In modernphysical cosmology, thecosmological principle is the notion that the spatial distribution of matter in the universe is uniformlyisotropic andhomogeneous when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course of evolution of the matter field that was initially laid down by theBig Bang.
The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.[1]
AsAndrew Liddle puts it, "the cosmological principle [means that] the universe looks the same whoever and wherever you are."[2]
The two testable structural consequences of the cosmological principle arehomogeneity andisotropy. Homogeneity – constant density– means that the same observational evidence is available to observers at different locations in the universe. Isotropy – looking the same in all directions – means that the same observational evidence is available by looking in any direction in the universe. Isotropy implies homogeneity, but an homogeneous universe could be anisotropic.[3]: 65
The cosmological principle is first clearly asserted in thePhilosophiæ Naturalis Principia Mathematica (1687) ofIsaac Newton.[dubious –discuss] In contrast to some earlier classical or medieval cosmologies, in which Earth rested at the center of universe, Newton conceptualized the Earth as a sphere in orbital motion around the Sun within an empty space that extended uniformly in all directions to immeasurably large distances. He then showed, through a series of mathematical proofs on detailed observational data of the motions of planets and comets, that their motions could be explained by a single principle of "universal gravitation" that applied as well to the orbits of theGalilean moons around Jupiter, the Moon around the Earth, the Earth around the Sun, and to falling bodies on Earth. That is, he asserted the equivalent material nature of all bodies within theSolar System, the identical nature of the Sun and distant stars, and thus the uniform extension of the physical laws of motion to a great distance beyond the observational location of Earth itself.
Since the 1990s, observations assuming the cosmological principle have concluded that around 68% of the mass–energy density of the universe can be attributed todark energy, which led to the development of theΛCDM model.[4][5][6]
Observations show that more distant galaxies are closer together and have lower content of chemical elements heavier than lithium.[citation needed] Applying the cosmological principle, this suggests that heavier elements were not created in the Big Bang but were produced bynucleosynthesis in giant stars and expelled via a series ofsupernovae and newstar formation from the supernova remnants, which means heavier elements would accumulate over time. Another observation is that the farthest galaxies (earlier time) are often more fragmentary, interacting and unusually shaped than local galaxies (recent time), suggesting evolution in galaxy structure as well.
A related implication of the cosmological principle is that the largest discrete structures in the universe should be inmechanical equilibrium. Homogeneity and isotropy of matter at the largest scales would suggest that the largest discrete structures are parts of a single non-discrete form, like the crumbs which make up the interior of a cake. At extreme cosmological distances, the property of mechanical equilibrium in surfaces lateral to the line of sight can be empirically tested; however, under the assumption of the cosmological principle, it cannot be detected parallel to the line of sight (seetimeline of the universe).
Cosmologists agree that in accordance with observations of distant galaxies, a universe must be non-static if it follows the cosmological principle. In 1923,Alexander Friedmann set out a variant ofAlbert Einstein's equations ofgeneral relativity that describe the dynamics of a homogeneous isotropic universe.[7][8] Independently,Georges Lemaître derived in 1927 the equations of an expanding universe from the General Relativity equations.[9] Thus, a non-static universe is also implied, independent of observations of distant galaxies, as a result of applying the cosmological principle togeneral relativity.
Karl Popper criticized the cosmological principle on the grounds that it makes "ourlack of knowledge a principle ofknowing something". He summarized his position as:
"The 'cosmological principles' were, I fear, dogmas that should not have been proposed."[10]
Although the universe is inhomogeneous at smaller scales, according to theΛCDM model it ought to be isotropic and statistically homogeneous on scales larger than 250 million light years. However, recent findings (theAxis of Evil for example) have suggested that violations of the cosmological principle exist in the universe and thus have called the ΛCDM model into question, with some authors suggesting that the cosmological principle is now obsolete and theFriedmann–Lemaître–Robertson–Walker metric breaks down in the late universe.[11]
Thecosmic microwave background (CMB) is predicted by the ΛCDM model to be isotropic, that is to say that its intensity is about the same whichever direction we look at.[14] Data from thePlanck Mission shows hemispheric bias in two respects: one with respect to average temperature (i.e. temperature fluctuations), the second with respect to larger variations in the degree of perturbations (i.e. densities).[15][16] The collaboration noted that these features are not strongly statistically inconsistent with isotropy.[17] Some authors say that the universe around Earth is isotropic at high significance by studies of thecosmic microwave background temperature maps.[18] There are however claims of isotropy violations fromgalaxy clusters,[12][13]quasars,[19] andtype Ia supernovae.[20]
The cosmological principle implies that at a sufficiently large scale, the universe ishomogeneous. Based onN-body simulations in a ΛCDM universe, Jaswant Yadav and his colleagues showed that the spatial distribution of galaxies is statistically homogeneous if averaged over scales of 260/h Mpc or more.[21]
A number of observations have been reported to be in conflict with predictions of maximal structure sizes:
TheClowes–Campusano LQG, discovered in 1991, has a length of 580 Mpc, and is marginally larger than the consistent scale.
