Braneworld

Braneworld refers to a class of theories in high-energy physics in which the observable universe is a 3+1 dimensional hypersurface, or brane, embedded in a higher dimensional space called the bulk. In these models, the Standard Model fields are confined to the brane, while gravity can propagate in the bulk. Braneworld ideas are motivated by string theory and M-theory, where D-branes and other p-branes provide natural realizations of lower-dimensional surfaces on which matter and gauge fields can reside.

Different braneworld scenarios address questions such as the hierarchy problem and alterations to gravity. The ADD model posits large extra dimensions in which gravity spreads, making it appear weaker on the brane. The Randall–Sundrum models employ warped extra dimensions, with RS I featuring two branes and RS II a single brane in an infinite bulk; both yield modified gravity at high energies or large distances. The DGP model introduces a brane-localized gravity term, producing a crossover between four-dimensional and higher-dimensional gravity.

Cosmology on the brane includes modified Friedmann equations and dynamics influenced by brane tension and bulk geometry. Experimental and observational probes include tests of gravity at short ranges, collider searches for Kaluza–Klein states, and cosmological data. To date there is no experimental confirmation of extra dimensions, and braneworld models face theoretical challenges such as stability and moduli stabilization. Nevertheless, braneworld concepts continue to inform approaches to quantum gravity, holography, and early-universe models such as brane inflation.