ADD Large Extra Dimensions
Up to seven flat extra dimensions, potentially as large as a millimeter.
Placeholder for a 3D visualisation of Braneworld & the Bulk. The interactive scene will land in Phase 3. String-theory-inspired models where our universe is a brane in a higher-dimensional bulk. The bulk pre-exists and persists. Variants differ in the bulk geometry: warped (RS), flat with large dimensions (ADD), or with parallel branes (Ekpyrotic).
In one sentence
The hierarchy problem is solved by allowing gravity to dilute into up to seven large extra dimensions.
The claim
Arkani-Hamed, Dimopoulos, and Dvali (ADD) proposed in 1998 that the hierarchy problem can be solved by allowing the extra dimensions to be very large, up to a millimeter, rather than warped or compactified at the Planck scale.
In ADD, our 3+1 dimensions are a 3-brane embedded in a higher-dimensional flat Minkowski bulk with up to 7 extra dimensions. The Standard Model is confined to our brane, but gravity propagates into the full higher-dimensional space. The fundamental gravity scale can be as low as TeV; gravity appears weak only because it is diluted into the extra-dimensional volume.
The family stance
A higher-dimensional spacetime, the bulk, with its own geometry. Different variants propose different bulk structures.
Predictions
- Deviations from Newton's law at the size of the extra dimensions
- Production of microscopic black holes at TeV colliders
- Kaluza-Klein graviton emission in scattering experiments
Evidence
- Elegant solution to hierarchy if extra dimensions are large
- Direct experimental tests via sub-millimeter gravity experiments
Counterpoints
- Sub-millimeter gravity tests have not seen deviations
- LHC has not produced microscopic black holes
- Simplest ADD scenarios excluded by collider searches
- Astrophysical constraints (supernova energy loss) further constrain it
Variants in this family
▸Go deeperTechnical detail with proper terminology
In ADD, the hierarchy emerges from volume dilution: M_Pl² ~ M*^(2+d) · R^d, where M_Pl is the 4D Planck mass, M* is the higher-dimensional fundamental scale, R is the size of the extra dimensions, and d is the number of large extra dimensions.
For M* ~ TeV and d = 2, R ~ 1mm. For d = 7, R is much smaller. The original "large extra dimensions" referred to R far larger than the Planck length.
Direct gravity tests probe R via measurements of the inverse-square law at sub-millimeter distances. Current bounds: R < ~30 micrometers for d = 2.
References
- EstablishedArkani-Hamed, Dimopoulos, Dvali (1998). Phys. Lett. B 429, 263
- EstablishedAntoniadis, Arkani-Hamed, Dimopoulos, Dvali (1998). Phys. Lett. B 436, 257
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