Second-order hydrodynamics and universality in non-conformal holographic fluids

We study second-order hydrodynamic transport in strongly coupled non-conformal field theories with holographic gravity duals in asymptotically anti-de sitter space. we first derive new kubo formulae for five second-order transport coefficients in non-conformal fluids in (3 + 1) dimensions. we then apply them to holographic rg flows induced by scalar operators of dimension δ = 3. for general background solutions of the dual bulk geometry, we find explicit expressions for the five transport coefficients at infinite coupling and show that a specific combination, $$ tilde{h}=2eta {tau}_{pi }-2left(kappa -{kappa}^{ast}right)-{lambda}_2 $$ , always vanishes. we prove analytically that the haack-yarom identity h = 2ητ π − 4λ1 − λ2 = 0, which is known to be true for conformal holographic fluids at infinite coupling, also holds when taking into account leading non-conformal corrections. the numerical results we obtain for two specific families of rg flows suggest that h vanishes regardless of conformal symmetry. our work provides further evidence that the haack-yarom identity h = 0 be universally satisfied by strongly coupled fluids.