Researchers have found an exact mathematical description of a “hairy” black brane — a black‑hole–like object that is translationally symmetric in two directions and carries a nontrivial scalar field. The solution lives in four‑dimensional Einstein gravity coupled to a neutral scalar and comes from a consistent truncation of type IIA supergravity that is used in holography to model the strongly coupled ABJM quantum field theory. The key result is that, although the microscopic theory breaks scale symmetry at high energy, the thermal, low‑energy behavior of the system is exactly that of a conformal fluid.

What the authors did was to write down and analyze an exact, two‑parameter family of static planar black brane solutions. One parameter controls the size of the horizon (call it m) and the other controls the amount of scalar “hair” (call it α). Because the solution is exact, the authors can compute thermodynamic quantities in closed form and treat the hair parameter α as a genuine thermodynamic variable. They show how mass, temperature and entropy densities depend on m and α, and they derive a generalized first law of thermodynamics and an Euler relation that include a conjugate to a quantity they identify as a thermodynamic central charge Ct.

How does a non‑conformal microscopic theory end up conformal in the infrared (IR, meaning low energies or long distances)? In this model the scalar field that breaks scale symmetry in the ultraviolet (UV, at high energy) runs logarithmically with scale. Yet the value of the scalar field at the black brane horizon depends only on the hair parameter α and not on the horizon size (or entropy). This decoupling forces the bulk viscosity to vanish and fixes the sound speed to the conformal value (in these three boundary dimensions the square of the speed of sound is 1/2). In other words, the thermal fluid described by the horizon behaves exactly like a conformal fluid, even though the underlying action is deformed away from scale invariance.