The cosmological principle states that, on a sufficiently large scale, the universe is both homogeneous and isotropic.
Which of the following observations provides the most direct evidence that the universe is homogeneous?
The cosmic microwave background radiation having an almost perfectly uniform temperature of 2.7 K2.7\text{ K}2.7 K in all directions across the sky.
The constant orbital speeds of stars in the outer regions of spiral galaxies regardless of their distance from the galactic center.
Deep-field galaxy redshift surveys showing a uniform average number density of galaxy clusters when comparing different massive volumes of space.
The cosmological redshift of light from distant galaxies being directly proportional to their distance from Earth.
