Editing Copernican principle (section)

== 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 the [[Geocentric model|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 heliocentrism|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]].<ref>{{cite book |last=Kuhn |first=Thomas S. |author-link=Thomas Kuhn |title=The Copernican Revolution: Planetary Astronomy in the Development of Western Thought |url=https://archive.org/details/copernicanrevolu0008kuhn |url-access=registration |publisher=[[Harvard University Press]] |year=1957 |isbn=978-0-674-17103-9 |bibcode=1957crpa.book.....K }}</ref>

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".<ref>{{cite journal |doi=10.1038/scientificamerican0301-24a |journal=Scientific American |year=2001 |volume=284 |issue=3 |page=24 |title=Copernican Counterrevolution |author-link=George Musser |last=Musser |first=George |url=http://www.scientificamerican.com/article.cfm?id=in-brief-2001-03 |bibcode=2001SciAm.284c..24M |url-access=subscription }}</ref><ref>{{cite journal |doi=10.1511/2009.76.50 |journal=American Scientist |year=2009 |volume=97 |issue=1 |pages=50–57 |title=The Bones of Copernicus |first=Dennis |last=Danielson}}</ref> 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."<ref>Sagan, Carl, ''Cosmos'' (1980) p. 193</ref>

While the Copernican principle is derived from the negation of past assumptions, such as [[geocentrism]], [[heliocentrism]], or [[galactocentrism]] which state that humans are at the center of the universe, the Copernican principle is stronger than ''acentrism'', 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."<ref name="RowanRobinson1996">{{cite book |last=Rowan-Robinson |first=Michael |author-link=Michael Rowan-Robinson |title=Cosmology |edition=3rd |publisher=[[Oxford University Press]] |pages=62–63  |year=1996 |isbn=978-0-19-851884-6}}</ref>

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 [[Isotropy|isotropic]] or the same in all directions from the vantage point of Earth, then one can infer that the universe is generally [[Homogeneity (physics)|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]].<ref name="RowanRobinson1996" />

