A typical observer will see a small cosmological constant. Many physicists are uncomfortable with this line of reasoning, and they are seeking the answer in different class of theories known as quintessence, after the Greek word for the fifth element. Modern physics, noted Dr. Paul Steinhardt, a theorist at Princeton, is replete with mysterious energy fields that would exhibit negative gravity. The trick, Dr. Steinhardt explained, is finding a field that would act like the dark energy without a lot of fudging on the part of theorists. Dark energy is an interesting problem.
Any solution is quite interesting.
Dr. Gia Dvali, Dr. Gregory Gabadadze and Dr. Massimo Porrati. Ordinary particles like electrons and quarks and forces like electromagnetism are confined to three dimensions, to the brane, but gravity is not. As a result, Dr. Dvali suggested that gravity could only travel so far through conventional space before it leaked off into the extra dimensions, thereby weakening itself. To an observer in the traditional three dimensions it looks as if the universe is accelerating. The cosmological constant, in effect, he said, is a kind of gravitational brane drain.