Four forces of nature - Strong force; Electromagnetic force; Weak force and gravitation, have varying strengths. Strong force is strongest and gravitation is the weakest (billion on billion times weaker than others). This extreme weakness of gravitation makes it difficult to comeup with a unifying theory of all four forces together - although the standard model does cover rest of the three. Scientists call this a "hierarchy problem". Dr.Lisa Randall, string theorist, Harvard professor and a very graceful 43 year old, developed a model of universe based on string theory(alongwith Dr. Raman Sundram), which explains why the gravitation is so weakly felt in our universe though in the beginning of universe all the four forces were indistinguishable from each other.
String theory (M-Theory) says that all particles and forces are nothing but a different vibrational states of thin stringlike structures which exist in 11 dimensions.
Universe consists of 3-dimensional membrane (jargon="brane") surrounded by a higher dimensions.
In their model called RS-1 and RS-2(published in 1999), they propounded that while all other forces are on the same "brane" as our universe, gravity resides on another "brane" separated from our universe by a five dimensional space with extra fifth dimension (time being the fourth). Gravitons (a loop of string), the particle associated with force of gravity is free to travel from gravity-brane to ours. However the space is warped in a negative way by the gravity-brane (anti De Sitter space), due to which though there is a large cluster of gravitons near gravity-brane only a small number are able to make it to our universe-brane. This explains why is the gravity so weak compared to other three forces which are resident on home-brane. Is it not familiar - Dogs bark louder near home-turf.
The extra 5th dimension of the 5-dimensional space separating us from gravity-brane is large enough but appears to be only 10-31cm wide to us because it is warped in negative fashion. Otherwise Newton law of gravitation would have been inverse cube law instead inverse square it actually is.
This theory and other competing ones (Arkani-Dvali-Dimopoulos= ADD) would be put to test in CERN's Large Hadron Collider (LHC) near Geneva, when it fires up in 2007.
Dr. Randall recently published "Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions" to convey to the lay readers the excitement of her work on RS-1 and RS-2.
Happy Reading!! Fources