Copernican principle

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 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.{{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 }}

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".{{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 }}{{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}} 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."Sagan, Carl, Cosmos (1980) p. 193

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."{{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}}

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.

In practice, astronomers observe that the universe has heterogeneous or non-uniform structures up to the scale of galactic superclusters, filaments and great voids. 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 parsecs.{{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}} However, recent evidence from galaxy clusters,{{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}}{{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}} quasars,{{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 }} and type Ia supernovae{{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}} 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,{{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}} U1.11, the Huge-LQG, the Hercules–Corona Borealis Great Wall,{{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 }} the Giant Arc,{{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}} and the Local HoleSergij 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. 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.{{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}} 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-based prediction of duration of ongoing events, a generalized version of the Doomsday argument.{{clarify|date=April 2017}}

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.{{Cite journal |last1=Clarkson |first1=C. |last2=Bassett |first2=B. |last3=Lu |first3=T. |title=A General Test of the Copernican Principle |doi=10.1103/PhysRevLett.101.011301 |journal=Physical Review Letters |volume=101 |issue=1 |page=011301 |year=2008 |pmid=18764099 |arxiv=0712.3457 |bibcode=2008PhRvL.101a1301C |s2cid=32735465 }}

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.

Recent and planned tests relevant to the cosmological and Copernican principles include:

