Difference between revisions of "Cosmological Principle"

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The Cosmological Principle and the Copernican Principle are philosophies which state that humans on the Earth or in the Solar System are not privileged observers of the universe. Due to this philosophy astronomical observations must consequently be interpreted under the context of current astronomical theories, regardless of what reality might otherwise suggest. Read the following quote carefully:

“ ‘For instance, if the universe were finite and had a centre, and if the Milky Way were not near that centre, then the total brightness of all the rest of the matter in the universe should be slightly greater on one side of the earth than the other. But the night sky beyond the Milky Way is not noticeably brighter in one direction than the other; therefore astronomers are forced to conclude that the universe extends indefinitely far in all directions. The only other possibility is that the earth is the centre of everything – one planet serving as a focal point for thousands and millions of galaxies and millions of millions of other probable planets. This alternative is so preposterous that cosmologists have ruled it out from the beginning. In fact, they have made it a fundamental tenet of their creed that the Milky Way’s position in the universe is not peculiar or untypical in any way. From this idea they derive a basic axiom known as the cosmological principle: namely, that the universe must be the same on average everywhere and in all directions. ”
—‘Space, Time and the Universe,’ The Universe, Time-Life Books, 1962, p.170.

Below are miscellaneous quotes from astronomers and authors remarking that we appear to be at the center of the universe.

Cosmological Principle

“ A fundamental presupposition of modern cosmology is the Copernican Principle, that we are not in a central, or otherwise special region of the Universe. Studies of Type Ia supernovae, together with the Copernican principle, have led to the inference that the Universe is accelerating in its expansion. The usual explanation for this is that there must exist a ‘Dark Energy,’ to drive the acceleration. Alternatively, it could be the case that the Copernican Principle is invalid, and that the data has been interpreted within an inappropriate theoretical framework. If we were to live in a special place in the Universe, near the centre of a void where the local matter density is low, then the supernovae observations would be accounted for without the addition of dark energy. ”
—Timothy Clifton, Oxford Astrophysics Member, BSc, PhD. (T. Clifton, et al, “Living in a Void: Testing the Copernican Principle with Distant Supernovae,” Phys. Rev. Lett. 101 (13): 1302 (Sep 2008).

“ Often the simplest of observations will have the most profound consequences. It has long been a cornerstone of modern science, to say nothing of man’s cosmic outlook, that the Earth attends a modest star that shines in an undistinguished part of a run-of-the-mill galaxy. Life arose spontaneously and man evolved on this miscellaneous clump of matter and now directs his own destiny without outside help. This cosmic model is supported by the Big-Bang and Expanding Universe concepts, which in turn are buttressed by the simple observation that astronomers see redshifts wherever they look. These redshifts are due, of course, to matter flying away from us under the impetus of the Big Bang. But redshifts can also arise from the gravitational attraction of mass. If the Earth were at the center of the universe, the attraction of the surrounding mass of stars would also produce redshifts wherever we looked! The argument advanced by George Ellis in this article is more complex than this, but his basic thrust is to put man back into a favored position in the cosmos. His new theory seems quite consistent with our astronomical observations, even though it clashes with the thought that we are godless and making it on our own. ”
—Editor of Nature Magazine, Paul C. W. Davies.

Cosmological Redshift

The galaxies we see are redshifted wherever we look and in a way that doesn't really make sense. The implication is that we are the center of the universe. The cosmological redshift is known as Hubble’s law, and postulated that the known universe is expanding. Hypothetical mechanisms were put in place to change the observation and its implications from redshift of light due to velicity into the metric expansion of space and time, otherwise known as the Expanding Universe.

See this quote from Edwin Hubble:

“ Such a condition [the red shifts] would imply that we occupy a unique position in the universe, analogous, in a sense, to the ancient conception of a central earth. The hypothesis cannot be disproved but it is unwelcome and would be accepted only as a last resort in order to save the phenomena. Therefore, we disregard this possibility and consider the alternative, namely, a distribution which thins out with distance.

