Arcminute Cosmology Bolometer Array Receiver

Science Goals

Determine the fine scale CMB Power Spectrum

simulated power spectrum	other experiments

CMB map at 150 GHz	CMB map at 220 GHz

A new generation of CMB experiments (BOOMERANG, DASI, and MAXIMA) are beginning to realize the potential of CMB anisotropy as a tool for precision cosmology. Simulated power spectra depend strongly on the values of the cosmological parameters (such as Ω_o, H_o, Λ, and Ω_B) and can be compared with observations to constrain cosmological models. The large chop and small beam of the ACBAR receiver make possible high resolution spectroscopy of spatial features of the CMB from angular scales corresponding to l=200 to 3000. ACBAR extends the frontier of CMB anisotropy measurement to angular scales more than a factor of two finer than that achieved with previous experiments. The top panel shows a simulated CMB power spectrum with the actual error bars achievable with ACBAR. This is a significant improvement in accuracy above l=1000 over previous experiments. (The real ACBAR CMB Power Spectrum analysis is in progress and will be published shortly.) The bottom left panel shows a map of one several deep ACBAR CMB fields at 150 GHz, taken during the 2001 South Pole winter season. The bottom right panel same field at 220 GHz. This particular CMB field is centered on QSO 0454-463. The 150GHz plot has an RMS of 13 μK in a 5’ beam. Note that the temperature differences of the CMB between hot and cold regions is around 300 μK, or approximately one part in ten thousand of the CMB temperature; in the center of the map, these fluctuations are measured with signal to noise >10.

Cosmological Parameter Estimation

The CMB Power Spectrum can be used to place limits on the certain cosmological parameters. A theoretical power spectrum may be calculated for any combination of Ω_tot, Ω_baryon, Ω_cdm, Λ, H₀, and so forth. By comparing these spectra with our actual experimental spectrum, we can determine the likelihood for each of these cosmological parameters to have certain values. The above plot shows likelihoods of some cosmological parameters calculated without ACBAR data.

CMB Foregrounds

This plot shows the various foreground contaminants that CMB experiments have to contend with on the angular scale versus frequency plane. The shaded regions are where dust, synchrotron, free-free, and radio point source emission exceed the predicted CMB anisotropy. The desirable region with the lowest foreground levels is the white swath through the middle of the plot. Also shown are the regions covered by many experiments; ACBAR’s coverage is the purple box. (from Max Tegmark's web page). ACBAR will provide constraints on foreground emission of future missions such as the Planck surveyor.

Map SZ Effect in known clusters:

The CMB photons interact with hot, ionized gas trapped inside large clusters of galaxies. This interaction causes a change in the CMB spectrum, and was first calculated theoretically by Sunyaev and Zel'dovich about 30 years ago. By measuring the CMB intensity at multiple frequencies, it is possible to separate the original CMB signal from what is now known as the Sunyaev-Zel'dovich (SZ) Effect. The first panel is a map of the Thermal SZ Effect from Galaxy Cluster Abell 3266 (z=.0545, T_x=6.2 keV, L_x=9.5x10⁴⁴) measured by ACBAR. For clusters of this angular size, it is necessary to use multi-frequency observations to remove contamination by CMB anisotropies which have comparable peak brightness. The white lines are contours in X-ray flux from ROSAT observations. This map, and others like it are being used to probe the distribution of hot gas bound to massive clusters of galaxies and provide an independent determination of baryon fraction and Hubble constant. The second panel is a plot of the frequencies covered by each of the three current ACBAR bands (blue, green & red lines) along with the CMB intensity change due to the SZ Effect (black line).

Search for Distant Galaxy Clusters:

The evolution of the number density of clusters of galaxies depends strongly on the value of the matter density. For a low value of matter density, clusters stop forming at low redshift, thus more had to form earlier to match the presently observed number density. With ACBAR's sensitivity and 4’ beam size , which is more sensitive to distant cluster emission than previous CMB experiments. The spectral response of ACBAR will allow us to distinguish between emission due to the SZ effect, CMB anisotropies and foreground emission. The analysis of the deep ACBAR maps for cluster candidates is in progress. This is an optimally-filtered frequency-subtracted plot with 5’ Beam Size. The RMS is ~4mK in 5’ Beam. Follow-up observations are in progress.