5.12.3 The Cosmological Principle

The cosmological principle states that:

The universe is isotropic, homogenous and the laws of physics are universal

• Isotropic means that the universe is the same in all directions to every observer
  • Although specific regions of space may be completely empty and other regions may contain galaxies or clusters of galaxies which clump together, over the entire volume of space the distribution of matter appears to be uniform
• Homogenous means that matter is uniformly distributed, the universe has a uniform density
  • Although specific regions of space may contain more matter and other regions may contain less matter, over the entire volume of space the density appears uniform
• At every point in the universe the laws of physics are universal
  • This means that the same laws and models apply as here on Earth

The Cosmological Principle is demonstrated using the three models of the universe:

• Model A shows a universe that is isotropic, but not homogenous
  • The universe is the same in all directions to every observer
  • However, there is not a uniform distribution of matter (uniform density) in all regions
• Model B shows a universe that is homogenous, but not isotropic
  • The universe has a uniform distribution of matter (uniform density) in all regions
  • However, the universe is not the same in all directions to every observer due to the orientation of the bricks
• Model C illustrates the Cosmological Principle, it is both isotropic and homogenous
  • The universe is the same in all directions to every observer
  • The universe has a uniform distribution of matter (uniform density) in all regions