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Cosmic journey

How dark energy influences the travels of light

By Deane Morrison

August 28, 2007

This is a companion piece to 'Researchers find absolutely nothing' Researchers Lawrence Rudnick, Shea Brown, and Liliya Williams found the void as a large spot where there were no galaxies or other large concentrations of matter. It coincides with the previously known location of a "cold spot" in a special type of light that fills the whole sky. Called the cosmic microwave background, or CMB, this light originated when the Universe was in its infancy and is considered an "echo of the Big Bang." The team figured out that it was no accident that the void and the CMB cold spot occurred in the same area of space. The explanation lies with dark energy, a powerful force that causes a different effect on light passing through such a large void than on light passing through areas rich in galaxies or other matter. Dark energy became a dominant force in the Universe very recently, when the Universe was already three-quarters of the size it is today. Dark energy works opposite gravity and is speeding up the expansion of the Universe. Thanks to dark energy, CMB light that passes through a large void just before arriving at Earth has less energy than light that passes through an area with a normal distribution of matter in the last leg of their journey. Here's why: In a simple expansion of the Universe, without dark energy, light approaching a large mass--such as a huge cluster of galaxies--picks up energy from its gravity. As the light pulls away, the gravity saps its energy, and it winds up with the same energy as when it started. But light passing through matter-rich space when dark energy becomes dominant doesn't fall back to its original energy level. Dark energy counteracts the influence of gravity, and so the large masses don't sap as much energy from the light as it pulls away. Thus, this light arrives at Earth with a slightly higher energy, or temperature, than it would in a dark energy-free Universe. It's like being on a landscape with no flat spaces, only hills and valleys. A source of gravity is like a valley; a wagon gains energy as it goes down a valley and loses energy as it pulls out of the valley. We see the wagon's energy gains and losses as changes in speed. As dark energy counteracts gravity, it stretches out the landscape, and the valleys get shallower. Light becomes like a wagon that never has to climb as far uphill as it rolled downhill. Thus, it shows a net gain of energy--but in the form of a higher temperature. For light passing through a large void, however, things are different. As it enters a void, light pulls away from whatever matter it is leaving behind. It loses energy, just like a wagon climbing a hill. But when dark energy stretches out the hill, the hill becomes smaller, and the wagon will never regain as much energy by rolling down as it lost in climbing up. Thus, the light suffers a net loss of energy, which shows up as a drop in temperature.