J. K. Cartwright

Degree Angular Scale Interferometer First Results: A Measurement of the Cosmic Microwave Background Angular Power Spectrum

We present measurements of anisotropy in the cosmic microwave background (CMB) from the first season of observations with the Degree Angular Scale Interferometer (DASI). The instrument was deployed at the South Pole in the austral summer 1999-2000, and we made observations throughout the following austral winter. We present a measurement of the CMB angular power spectrum in the range 100 < l < 900 in nine bands with fractional uncertainties in the range 10%-20% and dominated by sample variance. In this paper, we review the formalism used in the analysis, in particular the use of constraint matrices to project out contaminants such as ground and point source signals and to test for correlations with diffuse foreground templates. We find no evidence of foregrounds other than point sources in the data, and we find a maximum likelihood temperature spectral index β = -0.1 ± 0.2 (1 σ), consistent with CMB. We detect a first peak in the power spectrum at l ~ 200, in agreement with previous experiments. In addition, we detect a peak in the power spectrum at l ~ 550 and power of similar magnitude at l ~ 800, which are consistent with the second and third harmonic peaks predicted by adiabatic inflationary cosmological models.

The Anisotropy of the microwave background to l = 3500: Mosaic observations with the Cosmic Background Imager

We report measurements of anisotropy in the cosmic microwave background radiation over the multipole range l 200 3500 with the Cosmic Background Imager based on deep observations of three fields. These results confirm the drop in power with increasing l first reported in earlier measurements with this instrument and extend the observations of this decline in power out to l 2000. The decline in power is consistent with the predicted damping of primary anisotropies. At larger multipoles, l 1⁄4 2000 3500, the power is 3.1 greater than standard models for intrinsic microwave background anisotropy in this multipole range and 3.5 greater than zero. This excess power is not consistent with expected levels of residual radio source contamination but, for 8e1, is consistent with predicted levels of a secondary Sunyaev-Zeldovich anisotropy. Further observations are necessary to confirm the level of this excess and, if confirmed, determine its origin. Subject headings: cosmic microwave background — cosmology: observations

Cosmological Parameters from Cosmic Background Imager Observations and Comparisons with BOOMERANG, DASI, and MAXIMA

We report on the cosmological parameters derived from observations with the Cosmic Background Imager (CBI), covering 40 deg2 and the multipole range 300 l 3500. The angular scales probed by the CBI correspond to structures that cover the mass range from 1014 to 1017 M?, and the observations reveal, for the first time, the seeds that gave rise to clusters of galaxies. These unique, high-resolution observations also show damping in the power spectrum to l ~ 2000, which we interpret as being due to the finite width of the photon-baryon decoupling region and the viscosity operating at decoupling. Because the observations extend to much higher l, the CBI results provide information complementary to that probed by the BOOMERANG, DASI, MAXIMA, and VSA experiments. When the CBI observations are used in combination with those from COBE-DMR, we find evidence for a flat universe, ?tot = 1.00 (1 ?), a power-law index of primordial fluctuations, ns = 1.08, and densities in cold dark matter, ?cdmh2 = 0.16, and baryons, ?bh2 = 0.023. With the addition of large-scale structure priors the ?cdmh2 value is sharpened to 0.10, and we find ?? = 0.67. In the l < 1000 overlap region with the BOOMERANG, DASI, MAXIMA, and VSA experiments, the agreement between these four experiments is excellent, and we construct optimal power spectra in the CBI bands that demonstrate this agreement. We derive cosmological parameters for the combined cosmic microwave background (CMB) experiments and show that these parameter determinations are stable as we progress from the weak priors using only CMB observations and very broad restrictions on cosmic parameters, through the addition of information from large-scale structure surveys, Hubble parameter determinations, and Type Ia supernova results. The combination of these with CMB observations gives a vacuum energy estimate of ?? = 0.70, a Hubble parameter of h = 0.69 ? 0.04, and a cosmological age of 13.7 ? 0.2 Gyr. As the observations are pushed to higher multipoles, no anomalies relative to standard models appear, and extremely good consistency is found between the cosmological parameters derived for the CBI observations over the range 610 < l < 2000 and observations at lower l.

