The isotropy of the universe is not immediately obvious. In fact, on small
scales, the universe is blatantly anisotropic. Consider, for example, a sphere
3 meters in diameter, centered on your navel (Figure 2.2a). Within this
sphere, there is a preferred direction; it is the direction commonly referred
to as “down”. It is easy to determine the vector pointing down. Just let go
of a small dense object. The object doesn’t hover in midair, and it doesn’t
move in a random direction; it falls down, toward the center of the Earth.