The Cosmological Principle

The distribution of matter across the universe is approximately even.
The distribution of matter across the Universe is approximately even, homogeneous, when considered at large scales.

Albert Einstein's theory of General Relativity permits many possible types of universes. In applying the theory to describe the dynamics of our Universe, Einstein made a central empirical assumption to limit the number of possible solutions to the equations. He assumed that on very large scales the distribution of matter in the Universe is constant, making the Universe appear smooth. This idea is a form of the modern cosmological principle.

This principle is not exact since much of the Universe's matter is found clustered together in planets, stars, and galaxies, but when considered at sufficient scales the distribution of galaxies and clusters is approximately even. A useful analogy is a pane of glass which appears smooth to the touch but which is in fact on the microscopic level a highly irregular surface, dotted with peaks and valleys.

The Modern Model of the Universe

Dr. Edwin Hubble
Dr. Edwin Hubble

In 1929, astronomer Edwin Hubble made a truly startling discovery. By examining the light emitted from neighboring galaxies and making detailed observations of an electromagnetic property called redshift, Hubble showed that other galaxies appeared to be accelerating away from the Milky Way. Contrary to what Einstein had predicted, the Universe was actually expanding...

Sidequest [1.11a] Sidequest: Hubble's Law
Down the Rabbit Hole [1.11b] Down the Rabbit Hole: Expansion

The Big Bang
The Big Bang, Lance Akiyama

Hubble's discovery, coupled with the modern interpretation of the cosmological principle, led to the development and eventual acceptance of the Big Bang model. Based on the theoretical work of Alexander Freidmann and Georges Lemaitre, the model describes the fiery origins of the Universe as a "primordial atom" and its subsequent evolutionary history of expansion and cooling.

The Big Bang model is the most widely accepted scientific theory for the origin and evolution of our Universe. Since its inception in the mid-twentieth century, scientists have continued to make important theoretical and experimental discoveries which test and support the model's predictions. The current body of evidence for the Big Bang falls into four broad categories:

  • The expansion of the Universe
  • The presence of Cosmic Microwave Background Radiation
  • The various abundances of elements
  • The evolution of stars and galaxies

Down the Rabbit Hole [1.11c] Down the Rabbit Hole: The Steady State Theory

Cosmic Conundrums [1.11d] Cosmic Conundrums: Model of the Universe