Hubble's Law: Distance, Speed, & Upper Limit

[zuz]

I have read that Hubbles law states that a galaxys speed as it moves away from us is proportional to it's distance. Does this have an upper limit? If a galaxy 1 megaparsec is moving at 70 km/s, a galaxy 10,000,000 mgp away is moving faster then light. Or am I (most likely) mistaken?

 

[phinds]

I have read that Hubbles law states that a galaxys speed as it moves away from us is proportional to it's distance. Does this have an upper limit? If a galaxy 1 megaparsec is moving at 70 km/s, a galaxy 10,000,000 mgp away is moving faster then light. Or am I (most likely) mistaken?

You are correct, in principle although not in fact, and there is no upper limit. The objects at the edge of our observable universe are receding from us at about 3c. "Moves" however, is not really the right word to use here, having as it does, the connotation of proper motion, of which there is non in this situation. "Receding" is the right word to use.

The reason I say you are not right "in fact" is that an object 1MPsec away from us would not be receding at all since it would be inside the Local Group which is gravitationally bound.

I recommend the link in my signature.