So whats the difference between a prequel and a movie that takes place in the same universe? They could be taking this franchise in an entirely different direction and just use the same established universe.
If Marvel made a movie that had a scene with Tony Stark running around as a kid would you call it the Iron Man prequel?
Yes, that would be an "Iron Man" prequel. However, a movie about his baby sitter, while obviously taking place in the same universe, would not be a prequel unless it somehow lead to the events in "Iron Man".
A prequel is a story that takes place in the same universe, but specifically has elements that contribute/connect to the story that comes after it.
"In the same universe" can mean a lot of things. It could be something as simple as one character....like Detective John Munch appeared on "Homicide: Life on the Street"....the same character later appeared on "The Wire"....and oddly he also appeared on "X-Files". So all 3 of those shows are technically in the same universe, despite having no other connection.
"Prometheus" has too many ties to "Alien" to be called anything other than a prequel. Same derelict ship, same "Space Jockey" life form, same company that hired Ripley, same android like Bishop, etc. Some of the stuff you see in "Prometheus" is directly related to things that were later seen in "Alien".
Fred.