Copernican principle

In physical cosmology, the Copernican principle states that humans, on the Earth or in the Solar System, are not privileged observers of the universe,^[1] that observations from the Earth are representative of observations from the average position in the universe. Named for Copernican heliocentrism, it is a working assumption that arises from a modified cosmological extension of Copernicus' argument of a moving Earth.^[2]

Origin and implications

Most modern cosmology is based on the assumption that the cosmological principle is almost, but not exactly, true on the largest scales. The Copernican principle represents the irreducible philosophical assumption needed to justify this, when combined with the observations. If one assumes the Copernican principle and observes that the universe appears isotropic or the same in all directions from the vantage point of Earth, then one can infer that the universe is generally homogeneous or the same everywhere (at any given time) and is also isotropic about any given point. These two conditions make up the cosmological principle.^[7]

In practice, astronomers observe that the universe has

Since the 1990s the term has been used (interchangeably with "the Copernicus method") for J. Richard Gott's Bayesian-inference-based prediction of duration of ongoing events, a generalized version of the Doomsday argument.^{[clarification needed]}

Tests of the principle

The Copernican principle has never been proven, and in the most general sense cannot be proven, but it is implicit in many modern theories of physics. Cosmological models are often derived with reference to the cosmological principle, slightly more general than the Copernican principle, and many tests of these models can be considered tests of the Copernican principle.^[17]

Historical

Before the term Copernican principle was even coined, past assumptions, such as

• time drift of cosmological redshifts;[18]
• modelling the local gravitational potential using reflection of cosmic microwave background (CMB) photons;^[19]
• the
  supernovae;^[20]
• the kinetic Sunyaev–Zeldovich effect in relation to dark energy;^[21]
• cosmic neutrino background;^[22]
• the integrated Sachs–Wolfe effect^[23]
• testing the isotropy and homogeneity of the CMB;^{[24][25][26][27][28]}
• Some authors claim that the
  KBC Void violates the cosmological principle and thus the Copernican principle.^[29] However, other authors claim that the KBC void is consistent with the cosmological principle and the Copernican principle.^[30]

The standard model of cosmology, the Lambda-CDM model, assumes the Copernican principle and the more general cosmological principle. Some cosmologists and theoretical physicists have created models without the cosmological or Copernican principles to constrain the values of observational results, to address specific known issues in the Lambda-CDM model, and to propose tests to distinguish between current models and other possible models.

A prominent example in this context is

References