TheSloan Great Wall, discovered in 2003, has a length of 423 Mpc,[22] which is just barely consistent with the cosmological principle.
U1.11, alarge quasar group discovered in 2011, has a length of 780 Mpc, two times larger than the upper limit of the homogeneity scale.
TheHuge-LQG, discovered in 2012, is three times longer and two times wider than is predicted to be possible by current models.
In November 2013, a new structure 10 billion light years away measuring 2000–3000 Mpc (more than seven times that of the Sloan Great Wall) was discovered, theHercules–Corona Borealis Great Wall, putting further doubt on the validity of the cosmological principle.[23]
In September 2020, a 4.9σ conflict was found between the kinematic explanation of the CMB dipole and the measurement of the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars.[24]
In June 2021, theGiant Arc was discovered, a structure spanning approximately 1000 Mpc.[25] It is located 2820 Mpc away and consists of galaxies, galactic clusters, gas, and dust.
In January 2024, theBig Ring was discovered. It is located 9.2 billion light years away from Earth and has a diameter of 1.3 billion light years, giving it an angular size of 15 full moons as seen from Earth.[26]
However, as pointed out by Seshadri Nadathur in 2013 using statistical properties,[27] the existence of structures larger than the homogeneous scale (260/h Mpc by Yadav's estimation)[21] does not necessarily violate the cosmological principle in the ΛCDM model (seeHuge-LQG § Dispute).[28]
Is the CMB dipole purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle?[11]
Thecosmic microwave background (CMB) provides a snapshot of a largely isotropic and homogeneous universe. The largest-scale feature of the CMB is thedipoleanisotropy; it is typically subtracted from maps due to its large amplitude. The standard interpretation of the dipole is that it iskinematic, due to theDoppler effect caused by the motion of the solar system with respect to the CMB rest-frame.
Several studies have reported dipoles in the large-scale distribution of galaxies that align with the CMB dipole direction but that indicate a larger amplitude than would be caused by the CMB dipole velocity.[29] A similar dipole is seen in data ofradio galaxies; however, the amplitude of the dipole depends on the observing frequency, showing that these anomalous features cannot be purely kinematic.[30] Other authors have found radio dipoles consistent with the CMB expectation.[31] Further claims of anisotropy along the CMB dipole axis have been made with respect to theHubble diagram ofType Ia supernovae[32] andquasars.[33] Separately, the CMB dipole direction has emerged as a preferred direction in some studies of alignments in quasar polarizations[34] andstrong lensing time delay,[35] and in Type Ia supernovae[36] and otherstandard candles.[37] Some authors have argued that the correlation of distant effects with the dipole direction may indicate that its origin is not kinematic.
Alternatively,Planck data has been used to estimate the velocity with respect to the CMB independently of the dipole, by measuring the subtle aberrations and distortions of fluctuations caused byrelativistic beaming[38] and separately using theSunyaev–Zeldovich effect.[39] These studies found a velocity consistent with the value obtained from the dipole, indicating that it is consistent with being entirely kinematic. Measurements of thevelocity field of galaxies in the local universe show that on short scales galaxies are moving with theLocal Group, and that the average mean velocity decreases with increasing distance.[40] This follows the expectation if the CMB dipole were due to the local peculiar velocity field, it becomes more homogeneous on large scales. Surveys of the local volume have been used to reveal a low-density region in the opposite direction to the CMB dipole,[41] potentially explaining the origin of the localbulk flow.
Theperfect cosmological principle is an extension of the cosmological principle, and states that the universe is homogeneous and isotropic in spaceand time. In this view the universe looks not only the same everywhere in space (on large scales), but also the same as it always has and always will. The perfect cosmological principle underpinssteady state theory and emerges[clarification needed] fromchaotic inflation theory.[42][43][44]