A thinning out would be readily explained in either of two ways. The first is space absorption. If the nebulae were seen through a tenuous haze, they would fade away faster than could be accounted for by distance and red-shifts alone, and the distribution, even if it were uniform, would appear to thin out. The second explanation is a super-system of nebulae, isolated in a larger world, with our own nebula somewhere near the centre. In this case the real distribution would thin out after all the proper corrections had been applied.

Both explanations seem plausible, but neither is permitted by the observations.

The apparent departures from uniformity in the World Picture are fully compensated by the minimum possible corrections for redshifts on any interpretation. No margin is left for a thinning out. The true distribution must either be uniform or increase outward, leaving the observer in a unique position.

But the unwelcome supposition of a favoured location must be avoided at all costs… Such a favoured position, of course, is intolerable… Therefore, in order to restore homogeneity, and to escape the horror of a unique position, the departures from uniformity, which are introduced by the recession factors, must be compensated by the second term representing effects of spatial curvature. There seems to be no other escape. ”
— E. Hubble, The Observational Approach to Cosmology, 1937, p.58

Stephen Hawking says:

“ ...all this evidence that the universe looks the same whichever direction we look in might seem to suggest there is something special about our place in the universe. In particular, it might seem that if we observe all other galaxies to be moving away from us, then we must be at the center of the universe."

There is, however, an alternate explanation: the universe might look the same in every direction as seen from any other galaxy, too. This, as we have seen, was Friedmann’s second assumption. We have no scientific evidence for, or against, this assumption. We believe it only on grounds of modesty: it would be most remarkable if the universe looked the same in every direction around us, but not around other points in the universe. ”
—Steven Hawking, A Brief History of Time, p. 44 (Bantam, 1988).

“ This assumption is made because it is believed to be unreasonable that we should be near the center of the Universe.” ”
—George F. R. Ellis, “Is the Universe Expanding?” General Relativity and Gravitation 9 (2): 92 (1978).

“ If the Earth were at the center of the universe, the attraction of the surrounding mass of stars would also produce redshifts wherever we looked! This theory seems quite consistent with our astronomical observations ”
—Paul Davies in Nature

“ A widespread idea in cosmology is that the universe is homogeneous and isotropic above a certain scale. This hypothesis, usually called the cosmological principle, is thought to be a generalization of the Copernican principle that “the Earth is not in a central, specially favored position”. The assumption is that any observer at any place at the same epoch would see essentially the same picture of the large scale distribution of galaxies in the universe.

However, according to a Fourier analysis by Hartnett & Hirano, the galaxy number count N from redshift z data (N–z relation) indicates that galaxies have preferred periodic redshift spacings.........A natural interpretation is that concentric spherical shells of higher galaxy number densities surround us, with their individual centers situated at our location. ”
—Professor Shigeo Hirano, "Observational tests for oscillating expansion rate of the Universe" Physical Review D, 2010.

“ It is shown that the cosmological interpretation of the red shift in the spectra of quasars leads to yet another paradoxical result: namely, that the Earth is the center of the Universe.

...The Earth is indeed the center of the Universe. The arrangement of quasars on certain spherical shells is only with respect to the Earth. These shells would disappear if viewed from another galaxy or quasar. This means that the cosmological principle will have to go. Also it implies that a coordinate system fixed to the Earth will be a preferred frame of reference in the Universe. Consequently, both the Special and General Theory of Relativity must be abandoned for cosmological purposes. ”
—Y. P. Varshni, “The Red Shift Hypothesis for Quasars: Is the Earth the Center of the Universe?” Astrophysics and Space Science 43 (1): 3 (1976).

“ If the redshifts are a Doppler shift...the observations as they stand lead to the anomaly of a closed universe, curiously small and dense, and, it may be added, suspiciously young. ”
—Edwin Hubble, Monthly Notices of the Royal Astronomical Society, 17, 506, 1937.

