| Part of a series on |
| Physical cosmology |
|---|
 |
Early universe |
Components · Structure |
Inphysical cosmology, theCopernican principle states that humans are not privileged observers of theuniverse,[1] that observations from the Earth are representative of observations from the average position in the universe. Named forCopernican heliocentrism, it is a working assumption that arises from a modified cosmological extension ofCopernicus' argument of a moving Earth.[2]
Hermann Bondi named the principle after Copernicus in the mid-20th century, although the principle itself dates back to the 16th–17th centuryparadigm shift away from thePtolemaic system, which placedEarth at the center of theuniverse. 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 thegeocentrism. He argued that theapparent retrograde motion of the planets is an illusion caused by Earth's movement around theSun, which theCopernican 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 anymediocrity principle.[3]
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 centuryGiordano 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".[4][5] In the late 20th Century, Carl Sagan asked, "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."[6]
While the Copernican principle is derived from the negation of past assumptions, such asgeocentrism,heliocentrism, orgalactocentrism which state that humans are at the center of the universe, the Copernican principle is stronger thanacentrism, which merely states that humans are not at the center of the universe. The Copernican principle assumes acentrism and also states that human observers or observations from Earth are representative of observations from the average position in the universe.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 the Earth occupies a unique position in the universe."[7]
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 appearsisotropic or the same in all directions from the vantage point of Earth, then one can infer that the universe is generallyhomogeneous or the same everywhere (at any given time) and is also isotropic about any given point. These two conditions make up thecosmological principle.[7]
In practice, astronomers observe that the universe hasheterogeneous or non-uniform structures up to the scale ofgalactic superclusters,filaments andgreat voids. In the currentLambda-CDM model, the predominant model of cosmology in the modern era, the universe is predicted to become more and more homogeneous and isotropic when observed on larger and larger scales, with little detectable structure on scales of more than about 260 millionparsecs.[8] However, recent evidence fromgalaxy clusters,[9][10]quasars,[11] andtype Ia supernovae[12] suggests that isotropy is violated on large scales. Furthermore, various large-scale structures have been discovered, such as theClowes–Campusano LQG, theSloan Great Wall,[13]U1.11, theHuge-LQG, theHercules–Corona Borealis Great Wall,[14] theGiant Arc,[15] and theLocal Hole[16] all of which indicate that homogeneity might be violated.
On scales comparable to the radius of the observable universe, we see systematic changes with distance from Earth. For instance, at greater distances, galaxies contain more young stars and are less clustered, andquasars appear more numerous. If the Copernican principle is assumed, then it follows that this is evidence for the evolution of the universe with time: this distant light has taken most of the age of the universe to reach Earth and shows 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.
Bondi andThomas Gold used the Copernican principle to argue for theperfect cosmological principle which maintains that the universe is also homogeneous in time, and is the basis for thesteady-state cosmology.[17] However, this strongly conflicts with the evidence for cosmological evolution mentioned earlier: the universe has progressed from extremely different conditions at theBig Bang, and will continue to progress toward extremely different conditions, particularly under the rising influence ofdark energy, apparently toward theBig Freeze orBig Rip.
Since the 1990s the term has been used (interchangeably with "the Copernicus method") forJ. Richard Gott'sBayesian-inference-based prediction of duration of ongoing events, a generalized version of theDoomsday argument.[clarification needed]
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 thecosmological principle, slightly more general than the Copernican principle, and many tests of these models can be considered tests of the Copernican principle.[18]
Before the term Copernican principle was even coined, past assumptions, such asgeocentrism,heliocentrism, andgalactocentrism, 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. TheCopernican 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-shapedMilky 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 theBig Bang theory and the whole of moderncosmology.
Recent and planned tests relevant to the cosmological and Copernican principles include:
The standard model of cosmology, theLambda-CDM model, assumes the Copernican principle and the more generalcosmological 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 isinhomogeneous cosmology, to model the observedaccelerating universe andcosmological constant. Instead of using the current accepted idea ofdark 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.[32] To match observations the Solar System would have to be very close to the centre of this void, immediately contradicting the Copernican principle.
While theBig Bang model in cosmology is sometimes said to derive from the Copernican principle in conjunction with redshift observations, the Big Bang model can still be assumed to be valid in absence of the Copernican principle, because thecosmic microwave background, primordial gas clouds, and thestructure,evolution, and distribution ofgalaxies all provide evidence, independent of the Copernican principle, in favor of the Big Bang. However, the key tenets of the Big Bang model, such as the expansion of the universe, become assumptions themselves akin to the Copernican principle, rather than derived from the Copernican principle and observations.
{{cite book}}:ISBN / Date incompatibility (help)
