This particular line didnt have enough to put it in a proper context so I looked up his speech. The long "sweep" is referring to the progress America has made since it's beginnings. It could also be said as the way America has grown since it's beginings. Gary touched on it in his answer in that it refers to the evolution of America. Hope this helps. :)

My opinion is that it is deliberately ambiguous. It could mean the physical expanse of the country. It could refer to the evolution and history. I'm almost certain it doesn't have anything to do with research. What gave you that idea?