Cosmic Ripples Confirm Flat Universe

A telescope high in the Chilean Andes has opened a clear view of a long-sought hump in measurements of the universe's faint background radiation. The discovery, which will be reported in the 10 October Astrophysical Journal Letters, confirms a long-standing prediction about the universe's total mass and energy density and hints that a major part of the total may be a mysterious form of energy in empty space.

The "hump" measures the coarseness of the ripples imprinted on the cosmic microwave background (CMB) shortly after the big bang. In its first 100,000 years or so, the baby universe was so dense and hot that matter and light behaved like a single fluid, so that fluctuations in the density of the ionized gas created corresponding hot and cold spots in the radiation. But as the universe expanded and the gas cooled, light broke free from matter. A fossil record of those past fluctuations can still be seen, imprinted on the CMB.

Because cosmologists can calculate the actual size of the most common ripples, their apparent size on the sky should reflect the overall shape of space, just as the apparent size of objects seen through a lens depends on the shape of the glass. In a universe containing just enough matter and energy to make space flat, so that parallel light rays remain parallel, the hump would fall at an angular size of about 1 degree.

Finding the hump has proved difficult, however, because it requires extremely precise comparisons of the CMB's temperature at different points in the sky. Now, new measurements by the Microwave Anisotropy Telescope (MAT) team seem to have resolved that difficulty. Observing from the dedicated 85-centimeter telescope perched high on the southern slopes of Cerro Toco in northern Chile, the MAT team compared the CMB at different spots in the sky for almost 1200 hours. They found that the abundance of ripples clearly "rises and then falls" near 1 degree, says Princeton University astrophysicist Amber Miller.

"All by itself it shows the existence" of the 1-degree hump, says University of Chicago cosmologist Michael Turner. "We can claim to have a complete accounting of the matter and energy in the universe." If so, the universe that astronomers can see is seriously underweight. Simply counting up all of its stars, gas, and hidden "dark matter," astronomers have found only 30% of the necessary density. Theorists speculate that the rest takes the form of a cosmological constant, first proposed by Einstein--an energy in empty space that pushes on the fabric of space-time.