1. Jan 28, 2012

dpa

what is difference between
A. The Hubble radius is the distance at which objects recede from us at light speed. This comes from Hubble's Law: $v = Hr$: when $r=r_H=c/H$, the recession velocity is c. I'm not sure what you mean by spatial radius. Perhaps you are referring to the curvature scale, also called the radius of curvature. This is simply the inverse of the Gaussian curvature, K, of the spatial geometry: $R \sim 1/K$. The curvature scale is infinite for flat geometries.