哥白尼原则

原本托勒密模型的天空将地球放置在太阳系的中心，尼古拉·哥白尼则发现天体的运动可以不用地球（或别的）在中心的几何系统来解释，所以他假设从一个特别的位置来观测是可以转化至别的位置来说明。

哲学家伊曼努尔·康德用“哥白尼回转轮”来阐述他的《当代认识论》中肯的思想方法的效应。他把知识主题的外在情况和品质归咎于人的所有概念和经验主义的经验中心，并且克服了理性主义 - 经验主义的僵局，成为17和18世纪的特色。

将哥白尼原则应用在宇宙论上，也就是认为宇宙在大尺度下是均质和各向同性的。这个原则不仅仅是一种哲学上的声明，也是一种重大的认知：从均质和各向同性的观点看，在统计上产生大规模的偏差是不可能的；这也就是承认在观测上的发现，可以经由各种不同的观测加以印证。

在实务上，天文学家在超星系团、星系纤维和空洞的尺度上观测时，仍有不同的结构，但考虑到更大，在2亿秒差距的尺度时，宇宙基本上是均质的。但是，当这是真实的，则宇宙在大尺度的时间上不是均质和各向同性的，因为它是从条件极端不同下的大霹雳演化过来的，并且将继续往极端不同的情况发展下去，特别在暗能量的影响不断提升下，明显的朝向大冰冻或是大割裂发展。在非宇宙论时间尺度下的时间内宇宙是均质的，但是在基本粒子交互作用的时间尺度内不是各向同性的。在大尺度下各向异性的时间将导出最基本的近代物理的悬案。

2011年，中国科学院上海天文台研究员张鹏杰对哥白尼原理进行了检验，证实在径向尺度30亿光年以上哥白尼原则成立。

In physical cosmology, the Copernican principle states that humans are not privileged observers of the universe, 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.

Origin and implications

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 could be explained by reference to an assumption that the Sun is centrally located and stationary in contrast to geocentrism. 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.

Although the Copernican heliocentric model is often described as "demoting" Earth from its central role it had in the Ptolemaic geocentric model, it was successors to Copernicus, notably the 16th century Giordano Bruno, who adopted this new perspective. The Earth's central position had been interpreted as being in the "lowest and filthiest parts". Instead, as Galileo said, the Earth is part of the "dance of the stars" rather than the "sump where the universe's filth and ephemera collect". In the late 20th Century, Carl Sagan asked, "Who are we?

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.

Historical

Before the term Copernican principle was even coined, past assumptions, such as geocentrism, heliocentrism, and galactocentrism, which state that Earth, the Solar System, or the Milky Way respectively were located at the center of the universe, were shown to be false. The Copernican Revolution dethroned Earth to just one of many planets orbiting the Sun. Proper motion was mentioned by Halley. William Herschel found that the Solar System is moving through space within our disk-shaped Milky Way galaxy. Edwin Hubble showed that the Milky Way galaxy is just one of many galaxies in the universe. Examination of the galaxy's position and motion in the universe led to the Big Bang theory and the whole of modern cosmology.

Modern tests

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;

Physics without the principle

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 inhomogeneous cosmology, to model the observed accelerating universe and cosmological constant. Instead of using the current accepted idea of dark energy, inhomogeneous-cosmology models propose that the universe is much more inhomogeneous than currently assumed — for example, that the Solar System is in an extremely large low-density void. To match observations the Solar System would have to be very close to the centre of this void, immediately contradicting the Copernican principle.