Glossary
The Copernican Cosmological Principle

The Copernican Cosmological Principle is that
On a large scale, the universe is both homogeneous and isotropic (in 3-D space), and has/will always be so.

Note that the statement "has/will always be so" refers to the universe continuing to display the properties of homogeneity & isotropy. The Copernican Cosmological Principle does not imply that any actual observable parameter (e.g. the density of matter in the universe) will remain constant with time. Indeed, the Copernican Cosmological Principle allows the properties of the universe to evolve with time, but states that at any given time the universe will be both homogeneous and isotropic (in 3-D space).
(For an extension of the principle in the time domain, see the Perfect Cosmological Principle.)

Another way of expressing the principle is that all observers (in inertial frames)

  • will see identical properties & laws - homogeneity
  • will NOT see any preferred direction - isotropy

Origins

The principle can be considered a rather "idealised assumption" at first sight, and needs qualification and explanation. Indeed, it is

  • essentially a metaphysical statement
  • it is still unclear whether it is true (below) !!

Nevertheless it has been the foundation of (just about) all cosmologies because it offers a useful simplification and approximation of the real world. The impliction is that
the same laws of physics hold throughout the universe.

The Copernican Cosmological Principle (or often simply the "Cosmological Principle") is a logical extension of the the Copernican theory that the Earth is not the center of the universe. Thus the Earth is not "special", thus the "laws of nature" on (or around) Earth are not special.

As stated above, it is essentially a philosophical requirement/simplification necessary/assumed for all modern cosmologies:

  • if the laws of physics only apply to our region of the universe, then since these laws of physics will not apply to other regions (and/or the universe as a whole), it is unlikely we will be able to understand the universe as a whole
  • it is "justified" by Ockham's Razor

Observational Support For

The most often cited observational support for the Copernican Cosmolgical Principle is
  • the Cosmic Microwave Background (CMB), which is isotropic to 1 part in 105

Indeed if we observe homogeneity and isotropy in the characteristics of the universe, then one can argue

  • then surely the same physical laws must be at work (?)
    if the laws of physics are different in different regions of the universe, then surely this would have observational implications.

Observational Evidence Against

The obvious observational evidence against the Cosmolgical Principle seems to be
  • the structure seen in the universe on a variety of scales (stars, galaxies, clusters, super-clusters..)
This is why the qualifier "On a large scale.." is required to be added to the principle. The problem them becomes
  • a question of scale (now large is "large" ?)
  • whether the structures observed on large scales are indeed truely representative of the universe on these scales
    • Is what we can "see" (with current technology) a true representation of the universe on these scales ?
    • or, are the things we "see" (with current technology) only "perturbations" on the underlying universe.

An Analogy

A (small) sentient being living in the center of a "perfect" loaf of bread.
  • There may be obvious structure on small scales (air bubbles etc), but on the large scale the loaf can be considered uniform and isotropic
  • The laws of physics (e.g. which caused the dough to rise) are the same throughout the loaf.
  • The loaf might still be rising - but (in this perfect loaf) this happens uniformly & following the same laws throughout the loaf

Additional Notes