For a perfect fluid, the quantity defined through mixed components of the stress–energy tensor is independent on the choice of coordinates only for two values of the pressure to energy density ratio w = p/ρ: for radiation with w = 1/3, and for dark energy with w = −1. With other choices of w, the quantity is coordinate dependent, and only in the local rest frame of the fluid. We show that the same is true also for solid matter with shear stress Lam coefficient set to zero in a flat Friedmann–Lematre–Robertson–Walker Universe with perturbed metric as well as stress–energy tensor. We call the two different solids with coordinate independent radiation-like solid and dark energy-like solid, and we restrict ourselves to these two special cases. By analysing second order perturbations we discover two one parametric sets of such solid matter models containing special cases of radiation and dark energy as perfect fluids. We also study equations for perturbations up to the second order for both sets of models.

对于理想流体，通过应力-能量张量的混合分量定义的量与坐标选择无关，仅对于压力与能量密度比w = p / ρ 的两个值：对于w = 1/3 的辐射，对于w = -1 的暗能量。对于w 的其他选择，该数量取决于坐标，并且仅在流体的局部静止坐标系中。我们表明，对于具有扰动度量以及应力-能量张量的平坦 Friedmann-Lematre-Robertson-Walker Universe 中剪切应力 Lam 系数设置为零的固体物质也是如此。我们称这两种不同的实体具有坐标无关类辐射固体和类暗能量固体，我们仅限于这两种特殊情况。通过分析二阶扰动，我们发现了两个单参数的固体物质模型，其中包含作为完美流体的辐射和暗能量的特殊情况。我们还研究了两组模型的二阶扰动方程。