In physical cosmology, the Copernican principle, named after Nicolaus Copernicus, is a working assumption that arises from a modified cosmological extension of Copernicus' sun-centered heliocentric universe. The Copernican principle assumes that neither the Sun nor the Earth are in a central, specially favored position in the universe. More recently, the principle has been generalized to the relativistic concept that humans are not privileged observers of the universe. In this sense, it is equivalent to the mediocrity principle, with important implications for the philosophy of science.
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.
Origin and implications
Michael Rowan-Robinson emphasizes the Copernican principle as the threshold test for modern thought, asserting that: "It is evident that in the post-Copernican era of human history, no well-informed and rational person can imagine that Earth occupies a unique position in the universe.".
Hermann Bondi named the principle after Copernicus in the mid-20th century, although the principle itself dates back to the 16th-17th century paradigm shift away from the Ptolemaic system, which placed Earth at the center of the universe. Copernicus proposed that the motion of the planets can be explained by reference to an assumption that the Sun and not Earth is centrally located and stationary. He argued that the apparent retrograde motion of the planets is an illusion caused by Earth's movement around the Sun, which the Copernican model placed at the centre of the universe. Copernicus himself was mainly motivated by technical dissatisfaction with the earlier system and not by support for any mediocrity principle. In fact, although the Copernican heliocentric model is often described as "demoting" Earth from its central role it had in the Ptolemaic geocentric model, neither Copernicus nor other 15th- and 16th-century scientists and philosophers viewed it as such. It wasn't until the late 20th Century that Carl Sagan could claim "Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people.".
In cosmology, if one assumes the Copernican principle and observes that the universe appears isotropic or the same in all directions from our vantage-point on 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. In practice, astronomers observe that the universe has heterogeneous or non-uniform structures up to the scale of galactic superclusters, filaments and great voids. It becomes more and more homogeneous and isotropic when observed on larger and larger scales, with little detectable structure on scales of more than about 200 million parsecs. However, on scales comparable to the radius of the observable universe, we see systematic changes with distance from Earth. For instance, galaxies contain more young stars and are less clustered, and quasars appear more numerous. While this might suggest that Earth is at the center of the universe, the Copernican principle requires us to interpret it as evidence for the evolution of the universe with time: this distant light has taken most of the age of the universe to reach and shows us the universe when it was young. The most distant light of all, cosmic microwave background radiation, is isotropic to at least one part in a thousand.
Modern mathematical 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.
Bondi and Thomas Gold used the Copernican principle to argue for the perfect cosmological principle which maintains that the universe is also homogeneous in time, and is the basis for the steady-state cosmology. However, this strongly conflicts with the evidence for cosmological evolution mentioned earlier: the universe has progressed from extremely different conditions at the Big Bang, and will continue to progress toward extremely different conditions, particularly under the rising influence of dark energy, apparently toward the Big Freeze or Big Rip.
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.
Before the term Copernican principle was even coined, Earth was repeatedly shown not to have any special location in the universe. The Copernican Revolution dethroned Earth to just one of many planets orbiting the Sun. William Herschel found that the Solar System is moving through space within our disk-shaped Milky Way galaxy. Edwin Hubble showed that our galaxy is just one of many galaxies in the universe. Examination of our galaxy's position and motion in the universe led to the Big Bang theory and the whole of modern cosmology.
Michelson-Morley and Relativity
The Michelson-Morley experiments to measure Earth's motion through the luminiferous aether provided further indirect proof that Earth had no special position, although early interpretations implying a complete lack of motion in the ether would have dis-proven the principle. The final interpretations of the result in terms of the electromagnetic nature of light, Lorentz invariance, and ultimately the general and special theories of relativity, contained no contradictions of the Copernican principle although still no strong proof.
Ecliptic alignment of cosmic microwave background anisotropy
The Cosmic Microwave Background (CMB) radiation signature presents a direct large-scale view of the universe that can be used to identify whether our position or movement has any particular significance. There has been much publicity about analysis of results from the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck mission that show both expected and unexpected anisotropies in the CMB.
Some anomalies in the background radiation have been reported which are aligned with the plane of the Solar System, which contradicts the Copernican principle by suggesting that the Solar System's alignment is special. Land and Magueijo dubbed this alignment the "axis of evil" owing to the implications for current models of the cosmos, although several later studies have shown systematic errors in the collection of that data and the way it is processed. Various studies of the CMB anisotropy data either confirm the Copernican principle, model the alignments in a non-homogeneous universe still consistent with the principle, or attempt to explain them as local phenomena. Some of these alternate explanations were discussed by Copi, et al., who claimed that data from the Planck satellite could shed significant light on whether the preferred direction and alignments were spurious.
Coincidence is a possible explanation. Chief scientist from WMAP, Charles L. Bennett suggested coincidence and human psychology were involved, "I do think there is a bit of a psychological effect, people want to find unusual things." 
Recent and planned tests relevant to the cosmological and Copernican principles include:
- time drift of cosmological redshifts;
- modelling the local gravitational potential using reflection of cosmic microwave background (CMB) photons;
- the redshift dependence of the luminosity of supernovae;
- the kinetic Sunyaev-Zel’dovich effect in relation to dark energy;
- cosmic neutrino background;
- the integrated Sachs-Wolfe effect
- testing the isotropy and homogeneity of the CMB;
Physics without the principle
The standard model of cosmology, the Lambda-CDM model, assumes the Copernican principle and the more general Cosmological principle and observations are largely consistent but there are always unsolved problems. Some cosmologists and theoretical physicists design models lacking the Cosmological or Copernican principles, to constrain the valid values of observational results, to address specific known issues, and to propose tests to distinguish between current models and other possible models.
A prominent example in this context is the observed accelerating universe and the cosmological constant issue. An alternative proposal to dark energy is that the universe is much more inhomogeneous than currently assumed, and specifically that we are in an extremely large low-density void. To match observations we would have to be very close to the centre of this void, immediately contradicting the Copernican principle.
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