The Dark Energy Problem
The expansion rate of the Universe is increasing, not slowing down. How is that possible? One of the theories in place now involves studying the different densities left over from different times. This evidence will provide information about how the expansion of the universe occurred and how the density varied at different times. At an early time of the universe, the density was very high, so the expansion was not great. However, as the energy and matter density decreased, the universe began to expand faster. As the density across the universe decreases, the gravitational effect also gets weaker, so expansion gets faster.
Think of a rocket trying to get into space. The closer the rocket is to the earth, the denser the matter, so it takes a lot of energy to move the rocket into space. But once in space the gravity drops considerably, and the rocket can go faster, and its rate of travel can increase.
Scientists now believe that acoustic oscillations will provide the evidence for the expansion. Earlier periods of the universe should be denser than later periods. In other words, by measuring the length of the baryonic acoustic oscillations, scientists believe that with this information astronomers will be able to determine how dark energy has affected the expansion. They will have proof that the expansion rate changed over time. It was slower at the beginning, and faster now.
Initially it happened in the following manner. An overdense region of the primordial plasma existed.This overdensity attracts matter towards it through gravitational forces. However, the heat of photon-matter interactions does the opposite; it creates a large amount of outward pressure. It is the counteracting forces of gravity and pressure that create the oscillations, it is similar to how sound waves are created in the air by pressure differences.
About the Image: Anisotropies relates directly to variations in the density of galaxies as observed today. This is the property of being different, not the same in all directions in the cosmic microwave background, which originated when the universe was less than 400,000 years old.
You can read more about Dark Energy at Expanding Universe and the Mysterious Force of Dark Energy.
Image Credit: Berkley Lab News Center