Overview and Scientific Goals

MAXIMA (Millimeter Anisotropy eXperiment Imaging Array) is a balloon-borne millimeter-wave telescope designed to measure the angular power spectrum of fluctuations in the cosmic microwave background (CMB) over a wide range of angular scales. Such measurements are providing a powerful probe of the early universe. They can discriminate between competing cosmological models and have already ruled out some specific topological defect models. Inflationary cosmological models, which are currently favored, produce an angular power spectrum with a unique set of "acoustic peaks" between the angular scales of 2 degrees and 10'. The relative position and height and the detailed shape of these peaks can provide precise estimates of cosmological constants such as the total density Omega_tot of the universe, the baryonic mass density Omega_b, and the Hubble constant H0. In fact, CMB measurements are already putting constraints on Omega_tot.

MAXIMA has evolved from the successful MAX experiment. Our small team has built MAXIMA from the ground up culminating in two successful flights in 1998 and 1999. We are analyzing data from the 1998 flight, and plan to publish results very soon.

MAXIMA has an unprecedented combination of sensitivity, angular resolution, and control of systematic effects. The data from MAXIMA should reveal the presence of the predicted set of acoustic peaks in the angular power spectrum if they exist. Current data from other experiments show evidence for one peak at the ~1 degree scale, but the scatter is considerable. MAXIMA is sensitive from 4 degrees to the telescope's beam size of 10', which is an ideal range for probing the acoustic peaks. This angular scale is significantly smaller than the one explored by COBE (>7 degrees). The MAXIMA receiver has a 16-pixel array of bolometers cooled to a temperature of 100 mK to achieve high sensitivity. With all channels taken together, MAXIMA is the most sensitive CMB instrument yet fielded.

Raw sensitivity is only one aspect of an effective CMB anisotropy measurement. Systematic errors must be minimized by the scan strategy of the experiment. In MAXIMA the same pixel is observed on three time scales, which allows rejection of noise and spurious signals on any one time scale. For example, the same portion of sky is scanned twice with a time separation of >1 hour. These two scans occur at different angles, due to the earth's rotation, producing a cross-hatched pattern. This "cross-linking" of pixels allows a two dimensional map of the sky to be reconstructed from one dimensional scans in the presence of inevitable low-frequency noise in the detectors.

During our first overnight flight (MAXIMA-1, 1998) we observed 124 deg.2 of the sky which corresponds to >4,500 beam-size pixels, 6 times as many as the COBE-DMR satellite data set using the same pixelization scheme. The data are proving to be of high quality. We have produced maps of the sky from several different array elements and these maps show consistent structure from one to another. We have produced preliminary angular power spectra and are performing a battery of tests for systematic errors. Data from the second flight (MAXIMA-2, 1999) have nearly twice the area of sky coverage (230 deg.2) of the first flight.

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