# Help please! difference with clarification

1. Jan 28, 2012

### dpa

what is difference between

B.what are Flatness and horizon problems with standard cosmology model tried to overcome by string theory model.

2. Jan 30, 2012

### bapowell

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.

B. The horizon and flatness problems are well known and probably best looked up on Wikipedia or similar. String theory does not directly address these problems. The inflationary universe was developed to solve these (and other) problems.