“ A 2.29 GHz VLBI all-sky survey of ultra-compact radio sources has formed the basis of a number of cosmological investigations, which examine the relationship between angular-size and redshift. Here I use a sample of 468 such sources with 0.5<z<=3.787... This is interpreted as meaning that the Universe is not spatially homogeneous on the largest scales, and is better represented at late times by a spherically symmetric model with a density enhancement at its centre. ”
—Ultra-compact radio sources and the isotropy and homogeneity of the Universe J. C. Jackson (Archive)

Cosmic Microwave Background

“ In the Friedman universe, one possible interpretation of the coordinates is that the whole space is on the surface of an expanding balloon and has no center… [But] in such a universe, there is no cosmic microwave background (CMB) dipole, even in the presence of a peculiar velocity. In other words, the observation of a CMB dipole excludes such an interpretation of the coordinates for the Friedman universe. ”
—Y. Tomozawa, “The CMB Dipole and Existence of a Center for Expansion of the Universe,” Michigan Center for Theoretical Physics, University of Michigan, p. 2 (2 Feb 2008).

“ Additionally, we must take seriously the idea that the acceleration apparently indicated by supernova data could be due to large scale inhomogeneity with no dark energy. Observational tests of the latter possibility are as important as pursuing the dark energy (exotic physics) option in a homogeneous universe.

Theoretical prejudices as to the universe’s geometry, and our place in it, must bow to such observational tests. Precisely because of the foundational nature of the Copernican Principle for standard cosmology, we need to fully check this foundation. And one must emphasize here that standard CMB anisotropy studies do not prove the Copernican principle: they assume it at the start. ”
—George Ellis, “Inhomogeneity Effects in Cosmology,” arXiv:1103.2335v1 (Mar 2011).

“ Studies of the cosmic background radiation have confirmed the isotropy of the radiation, or its complete uniformity in all directions. If the universe possesses a center, we must be very close to it…otherwise, excessive observable anisotropy in the radiation intensity would be produced, and we would detect more radiation from one direction than from the opposite direction. ”
—Joseph Silk of the University of California, "The Big Bang: The Creation and Evolution of the Universe", p. 53 (W. H. Freeman, 1980).

“ But when you look at CMB map, you also see that the structure that is observed, is in fact, in a weird way, correlated with the plane of the earth around the sun. Is this Copernicus coming back to haunt us? That's crazy. We're looking out at the whole universe. There's no way there should be a correlation of structure with our motion of the earth around the sun - the plane of the earth around the sun - the ecliptic. That would say we are truly the center of the universe. The new results are either telling us that all of science is wrong and we're the center of the universe, or maybe the data is simply incorrect ”
—A Talk With Lawrence M. Kraus, Edge Magazine, July 5, 2006

From the paper - Why is the Solar System Cosmically Aligned? (Archive):

“ Developing the multi- pole vectors allowed us to examine how the CMB’s large-scale features align with each other and the ecliptic — the plane of Earth’s orbit around the Sun. ”

Dark Energy

“ Although dark energy may seem a bit contrived to some, the Oxford theorists are proposing an even more outrageous alternative. They point out that it's possible that we simply live in a very special place in the universe - specifically, we're in a huge void where the density of matter is particularly low. The suggestion flies in the face of the Copernican Principle, which is one of the most useful and widely held tenets in physics.

Copernicus was among the first scientists to argue that we're not in a special place in the universe, and that any theory that suggests that we're special is most likely wrong. The principle led directly to the replacement of the Earth-centered concept of the solar system with the more elegant sun-centered model.

Dark energy may seem like a stretch, but it's consistent with the venerable Copernican Principle. The proposal that we live in a special place in the universe, on the other hand, is likely to shock many scientists. ”
—Dark Energy: Is It Merely An Illusion? ScienceDaily (Sep. 26, 2008) (Archive)

“ An alternative to admitting the existence of dark energy is to review the postulates that necessitate its introduction. In particular, it has been proposed that the SNe observations could be accounted for without dark energy if our local environment were emptier than the surrounding Universe, i.e., if we were to live in a void. This explanation for the apparent acceleration does not invoke any exotic substances, extra dimensions, or modifications to gravity – but it does require a rejection of the Copernican Principle. We would be required to live near the center of a spherically symmetric under-density, on a scale of the same order of magnitude as the observable Universe. Such a situation would have profound consequences for the interpretation of all cosmological observations, and would ultimately mean that we could not infer the properties of the Universe at large from what we observe locally.

