Brane world perspective on the cosmological constant and the hierarchy problems - art. no. 045007

We elaborate on the recently proposed static brane world scenario, where th e effective 4D cosmological constant is exponentially small when parallel 3 -branes are far apart. We extend this result to a compactified model with t wo positive tension branes. In addition to an exponentially small effective 4D cosmological constant, this model incorporates a Randall-Sundrum-like s olution to the hierarchy problem. Furthermore, the exponential factors for the hierarchy problem and the cosmological constant problem obey an inequal ity that is satisfied in nature. This inequality implies that the cosmologi cal constant problem can be explained if the hierarchy problem is understoo d. The basic idea generalizes to the multibrane world scenario. We discuss models with piecewise adjustable bulk cosmological constants (to be determi ned by the 5-dimensional Einstein equation), a key element of the scenario. We also discuss the global structure of this scenario and clarify the phys ical properties of the particle (Rindler) horizons that are present. Finall y, we derive a 4D effective theory in which all observers on all branes not separated by particle horizons measure the same Newton's constant and 4D c osmological constant.