ronald Linde (total unknown)
   • Cecil and Sally Drinkward (total unknown)
   • Barbara and Stanley Rawn Jr. (total unknown)
   • Kavli Operating Institute (total unknown)
   • Canadian Institute for Advanced Research (total unknown)

6. Construction history and cost
   A proposal made to NSF ATI program in 1994 was funded in 1995 providing funds for construction (AST 9413935 Readhead $2.00M 1995-1997)
   • 1994 proposal to NSF MRI
   • 1995-1998 construction at Caltech
   • 1998 assembled and tested at Caltech
   • 1999 August shipped to Chile
   • 1999 November "first light" in Chile

7. Operational history and cost
   From 1998-2004, 3 NSF grants totalling $5.25M supported CBI operations.
   AST/ATI 9802989 Readhead $2.71M 1998-2002 Program: ATI, EXGAL & COSMO
   AST/MPS 0098734 Readhead $1.38M 2001-2004 Program: PARTICLE ASTROPHYSICS
   AST 0206416 Readhead $1.16M 2002-2004 Program: EXGAL & COSM
   
   • 2000 January routine CMB observing begins
   • 2002 polarization upgrade - achromatic polarizers constructed in Chicago
   • 2005 continued operation with funds from new sources

II. Technical details

1. Specifics of telescope/instrument
   
   • 13 elements, each 90-cm Cassegrain reflectors
   • elements mounted on planar platform, reconfigurable
   • baseline lengths 1-5.5 m
   • 26-36 GHz in ten 1 GHz wide channels
   • field-of-view 45' (at 31 GHz)
   • resolution 4'-10' (configuration dependent)
   • point source sensitivity rms 4 mJy/beam in 900 s at beam center
   • brightness rms 0.01-0.04 mK in mosaic of 900 s/pt (config dependent)
   • single polarization, R or L (switchable for each element)
   • 780 instantaneous visibilities (baseline channels)
   • alt-az mount pointable to 45 deg elevation
   • platform rotatable about optical axis (rotate baselines in uv-plane)
   • pointing accuracy <10" (3" rms typical)

2. New capabilities anticipated/planned in next 5-10 years
   Plans are for testing of QUIET on the CBI platform starting 2005 or 2006. No plans for upgrading CBI itself, with CBI running in current state through 2006.

III. User profile

1. % of "open skies" time
   None, but 10% to Chilean radio astronomers

2. Institutional affiliations of users
   Participants in CBI are from Caltech, NRAO, CITA, U. Chile and U. Concepcion. See: http://www.astro.caltech.edu/~tjp/CBI/personnel/ for full collaboration list.

3. Student access, involvement, usage
   5 graduate students (3 from Caltech, 2 from U. Concepcion) are basing their Ph.D. dissertation research on CBI. 3 other graduate students (2 from U. Chile, 1 from Caltech) are also currently working on CBI. Additionally, an undergraduate student from U. Wisconsin worked on CBI science under the NRAO NSF-REU program.

IV. Science Overview

1. Current forefront scientific programs
   
   • Measurement of the polarization and total intensity of the CMB for l=300-2000.
   • Measurement of the Sunyaev-Zeldovich Effect in a complete sample of nearby (z < 0.1) X-ray luminous clusters of galaxies.
   • Characterization of galactic microwave emission at 30 GHz on angular scales from 5' to 45'.

2. Major discoveries (through 1999)
   
   • The acoustic peaks in the CMB EE polarization power spectrum are out of phase with respect to the TT power spectrum, as predicted by standard models. This implies that the acoustic peaks are indeed introduced at last scattering by electron motions in response to (nearly) adiabatic density fluctuations.
   • There is an excess of power at l>2000 in the CMB temperature power spectrum over the level expected from primary CMB anisotropies. This excess is consistent with the level expected by the SZ effect from large-scale structure out to high redshift, though no individual clusters can be identified in the data.
   • The CMB temperature power spectrum at l=500-3500 shows the expected signatures of the "damping tail" (due to the finite "thickness" of the last scattering surface). This was seen for the first time to l=1500 by the CBI in 2000 and was greatly improved with the first full year and 2-year power spectra, where the higher-order acoustic peaks were measured and located.

3. Science highlights of last 5 years
   In addition to the above:
   • Anomalous microwave emission correlated with infrared dust emission, possibly due to spinning magnetized dust, has been seen by the CBI in the Helix nebula.
   • The pulsar wind nebula of the Vela pulsar was investiged by CBI observations at 31 GHz. The Vela X region, in particular the filaments, are likely to be powered by the pulsar wind (similar to the effect seen in plerions).
   • The Sunyaev-Zeldovich effect was measured by the CBI for a sub-sample of 7 nearby (z<0.1) clusters. Combination with X-ray data from ROSAT gives H_o=67 (+30+15,-18-6) km/s/Mpc (statistical & systematic estimated errors) for the Hubble constant. These clusters have a variety of X-ray morphologies and dynamical states, and further CBI and X-ray analysis should be able to ascertain the state of the ICM gas in these clusters.

4. Main future science questions to be addressed
   
   • Is the CMB polarization contaminated by foreground emission at 30 GHz to levels of 0.8 microKelvin at l~700? (this would limit future experiments looking for the B-mode from gravitational lensing)
   • CBI will morph into a QUIET testbed, which will investigate B-mode lensing (as well as do cosmology with precisely measured E-modes).

5. Synergies with other major forefront facilities
   
   • Complementary angular scales to large-angle CMB experiments such as WMAP, DASI, Boomerang, VSA. Example: CBI data is included in the WMAP+"ext" CMB power spectrum, See http://lambda.gsfc.nasa.gov/product/map/map_images/pub_images/f12_spectrum.pdf
   • CBI SZE observations of X-ray luminous clusters of galaxies - when combined with X-ray images from Chandra or XMM will yield improved constraints on the state of the dark matter and baryons in the intracluster medium and possibly on cosmological parameters such as the Hubble constant: see Myers et al. presentation http://www.aoc.nrao.edu/~smyers/cbi/pdftalks/MyersXRR06Feb04.pdf

6. Unique contributions
   By pushing the envelope to high multipoles the CBI early on demonstrated the importance of this region of parameter-space for providing independent confirmation of the cosmological model and searching for power due to secondary anisotropies. If the interpretation of the high-l "excess" measured by the CBI in terms of Pop III stars is correct, then the CBI will have provided the first observational evidence of the existence Pop III stars.

V. Education/Outreach activities

1. Visitor facility
   NA.

2. Student programs
   As noted above, graduate students from several universities are involved, and an undergraduate student worked on CBI under the NRAO NSF-REU program.

3. Other (as apply)
   Readhead has begun a collaboration with the NOVA (WGBH Boston) program to develop K-12 education materials. The WGBH/Nova program "Origins" featured the CBI prominently

VI. Documentation/website URLs

1. URL of facility website
   http://www.astro.caltech.edu/~tjp/CBI/

2. URL of EPO website
   
   • http://www.astro.caltech.edu/~tjp/CBI/press2/
   • http://www.astro.caltech.edu/~tjp/CBI/press1/

3. URL(s) of any brief overviews of project/facility
   http://www.astro.caltech.edu/~tjp/CBI/descrip/

4. URL(s) of miscellaneous documentation
   See recent CBI talk by S. Myers