The accelerating universe theory implies that not only is the universe expanding, it is beginning to expand at every increasing rate. According to simpler versions of the Big Bang theory, the universe has expanded since its explosive birth. Gravity has gradually slowed the expansion of the universe so even if it continues to expand forever it should do so at an ever steadily decreasing rate.
However, recent observations of exploding stars, Type Ia supernovae, suggest that the universe is expanding at an accelerating rate. The implications of an accelerating universe defy the current gravitational laws of the universe and bring back Albert Einstein�s belief of a cosmological constant.
So how does a supposedly decelerating universe begin to accelerate?
Cosmologists attribute the acceleration to an unusual form of energy now considered the cosmological constant, lambda Originally, the cosmological constant was a term invoked by Albert Einstein back in 1917 when he realized his theory of gravity predicted an universe that is either expanding or contracting and not static as he had hoped. Einstein understood a universe could remain static if there was a cosmic repulsion that exactly balanced matter�s gravitational attraction. When experiments began to show the universe was expanding Einstein recanted his idea of a cosmological constant calling it �my greatest blunder�.
Now cosmologists have re-invoked the cosmological constant, not to allow for a static universe, but to allow for an accelerating one. Though poorly understood, the cosmological constant is believed to be energy density of the vacuum. The energy of the cosmological constant is a property of space-time and is the same everywhere. A sufficiently large Lambda will cause galaxies to accelerate away from each other contrasting ordinary forms of energy�s tendency to slow down receding objects. This appears to be exactly what is happening.
