The Cosmological Principle

Together, the homogeneity and isotropy of the universe make up what we call the cosmological principle: a cornerstone assumption in modern cosmology. If we could not make this assumption (based on observation), then our cosmology might only apply to a very local part of the universe. But the cosmological principle allows us to extrapolate our conclusions drawn from our local viewpoint to the whole universe. And consider these implications: A homogeneous universe can have no border or edge (since it is the same in any volume), nor can it have a center (since it should look the same in all directions from any viewpoint).

Perhaps the best model of the expanding universe is a toy balloon, on which we randomly draw some dots with a magic marker. But a warning first: If the universe is represented by the surface of a balloon, then we are talking about a two-dimensional universe. There is no inside of the balloon if you can draw only on its surface. Magic-marker dots on the balloon's surface represent galaxies.

As you look out into the universe, that is, out into the balloon's surface, you see no center and no edge—just like our balloon model. And if the balloon is inflated, the dots move away from one another. Regardless of their point of view, all the dots "see" all the others as moving away.

Telescopes Mastery

Telescopes Mastery

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