 Recent work, codified by Harada et al. (2018), shows just how different such cosmological singularities can be. For homogeneous cosmological models filled with perfect fluids with a linear equation of state—the standard cosmological model—certain values of the barotropic index yield past, future, or past and future big rips that are such that every timelike geodesic runs into them, but every null geodesic avoids them. (See  note 7  for an explanation of the barotropic index.) In other words, any body traveling more slowly than light will run into the singularity, but every light ray will escape to infinity. This is not a situation that lends itself to easy and perspicuous physical interpretation. 