The cosmological principle states that, on a sufficiently large scale, the universe is both homogeneous and isotropic.
Which of the following observational properties most directly supports the claim that the universe is isotropic?
The recession velocity of a distant galaxy is proportional to its distance from Earth.
The Cosmic Microwave Background (CMB) radiation has an almost perfectly uniform temperature of approximately 2.7 K2.7\text{ K}2.7 K in all directions.
The orbital speeds of stars in the outer regions of spiral galaxies remain constant with distance from the galactic center.
Redshift observations of Type Ia supernovae show that the expansion of the universe is accelerating.
