# Hubble drag

28 Oct 2020The growth equation for linear structure growth in the overdensity \(\delta\), \(\ddot{\delta} + 2H\dot{\delta} = 4\pi G \bar{\rho}_m\delta\), features the term \(2H\dot{\delta}\), which is referred to as Hubble-drag, supposedly slowing structure formation. Can this be? And in a contracting universe with \(H<0\), would the term actually accelerate structure formation?