All the matter and radiation in the universe was created by the big bang. The radiation stopped interacting with the matter about 300,000 years later - when the universe had cooled down enough for electrons and protons to form hydrogen atoms. The fact that the CMB has a perfect black-body spectrum with a temperature of around 2.73 Kelvin is one of the key pieces of evidence for the big bang. However, since the CMB is related to the state of the universe 300,000 years after the big bang, we expect to find tiny variations in the background temperature across the sky that correspond to slight variations in the distribution of matter at the time. These irregularities later became galaxies and clusters of galaxies. The fluctuations in the CMB also contain information about the total energy density and curvature of the universe.

In 1991 the COBE satellite measured large-scale fluctuations in the CMB for the first time. The Boomerang experiment has now measured these fluctuations with a sensitivity of better than one ten thousandth of a degree. The Boomerang team has confirmed, to within 10%, that the universe is flat and should therefore expand forever. The data also confirm that the patterns caused by sound waves speeding through the early universe helped to create giant clusters of galaxies.

Boomerang is a balloon-based telescope that was launched from Antarctica on 29 December 1998. The telescope took readings at an altitude of 37 kilometres to reduce the absorption of millimetre wave radiation by water vapour in the atmosphere. Over a period of almost 11 days the experiment measured the CMB at four wavelengths -- 0.75 mm, 1.25 mm, 2 mm and 3.33 mm -- across about 3% of the sky. By comparing the results at different wavelengths, the Boomerang team was able to remove signals due to the instruments, dust and other sources, and then derive a "power spectrum" - a curve showing how the size of the fluctuations varied with angle. "The idea was to ignore all the other stuff and find just the contribution of the cosmic microwave background," said Andrew Jaffe from the University of California at Berkeley.

"It is really exciting to be able to see some of the fundamental structures of the universe in their embryonic state," said Paolo de Bernardis of the University of Rome La Sapienza. "The light we have detected has travelled across the entire universe before reaching us, and we are perfectly able to distinguish it from the light generated in our own galaxy."

"These images represent the ultimate limit of our vision," said Andrew Lange from the California Institute of Technology. "The enormous structures that they reveal predate the first star or galaxy in the universe. It is an incredible triumph of modern cosmology to have predicted their basic form so accurately."