This paper reviews a family of five-dimensional models that interpolate between two well-known warped geometries. At one end is anti–de Sitter space (AdS), familiar from the Randall–Sundrum proposal, and at the other is the linear dilaton (LD) background, which has a holographic link to so-called Little String Theory. The family is labeled by a real parameter ν: ν=0 gives AdS, ν=1 gives the linear dilaton case. The authors show how these backgrounds change the behavior of fields that live in the extra dimension and explore consequences for particle physics and cosmology.

Technically, the paper works in a five-dimensional gravity plus scalar (dilaton) system and solves for the background metric and scalar profile. For large values of the scalar the solutions end at a real singularity at a finite coordinate y=ys (in one useful normalization ys=1/η). The different values of ν control the spectrum of excitations that propagate in the bulk: for ν<1 the spectrum is continuous with no mass gap, for ν>1 it is discrete with a mass gap, and the critical ν=1 (the LD case) gives a continuous spectrum with a mass gap. The authors give explicit formulae for the warp factor A(y) and for the scalar, and they identify a characteristic mass gap for bulk states; in one presentation the gap scale is written as mg = 3η/2.

For particle physics the LD-type backgrounds open the door to lowering the five-dimensional Planck scale M5 below the usual four-dimensional Planck scale M4. The review highlights a solution where M5 can be roughly 10^-4 times M4, while the low-energy scale on the brane remains near the TeV range. The warp factor is used to separate the TeV scale from M5; the authors note, however, that string theory or other UV input is still needed to explain the remaining hierarchy between M5 and M4.

For cosmology the paper studies “brane world” setups that include a five-dimensional black hole. In that case the extra-dimensional geometry sources a holographic fluid on the brane. For an AdS background this fluid behaves like extra radiation, while for the linear dilaton background it behaves like pressureless matter. The authors point out that this pressureless holographic fluid is weakly coupled to Standard Model particles (only gravitationally) and could act as dark matter. Under the assumption that the inflaton couples only to Standard Model fields, the fluid would be absent after inflation and reheating but can be repopulated by a freeze-in mechanism. That freeze-in comes from leakage of gravitons into the bulk and is controlled by the reheating temperature.