Reprinted by permission from Nature 404:27 April (2000), copyright 2000 Macmillan Magazines Ltd.

April 26, 2000, was a historic day in scientific cosmology. In an online-video news conference and a cover story in Nature, the international BOOMERANG consortium, led by Andrew Lange of the California Institute of Technology and Paolo de Bernardis of Universit� di Roma La Sapienza, announced results of the most detailed measurement yet made of the cosmic microwave background radiation (CMB).

BOOMERANG, which stands for “balloon observations of millimetric extragalactic radiation and geophysics,” revealed that the curvature of the Universe is not positive or negative but flat. Much of the data analysis was performed at NERSC by astrophysicist Julian Borrill, a BOOMERANG team member and NERSC staff scientist.

During BOOMERANG’s 10-day flight around the South Pole in January 1999, it made close to one billion measurements of tiny variations in the temperature of the CMB across a wide swath of the sky. From this dataset, the research team was able to make the most detailed map of the CMB’s temperature fluctuations ever seen.

From a map of these temperature fluctuations, the researchers derived a “power spectrum,” a curve that registers the strength of the fluctuations on different angular scales, and which contains information on such characteristics of the Universe as its geometry and how much matter and energy it contains. Julian calculated the power spectrum using 50,000 hours of processor time on NERSC’s 696-node Cray T3E, employing software he developed called MADCAP (Microwave Anisotropy Dataset Computational Analysis Package).

	By observing the characteristic size of hot and cold spots in the BOOMERANG images, the geometry of space can be determined. Cosmological simulations predict that if our Universe has a flat geometry (in which standard high school geometry applies), then the BOOMERANG images will be dominated by hot and cold spots of around 1� in size (bottom center). If, on the other hand, the geometry of space is curved, then the bending of light by this curvature of space will distort the images. If the universe is closed, so that parallel lines converge, then the images will be magnified by this curvature, and structures will appear larger than 1� on the sky (bottom left). Conversely, if the universe is open and parallel lines diverge, then structures in the images will appear smaller (bottom right). Comparison with the BOOMERANG image (top) indicates that space is very nearly flat.

Combined with other cosmological measurements, such as studies of distant supernovae by the Supernova Cosmology Project headquartered at Berkeley Lab, the BOOMERANG results support the emerging “concordance model” of a flat Universe filled with dark energy, which may correspond to the cosmological constant first proposed by Albert Einstein in 1917. For more details, see page 67.

Julian Borrill