Ultimate Claustrophobia

From Jeremy Bernstein's Einstein: One way of understanding the universe is "as a sort of sphere in space, filled, on the average, with a small uniform density of matter. A light ray started at any point in the universe will return to its starting point . . .in about 10 billion years" (156). Talk about enclosures! A ten-billion-year wide coffin. And, of course: what's outside the sphere?