The paper "Accelerating Universes With Scaling Dark Matter" by Michel Chevallier and David Polarski explores Friedmann-Robertson-Walker universes with a significant fraction of energy density stored in an $X$-component, characterized by an equation of state $w_X < -1/3$. The authors identify all critical points of the system for constant equations of state and investigate background quantities that can distinguish models with different $w_X$ values. They use a toy model with a varying equation of state to show that even a large variation in $w_X$ at small redshifts is difficult to observe with luminosity distance measurements up to $z \sim 1$. Accurate measurements in the range $1 < z < 2$ and independent, accurate knowledge of $\Omega_{m,0}$ (and/or $\Omega_{X,0}$) are required to resolve a variable $w_X$ from a constant $w_X$. The paper also discusses the implications of these findings for observational constraints, such as the age of the universe, age in function of redshift, and luminosity distance measurements, emphasizing the need for high-precision data to distinguish between different models.The paper "Accelerating Universes With Scaling Dark Matter" by Michel Chevallier and David Polarski explores Friedmann-Robertson-Walker universes with a significant fraction of energy density stored in an $X$-component, characterized by an equation of state $w_X < -1/3$. The authors identify all critical points of the system for constant equations of state and investigate background quantities that can distinguish models with different $w_X$ values. They use a toy model with a varying equation of state to show that even a large variation in $w_X$ at small redshifts is difficult to observe with luminosity distance measurements up to $z \sim 1$. Accurate measurements in the range $1 < z < 2$ and independent, accurate knowledge of $\Omega_{m,0}$ (and/or $\Omega_{X,0}$) are required to resolve a variable $w_X$ from a constant $w_X$. The paper also discusses the implications of these findings for observational constraints, such as the age of the universe, age in function of redshift, and luminosity distance measurements, emphasizing the need for high-precision data to distinguish between different models.