Mapping to MicroKelvin Precision

The key to realizing the full potential of the CMB is the ability to map its anisotropy with microKelvin precision. The accuracy of an experiment is limited by: (i) instrumental sensitivity; (ii) systematics such as ambient temperature variations on the detectors, atmospheric emission and absorption, and spill into the beam from nearby warm objects such as the Earth, sun and moon; (iii) astrophysical foregrounds such as the Galaxy and extragalactic sources; and (iv) sampling variance. The last of these provides a fundamental limit: because there are but multipole moments for a given , the variance of the multipole moments can only be estimated to a precision of . This is often called cosmic variance because it is the variance in the 's measured by an ensemble of observers studying different CMB skies.

Fig.3: Known foregrounds confronting COBE and its high-precision successors (MAP and Planck) as a function of frequency and angular scale (multipole ). The crosshatched regions indicate where the foregrounds are significant problems: Galactic synchrotron emission (blue); Galactic bremsstrahlung emission (purple); Galactic dust emission (red); and extragalactic point sources (green). The sensitivity of COBE DMR, MAP, and Planck in the frequency - multipole plane is also shown. (Adapted from a figure by George Efstathiou and Max Tegmark.)

In the three decades since the Penzias-Wilson discovery there have been enormous technological advances in detectors. Two common techniques are used today: microwave amplifiers using high electron-mobility transistors (HEMTs) and bolometrs which measure the heating of a small amount of material by CMB photons. Historically HEMT amplifiers have been used at lower microwave frequencies and bolometers have been used at higher frequencies, but this distinction is not as strong as it once was. The improvement in detector sensitivity is nothing short of remarkable: Using today's state-of-the-art HEMTs, the sensitivity achieved by COBE's DMR in four years could have been reached in ten days!

The first line of defense against systematic error is measuring differentially. A gain fluctuation acts equally on both elements of the differential signal and cancels out. The COBE DMR measured temperature differences between points on the sky separated by sixty degrees and the matrix of differences was inverted to obtain a true map of the CMB sky. Other techniques to minimize systematic errors include under-illumination of optics to reduce off-axis signal pick-up from warm objects such as Earth and graduate students, and designs that are as symmetric as possible between the main and reference signal paths.

The signal from our Milky Way galaxy is dominated by synchrotron and bremsstrahlung emission at low frequencies and by dust emission at high frequencies. Happily when one looks away from the Galactic plane the CMB anisotropy dominates the Galactic emission between about 30 GHz and 120 GHz. Furthermore the obscuring foregrounds fall off more rapidly than the CMB at high . Extragalactic sources such as galaxies and quasars are only troublesome for experiments with angular resolution of much better than one degree. By observing at high Galactic latitudes, where Milky Way emission is weakest, and by measuring at several different frequencies, the Galactic signal can be separated from CMB anisotropy (see Fig.3).

Cosmic anisotropy experiments have been carried out from the ground (including the South Pole and mountain tops), from balloon platforms, and from two satellites in space, COBE and the Soviet Relict 1 in 1983. There are advantages and trade-offs for each: ground-based experiments allow easy access but must deal with atmospheric emission and absorption; balloons can lift experiments above most of the atmosphere, but duration and flight opportunities are limited; and satellites eliminate atmospheric problems and allow full-sky access, but opportunities are even more limited and more expensive.

As the Table illustrates, experimenters have taken up the challenge to map the CMB sky with microKelvin precision. A new generation of long-duration balloon experiments will make maps of patches of the sky with sub-degree angular resolution. Ground-based interferometers will make higher resolution maps of smaller regions. Two new space missions are now planned. NASA has approved the Microwave Anisotropy Probe (MAP) and ESA has approved Planck (formerly COBRAS/SAMBA). Both satellites will make full-sky maps with angular resolutions of 0.2 and 0.1 degrees respectively. That's more than 30 times better than the angular resolution of the COBE map. Both will use amplifiers with HEMTs for microwave frequencies between 20 and 100GHz.

MAP, which is scheduled to launch in the Fall of 2000, is a passively cooled differential microwave radiometer like COBE, with back-to-back 1.4x1.6m reflectors. Planck, scheduled for launch 5 years later will in addition to the HEMTs have 6 higher frequency channels (from 100 to 857 GHz) that use bolometers cooled by liquid Helium. Both satellites will be placed in orbit at the remote L2 Lagrange point, 1.5 million km anti-sunward from the Earth.