If you take a box that’s a hundred million light-years on each side and plunk it down here, take another such box and plunk it down way over there (say, a billion light-years from here), and then measure the average overall properties inside each box—average number of galaxies, average amount of matter, average temperature, and so on—you’ll find it difficult to distinguish between the two. In short, if you’ve seen one 100-million-light-year chunk of the cosmos, you’ve pretty much seen them all.
Cosmological Principle
