||The results of the first flight of the MAXIMA
balloon-borne study of the cosmic microwave background radiation, released
May 9, 2000, agree with the results of the BOOMERANG Antarctic flight,
announced April 26, in the all-important conclusion that the universe is
"A subset of cosmological theories, those involving inflation, dark matter, and a cosmological constant, fit our data extremely well," says MAXIMA team leader Paul L. Richards, a professor of physics at the University of California at Berkeley and a member of Berkeley Lab's Materials Sciences Division.
Richards added that MAXIMA's results not only generally agree with BOOMERANG's but extend them to finer resolution. Moreover, the two experiments, which differed in many technical and logistic features, also show intriguing differences in the details of their data analysis.
In its flight on August 2, 1998, MAXIMA (for "millimeter anistropy experiment imaging array") flew for seven hours over East Texas and observed a patch of the northern sky about 20 times the size of the full moon -- three-tenths of a percent of the whole sky -- to a resolution of one-sixth of a degree.
BOOMERANG (for "balloon observations of millimetric extragalactic radiation and geophysics") completely circled the South Pole in 10 and a half days, riding the stratospheric polar vortex and returning to its starting place (the real meaning of the BOOMERANG acronym) early in January, 1999, after observing three percent of the whole sky at somewhat lower resolution.
The goal of both experiments was to take the temperature of the CMB, whose variations, or anisotropy, amount to a snapshot of the entire universe 300,000 years after the Big Bang. Before that time the universe was too hot for atoms to form, and photons were trapped in a dense fog of electrically charged particles -- so dense that pressure waves reverberated through the mass like sound waves in water.
When the universe cooled enough for electrons and protons to form neutral hydrogen atoms -- a moment known as recombination -- highly energetic photons were freed. In the billions of years since, they have cooled to less than three degrees Kelvin above absolute zero, equivalent to microwave frequencies. Slight variations in that remarkably uniform background temperature, typically only a hundred millionths of a degree, record the pattern of the "sound waves" at the moment the universe became transparent.
From their measurements, the CMB experimenters first constructed maps of the temperature differences in the portions of sky they were studying. To a practiced investigator of the CMB, a map alone is revealing.
"One can readily see structure in the map on roughly a one-degree angular scale," says MAXIMA team member George Smoot, of Berkeley Lab's Physics Division and UC Berkeley, who pioneered studies of CMB anisotropies with an experiment aboard the COBE satellite in the early 1990s. "Structures in the early universe with a size of nearly one degree show that the universe has a geometry that is very nearly flat -- the same geometry, Euclid's, that we all studied in high school."
The best explanation for a flat universe is inflation theory, which posits that a small, causally connected portion of the entire cosmos expanded rapidly in the first instant after the Big Bang to form the almost-but-not-quite perfectly smooth universe we observe today.
Until CMB maps have been analyzed to yield spectra showing the relative numbers of all features at various angular sizes, however, subtler information is hidden.
The strongest peak in the CMB power spectrum -- the one at one angular degree -- represents the fundamental "pitch" of the ringing universe, directly related to its size at that moment. Just as the tone of a ringing bell incorporates harmonics -- higher frequency sounds, integer multiples of the pure pitch -- there should be groups of smaller features in the CMB yielding a second peak in the power spectrum, a third peak, and so on. Their frequency and amplitude differ with different models of the early universe.
Much of the analysis for the enormous datasets of both MAXIMA and BOOMERANG was performed at NERSC. Andrew Jaffe of UC Berkeley and his colleagues devised an algorithm for finding the most likely map or power spectrum given the data, an algorithm that was implemented by Julian Borrill of Berkeley Lab and UC Berkeley, using software he devised called MADCAP ("microwave anisotropy dataset computational analysis package").
Jaffe and Borrill, who like other MAXIMA and BOOMERANG collaborators are members of both teams, nevertheless managed to maintain the complete independence of their results. The differences and similarities in the spectra from the two experiments raise intriguing questions.
Especially when the CMB measurements are compared with other, wholly indendent cosmological observations, including estimates of the mass of galactic clusters and supernova studies indicating that the expansion of the universe is accelerating, MAXIMA and BOOMERANG agree on the essentials. Inflation made the universe flat. About a third of its density is due to matter, most of it dark, and the rest is due to an unknown form of energy often called the cosmological constant.
One surprise of the BOOMERANG announcement was that its power spectrum showed no pronounced harmonic at about half an angular degree, where one would be expected if the most straightforward models of the inflationary universe are correct. MAXIMA's power spectrum, with higher resolution at small angles, confirms that there is no sharply defined harmonic at half a degree, and it allows only a slightly larger harmonic at a third of a degree.
Where are the harmonics? "There are factors that could either distort the harmonics themselves or the way we see those harmonics today," Smoot says, naming a number of possibilities including topological defects in the early universe; a universe that contains more ordinary matter today than our best estimates; a universe in which some matter decayed before the moment of recombination; or an unsuspected role for neutrinos. "There is no agreement on any of these," he says. "It's new physics."
An area where Paul Richards sees a difference between the MAXIMA and BOOMERANG results is in the calculation of "tilt," which describes the initial clumping of matter in space on different scales. This clumping in turn produced the galaxies and clusters of galaxies we see today.
If the amplitude of these tiny, "primordial" perturbations was the same on all physical scales, leading to the formation of similar structures on all scales, their spectrum is said to have no tilt.. The pattern of clumping is also reflected in the pattern of the microwave background, and thus in the angular power spectrum measured by MAXIMA and BOOMERANG.
"Inflationary theories make two main predictions," says Paul Richards. "One is a flat universe. The other is that the power spectrum has no tilt. MAXIMA supports both these predictions."
Andrew Jaffe cautions that no tilt "is a very 'natural' value for the universe's density perturbations at any given time," and -- although a spectrum with no tilt is predicted by inflation -- "because it is a very natural spectrum, other possible theories, such as cosmic strings and other so-called topological defects, are also characterized by no tilt, although their spectra differ in other ways that our data can probe."
The problem of the "physics of the bumps and wiggles" in the CMB power spectrum remains.
In close collaboration with their university colleagues, staff scientists and guests at Berkeley Lab, several with joint appointments at UC Berkeley, contributed significantly to MAXIMA's experimental hardware as well as its data processing analysis through programs supported by the Department of Energy. See http://aether.lbl.gov/.