Physics News Update, Number 481

Number 481, April 27, 2000 by Phillip F. Schewe and Ben Stein

BEST MAP YET OF THE COSMIC MICROWAVE BACKGROUND (CMB). The CMB is a redshifted picture of the universe at the moment photons and newly formed hydrogen atoms parted company roughly 300,000 years after the big bang. First detected in the 1960s, the CMB appeared to be utterly uniform until, eight years ago, the COBE satellite provided the first hint of slight temperature variations, on a coarse scale, with an angular resolution of about 7 degrees. Since then several detectors have obtained resolutions of better than 1 degree.

Actually, the contribution to small-scale fluctuations in the CMB is customarily rendered in terms of multipoles (a sort of coefficient), denoted by the letter l. The contribution to the temperature fluctuations in the CMB for a multipole value of l comes from patches on the sky with an angular size of pi/l. COBE's CMB measurements extended to a multipole of only about 20, but a major new map, made using a detector mounted on a balloon blown all the way around the Antarctic continent, covers the multipole range from 50 to 600, thus probing CMB fluctuations with much finer angular detail, over about 3% of the sky.

The 36-member, international "Boomerang" (Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetics) collaboration, led by Andrew Lange of Caltech and Paolo de Bernardis of the University of Rome, confirms that a plot of CMB strength peaks at a multipole value of about 197 (corresponding to CMB patches about one degree in angular spread), very close to what theorists had predicted for a cosmology in which the universe's overall curvature is zero and the existence of cold dark matter is invoked.

The absence of any noticeable subsidiary peaks (higher harmonics) in the data, however, was not in accord with theory. The shape of the observed pattern of temperature variations suggests that a disturbance very like a sound wave moving through air passed through the high-density primordial fluid and that the CMB map can be can be thought of as a sort of sonogram of the infant universe. (de Bernardis et al., Nature, 27 April 2000.)

QUANTUM HEAT. The movement of electrons down a wire becomes a quantum affair when the electron wavelength (the size of the quantum wave counterpart of the particulate electron) is comparable in size to the width of the wire. Theorists have thought the same would be true of "particles" of heat (phonons) moving down a wire. In the case of electrons, quantum reality manifests itself in the form of quantization: the electrons can only have conduction values in multiples of a basic unit equal to 2 times the electric charge squared, divided by Planck's constant. In the case of heat, the unit of thermal conduction would equal the temperature times pi squared times the square of Boltzmann's constant, divided by three times Planck's constant. Such quantized thermal conduction has now been seen for the first time by physicists at Caltech (Michael Roukes, roukes@caltech.edu), where heat added to a tiny (4x4 micron) silicon nitride "phonon cavity" can depart only across narrow bridges, essentially wires only 500 atoms wide (Schwab et al., Nature, 27 April 2000). Heat is added, and the temperature of the cavity monitored, by tiny gold circuits leading to SQUIDs (superconducting quantum interference devices). With further refinements, the researchers hope to explore the particle nature of heat, in effect a sort of "quantum phonon optics." In the same issue, commentators Leo Kouwenhoven and Liesbeth Venema refer to the Caltech observations as "the first demonstration of quantum physics in nanomechanical structures."