FIRST INTRINSIC ANISOTROPY OBSERVATIONS WITH THE COSMIC BACKGROUND IMAGER

We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in Cl in the range l = 400-1500. The broadband amplitudes we have measured are δTband = 58.7 μK for l = 603 and δTband = 29.7 μK for l = 1190, where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The Cl we have measured enables us to place limits on the density parameter, Ωtot ≤ 0.4 or Ωtot ≥ 0.7 (90% confidence).

The Cosmic Background Imager

Design and performance details are given for the Cosmic Background Imager (CBI), an interferometer array that is measuring the power spectrum of fluctuations in the cosmic microwave background radiation (CMBR) for multipoles in the range 400<l< 3500. The CBI is located at an altitude of 5000 m in the Atacama Desert in northern Chile. It is a planar synthesis array with 13 0.9 m diameter antennas on a 6 m diameter tracking platform. Each antenna has a cooled, low-noise receiver operating in the 26-36 GHz band. Signals are cross-correlated in an analog filterbank correlator with 10 1 GHz bands. This allows spectral index measurements that can be used to distinguish CMBR signals from diffuse galactic foregrounds. A 1.2 kHz 180° phase-switching scheme is used to reject cross talk and low-frequency pick-up in the signal processing system. The CBI has a three-axis mount that allows the tracking platform to be rotated about the optical axis, providing improved (u, v) coverage and a powerful discriminant against false signals generated in the receiving electronics. Rotating the tracking platform also permits polarization measurements when some of the antennas are configured for the orthogonal polarization.

Experiment Design and First Season Observations with the Degree Angular Scale Interferometer

We describe the instrumentation, experiment design, and data reduction for the first season of observations with the Degree Angular Scale Interferometer (DASI), a compact microwave interferometer designed to measure anisotropy of the cosmic microwave background (CMB) on degree and subdegree scales (l 100-900). The telescope was deployed at the Amundsen-Scott South Pole Research Station during the 1999-2000 austral summer, and we conducted observations of the CMB throughout the following austral winter. In its first season of observations, DASI has mapped CMB fluctuations in 32 fields, each 34 across, with high sensitivity.

A wideband analog correlator for microwave background observations

Design and performance details are given for a 10-GHz bandwidth analog filterbank correlator with 1-GHz spectral resolution. The correlator is based on a 1-GHz bandwidth hybrid module which contains all the multipliers and signal distribution for a 13-antenna array.

A measurement of the coupling between close-packed shielded cassegrain antennas

Design and performance details are given for a 0.9-m diameter shielded cassegrain antenna, which will be used in a 13-element close-packed array, The array is designed to make images of brightness fluctuations in the cosmic microwave background radiation. Coupling between a pair of the shielded cassegrain antennas with a separation of 1 m is in the range -110 to -130 dB over the 26- to 36-GHz band.

First Results from the CBI

The Cosmic Background Imager (CBI) is an instrument designed to measure intrinsic anisotropies in the cosmic microwave background (CMB) on angular scales from about 3 arc minutes to one degree (spherical harmonics of l~4250 to l~400). The CBI is a 13 element interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency bands from 26 to 36 GHz. We present a review of the capabilities of the instrument and a discussion of observations which have been taken over the past year from the Atacama desert of Chile. We also present first results from the CBI which show a strong cutoff in the power spectrum between l=600 and l=1200 which is consistent with the photon-diffusive damping predicted by most models of structure formation in the early universe. We discuss future topics which the CBI will address.

Preliminary Results from the Cosmic Background Imager

The Cosmic Background Imager (CBI) is a 13-element interferometer designed to image intrinsic anisotropies in the cosmic microwave background (CMB) on arcminute scales. A review of the capabilities of the instrument is presented, together with a discussion of observations which have been taken over the past 9 months from the Atacama desert of Chile. We present preliminary high-resolution mosaiced images of the CMB obtained from recent CBI data and discuss topics which the CBI will address in the near future.