• time drift of cosmological redshifts;{{Cite journal |last1=Uzan |first1=J. P. |last2=Clarkson |first2=C. |last3=Ellis |first3=G. |title=Time Drift of Cosmological Redshifts as a Test of the Copernican Principle |doi=10.1103/PhysRevLett.100.191303 |journal=Physical Review Letters |volume=100 |issue=19 |page=191303 |year=2008 |pmid=18518435 |arxiv=0801.0068 |bibcode=2008PhRvL.100s1303U |s2cid=31455609}}
• modelling the local gravitational potential using reflection of cosmic microwave background (CMB) photons;{{Cite journal |last1=Caldwell |first1=R. |last2=Stebbins |first2=A. |doi=10.1103/PhysRevLett.100.191302 |title=A Test of the Copernican Principle |journal=Physical Review Letters |volume=100 |issue=19 |page=191302 |year=2008 |pmid=18518434 |arxiv=0711.3459 |bibcode=2008PhRvL.100s1302C |s2cid=5468549}}
• the redshift dependence of the luminosity of supernovae;{{Cite journal |last1=Clifton |first1=T. |last2=Ferreira |first2=P. |last3=Land |first3=K. |title=Living in a Void: Testing the Copernican Principle with Distant Supernovae |doi=10.1103/PhysRevLett.101.131302 |journal=Physical Review Letters |volume=101 |issue=13 |year=2008 |pmid=18851434 |arxiv=0807.1443 |bibcode=2008PhRvL.101m1302C |page=131302 |s2cid=17421918}}
• the kinetic Sunyaev–Zeldovich effect in relation to dark energy;{{Cite journal |last1=Zhang |first1=P. |last2=Stebbins |first2=A. |doi=10.1098/rsta.2011.0294 |title=Confirmation of the Copernican principle through the anisotropic kinetic Sunyaev Zel'dovich effect |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=369 |issue=1957 |pages=5138–5145 |year=2011 |pmid=22084299 |bibcode=2011RSPTA.369.5138Z |doi-access=free }}
• cosmic neutrino background;{{Cite journal |last1=Jia |first1=J. |last2=Zhang |first2=H. |doi=10.1088/1475-7516/2008/12/002 |title=Can the Copernican principle be tested using the cosmic neutrino background? |journal=Journal of Cosmology and Astroparticle Physics |volume=2008 |issue=12 |page=002 |year=2008 |arxiv=0809.2597 |bibcode=2008JCAP...12..002J |s2cid=14320348}}
• the integrated Sachs–Wolfe effect;{{Cite journal |last1=Tomita |first1=K. |last2=Inoue |first2=K. |doi=10.1103/PhysRevD.79.103505 |title=Probing violation of the Copernican principle via the integrated Sachs–Wolfe effect |journal=Physical Review D |volume=79 |issue=10 |page=103505 |year=2009 |arxiv=0903.1541 |bibcode=2009PhRvD..79j3505T |s2cid=118478786}}
• testing the isotropy and homogeneity of the CMB;{{Cite journal |last1=Clifton |first1=T. |last2=Clarkson |first2=C. |last3=Bull |first3=P. |title=Isotropic Blackbody Cosmic Microwave Background Radiation as Evidence for a Homogeneous Universe |doi=10.1103/PhysRevLett.109.051303 |journal=Physical Review Letters |volume=109 |issue=5 |year=2012 |pmid=23006164 |arxiv=1111.3794 |bibcode=2012PhRvL.109e1303C |page=051303 |s2cid=119278505}}{{Cite journal |last1=Kim |first1=J. |last2=Naselsky |first2=P. |doi=10.1088/0004-637X/739/2/79 |title=Lack of Angular Correlation and Odd-Parity Preference in Cosmic Microwave Background Data |journal=The Astrophysical Journal |volume=739 |issue=2 |page=79 |year=2011 |arxiv=1011.0377 |bibcode=2011ApJ...739...79K |s2cid=118580902}}{{Cite journal |last1=Copi |first1=C. J. |last2=Huterer |first2=D. |last3=Schwarz |first3=D. J. |last4=Starkman |first4=G. D. |title=Large-Angle Anomalies in the CMB |doi=10.1155/2010/847541 |journal=Advances in Astronomy |volume=2010 |pages=1–17 |year=2010 |arxiv=1004.5602 |bibcode=2010AdAst2010E..92C |s2cid=13823900 |doi-access=free}}{{cite journal |arxiv=1303.5083 |collaboration=Planck Collaboration |last=Ade |title=Planck 2013 results. XXIII. Isotropy and Statistics of the CMB |year=2013 |doi=10.1051/0004-6361/201321534 |volume=571 |journal=Astronomy & Astrophysics |page=A23 |bibcode=2014A&A...571A..23P |s2cid=13037411}}{{cite arXiv |eprint=astro-ph/0703325 |last=Longo |first=Michael |title=Does the Universe Have a Handedness? |year=2007}}
• observation of the KBC Void – some authors claim it violates the cosmological principle and thus the Copernican principle,{{Cite journal |last1=Haslbauer |first1=M. |last2=Banik |first2=I. |last3=Kroupa |first3=P. |date=2020-12-21 |title=The KBC void and Hubble tension contradict LCDM on a Gpc scale – Milgromian dynamics as a possible solution |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=2 |pages=2845–2883 |arxiv=2009.11292 |bibcode=2020MNRAS.499.2845H |doi=10.1093/mnras/staa2348 |issn=0035-8711 |doi-access=free}} while others claim that it is consistent with them.{{Cite journal |last1=Sahlén |first1=Martin |last2=Zubeldía |first2=Íñigo |last3=Silk |first3=Joseph |date=2016 |title=Cluster–Void Degeneracy Breaking: Dark Energy, Planck, and the Largest Cluster and Void |journal=The Astrophysical Journal Letters |volume=820 |issue=1 |pages=L7 |doi=10.3847/2041-8205/820/1/L7 |issn=2041-8205 |arxiv=1511.04075 |bibcode=2016ApJ...820L...7S |s2cid=119286482 |doi-access=free }}

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 we are in an extremely large low-density void.{{Cite journal | last1 = February | first1 = S. | last2 = Larena | first2 = J. | last3 = Smith | first3 = M. | last4 = Clarkson | first4 = C. | title = Rendering dark energy void | doi = 10.1111/j.1365-2966.2010.16627.x | journal = Monthly Notices of the Royal Astronomical Society | volume = 405 | issue = 4 | page = 2231 | year = 2010 | doi-access = free |arxiv = 0909.1479 |bibcode = 2010MNRAS.405.2231F | s2cid = 118518082 }} To match observations we would have to be very close to the centre of this void, immediately contradicting the Copernican principle.

While the Big 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 the cosmic microwave background, primordial gas clouds, and the structure, evolution, and distribution of galaxies 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.

{{div col|colwidth=20em}}

• Absolute time and space
• Anthropic principle
• Axis of evil (cosmology)
• Hubble Bubble (astronomy)
• Mediocrity principle
• Particle chauvinism
• P symmetry
• Rare Earth hypothesis
• The Principle (2014 film)
• Cosmological principle{{div col end}}

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