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In 2003 astronomers released results from the Wilkinson Microwave Anisotropy Probe (WMAP) that thoroughly confirmed existing ideas of cosmology and also produced several revelations about the nature of the universe. The probe studied the distribution of the ripples in the cosmic background radiation. A major conclusion from WMAP data linked with other observations is that the universe follows Euclidean geometry—that is, given any line in the universe, one and only one parallel line may be drawn through any point not on the original line. Such a universe is known as 'flat,' although it extends infinitely in all directions. If the universe is flat, it must be at the critical density that marks the boundary between an open and closed universe.

WMAP results also confirmed that the density of baryons—the elementary particles that make up regular matter—account for only 4 percent of the critical density. The probe further showed that another 23 percent of the universe consists of dark matter, a mysterious substance that does not shine in any part of the spectrum. The gravity of dark matter, however, is detectable. It binds clusters of galaxies together and causes the outer portions of galaxies to rotate faster than they would otherwise. Astronomers do not know the composition of dark matter, but they can theorize what it might be like.

A slowly moving, cold dark matter, for example, could consist of not-yet-discovered particles that have names such as axions and weakly interacting massive particles (WIMPs). A rapidly moving, hot dark matter could be made up of particles called neutrinos, but measurements of neutrino mass indicate that they are too lightweight to account for much of the dark matter.

Since normal matter and dark matter account for only 27 percent of the material necessary for the universe to be at the critical density, the remaining 73 percent of the universe must be composed of a still more mysterious substance that astronomers have named 'dark energy.' The composition of dark energy is not known, but its effect on the universe is detectable. Dark energy exerts a negative pressure that acts as antigravity, accelerating the universe's expansion. The effect of dark energy was smaller in the past, allowing gravity to slow the universe's expansion, but on the largest scale the repulsive force of dark energy now overwhelms the attractive force of gravity.

WMAP results also showed that the universe is 13.7 billion years old, with an uncertainty of only 0.2 billion years, and that the cosmic background radiation was set free 389,000 years after the big bang, a value uncertain by only 8,000 years. WMAP estimated a value for the Hubble constant of 71 kilometers per second per megaparsec (44 miles per second per magaparsec), in near agreement with the value predicted by the Hubble Key Project. WMAP’s wide-ranging results will be refined as the spacecraft makes additional observations. Observations made by the European Space Agency's Planck spacecraft, scheduled for launch in 2007, will be even more precise.


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