In practice, astronomers observe that the universe has [[Homogeneity and heterogeneity|heterogeneous]] or non-uniform structures up to the scale of [[galactic supercluster]]s, [[Galaxy filament|filaments]] and [[Void (astronomy)|great void]]s. In the current [[Lambda-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 million [[parsec]]s.<ref name="Yadav">{{cite journal |last1=Yadav |first1=Jaswant |last2=Bagla |first2=J. S. |last3=Khandai |first3=Nishikanta |date=25 February 2010 |title=Fractal dimension as a measure of the scale of homogeneity |journal=Monthly Notices of the Royal Astronomical Society |volume=405 |issue=3 |pages=2009–2015 |arxiv=1001.0617 |bibcode=2010MNRAS.405.2009Y |doi=10.1111/j.1365-2966.2010.16612.x |doi-access=free |s2cid=118603499}}</ref> However, recent evidence from [[galaxy cluster]]s,<ref name="Billings">{{cite web |author=Billings |first=Lee |date=April 15, 2020 |title=Do We Live in a Lopsided Universe? |url=https://www.scientificamerican.com/article/do-we-live-in-a-lopsided-universe1/ |access-date=March 24, 2022 |website=[[Scientific American]]}}</ref><ref name="Migkas et al">{{cite journal |url=https://www.aanda.org/articles/aa/full_html/2020/04/aa36602-19/aa36602-19.html |title=Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX-T scaling relation |last1=Migkas |first1=K. |last2=Schellenberger |first2=G. |last3=Reiprich |first3=T. H. |last4=Pacaud |first4=F. |last5=Ramos-Ceja |first5=M. E. |last6=Lovisari |first6=L. |journal=Astronomy & Astrophysics |volume=636 |issue=April 2020 |page=42 |doi=10.1051/0004-6361/201936602 |date=8 April 2020 |arxiv=2004.03305 |bibcode=2020A&A...636A..15M |s2cid=215238834 |access-date=24 March 2022}}</ref> [[quasar]]s,<ref>{{cite journal |last1=Secrest |first1=Nathan J. |last2=von Hausegger |first2=Sebastian |last3=Rameez |first3=Mohamed |last4=Mohayaee |first4=Roya |last5=Sarkar |first5=Subir |last6=Colin |first6=Jacques |date=February 25, 2021 |title=A Test of the Cosmological Principle with Quasars |journal=The Astrophysical Journal Letters |volume=908 |issue=2 |pages=L51 |arxiv=2009.14826 |bibcode=2021ApJ...908L..51S |doi=10.3847/2041-8213/abdd40 |s2cid=222066749 |doi-access=free }}</ref> and [[type Ia supernova]]e<ref>{{cite journal |last1=Javanmardi |first1=B. |last2=Porciani |first2=C. |last3=Kroupa |first3=P. |last4=Pflamm-Altenburg |first4=J. |date=August 27, 2015 |title=Probing the Isotropy of Cosmic Acceleration Traced By Type Ia Supernovae |url=https://iopscience.iop.org/article/10.1088/0004-637X/810/1/47 |journal=The Astrophysical Journal Letters |volume=810 |issue=1 |page=47 |arxiv=1507.07560 |bibcode=2015ApJ...810...47J |doi=10.1088/0004-637X/810/1/47 |s2cid=54958680 |access-date=March 24, 2022}}</ref> suggests that isotropy is violated on large scales. Furthermore, various large-scale structures have been discovered, such as the [[Clowes–Campusano LQG]], the [[Sloan Great Wall]],<ref name=apj624_2_463>{{Cite journal |display-authors=1 |last1=Gott |first1=J. Richard III |last2=Jurić |first2=Mario |last3=Schlegel |first3=David |last4=Hoyle |first4=Fiona |last5=Vogeley |first5=Michael |last6=Tegmark |first6=Max |last7=Bahcall |first7=Neta |last8=Brinkmann |first8=Jon |title=A Map of the Universe |journal=The Astrophysical Journal |volume=624 |issue=2 |pages=463–484 |date=May 2005 |doi=10.1086/428890 |bibcode=2005ApJ...624..463G |arxiv=astro-ph/0310571 |s2cid=9654355}}</ref> [[U1.11]], the [[Huge-LQG]], the [[Hercules–Corona Borealis Great Wall]],<ref>{{cite arXiv |eprint=1311.1104 |last1=Horvath |first1=I. |title=The largest structure of the Universe, defined by Gamma-Ray Bursts |last2=Hakkila |first2=J. |last3=Bagoly |first3=Z. |year=2013|class=astro-ph.CO }}</ref> the [[Giant Arc]],<ref>{{Cite web |url=https://www.newscientist.com/article/2280076-line-of-galaxies-is-so-big-it-breaks-our-understanding-of-the-universe/ |title=Line of galaxies is so big it breaks our understanding of the universe}}</ref> and the [[Local Hole]]<ref>Sergij Mazurenko et al., “A Simultaneous Solution to the Hubble Tension and Observed Bulk Flow within 250 H−1 Mpc,” Monthly Notices of the Royal Astronomical Society 527, no. 3 (January 21, 2024): 4388–96, https://doi.org/10.1093/mnras/stad3357; Moritz Haslbauer, Indranil Banik, and Pavel Kroupa, “The KBC Void and Hubble Tension Contradict ΛCDM on a Gpc Scale − Milgromian Dynamics as a Possible Solution,” Monthly Notices of the Royal Astronomical Society 499, no. 2 (October 28, 2020): 2845–83, https://doi.org/10.1093/mnras/staa2348.</ref> 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, and [[quasars]] 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 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]].<ref name="BondiGold1948">{{cite journal |last=Bondi |first=H. |last2=Gold |first2=T. |title=The Steady-State Theory of the Expanding Universe |journal=[[Monthly Notices of the Royal Astronomical Society]] |year=1948 |volume=108 |issue=3 |pages=252–270 |bibcode=1948MNRAS.108..252B |doi=10.1093/mnras/108.3.252 |doi-access=free}}</ref> 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]].

Since the 1990s the term has been used (interchangeably with "the Copernicus method") for [[J. Richard Gott]]'s [[Bayesian inference|Bayesian-inference]]-based prediction of duration of ongoing events, a generalized version of the [[Doomsday argument]].{{clarify|date=April 2017}}