Within the standard inflationary cosmological model the probability of large, deep voids occurring is extremely small. However, it can be argued that the center of a large underdensity is the most likely place for observers to find themselves. In this case, finding ourselves in the center of a giant void would violate the Copernican principle, that we are not in a special place… ”
—“Living in a Void: Testing the Copernican Principle with Distant Supernovae,” Physical Review Letters, 101, 131302 (2008) DOI: 10.1103/PhysRevLett.101.131302.with citation from A. D. Linde, D. A. Linde and A. Mezhlumian in Physical Letters B345, 203 (1995) and S. Alexander, T. Biswas and A. Notari at [arXiv:0712.0370]; and H. Alnes, M. Amarzguioui and Ø. Grøn in Physical Review D73, 083519 (2006); and J. Garcia‐Dellido & T. Jaugboelle in Journal of Cosmology and Astroparticle Physics 04, 003 (2008).

“ Dark energy is an illusion if their equations are right, and the universe, at least 27.2 billion light years across, is spreading at an increasing rate into an even bigger vacuum empty of any matter, propelled by the energy of the Big Bang. The only problem is that for the equations to work, we must be "literally at the center of the universe, which is, to say the least, unusual," says physicist Lawrence Krauss of Arizona State University in Tempe. ”
—"Mystery solved: Dark energy isn't there", USA Today, August 17, 2009 (Archive)

“ Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible. ”
—Report of the Dark Energy Task Force, 2006 (Source)

Gamma Ray Bursts

“ The uniform distribution of burst arrival directions tells us that the distribution of gamma-ray-burst sources in space is a sphere or spherical shell, with us at the center (some other extremely contrived and implausible distributions are also possible). But Copernicus taught us that we are not in a special preferred position in the universe; Earth is not at the center of the solar system, the Sun is not at the center of the galaxy, and so forth. There is no reason to believe we are at the center of the distribution of gamma-ray bursts. If our instruments are sensitive enough to detect bursts at the edge of the spatial distribution, then they should not be isotropic on the sky, contrary to observation; if our instruments are less sensitive, then the N ∝ S-3/2 law should hold, also contrary to observation. That is the Copernican dilemma. To this day, after the detection of several thousand bursts, and despite earnest efforts to show the contrary, no deviation from a uniform random distribution (isotropy) in the directions of gamma-ray bursts on the sky has ever been convincingly demonstrated. ”
—Jonathan I. Katz, The Biggest Bangs: The Mystery of Gamma-Ray Bursts, The Most Violent Explosions in the Universe, pp. 84, 90-91 (Oxford University Press, 2002).

Probability

“ "Perhaps there is a large scale inhomogeneity of the observable universe such as that described by the Lemaıtre-Tolman-Bondi (LTB) pressure-free spherically symmetric models, and we are near the centre of a void. The idea that such models can explain the supernova observations without any dark energy is discussed by Celerier at this meeting (and see also Celerier [7]).....This freedom enables us to fit the supernova observations with no dark energy or other exotic physics"

"A typical observationally viable model is one in which we live roughly centrally (within 10% of the central position) in a large void.....and no dark energy or quintessence field"

"Many dismiss these models on probability grounds: It is improbable we are near the centre of such a model. But there is always improbability in cosmology......In essence, there simply is no proof the universe is probable; that is a philosophical assumption, which may not be true. The universe may be improbable!! Secondly, there is no well-justified measure for any such probability proposal even if we ignore the first problem. This is still an issue of debate....And thirdly, a study by Linde et al [22] shows that (given a particular choice of measure) this kind of inhomogeneity actually is a probable outcome of inflationary theory, with ourselves being located near the centre! One cannot dismiss such models out of hand for probability reasons. ”
—George F R Ellis, "Dark matter and dark energy proposals:maintaining cosmology as a true science?" November 21, 2008 (Archive)

Copernican Principle

“ However we are not able to make cosmological models without some admixture of ideology. In the earliest cosmologles, man placed himself in a commanding position at the centre of the universe. Since the time of Copernicus we have been steadily demoted to a medium sized planet going round a medium sized star on the outer edge of a fairly average galaxy, which is itself simply one of a local group of galaxies. Indeed we are now so democratic that we would not claim that our position in space is specially distinguished in any way. We shall, following Bondi (1960), call this assumption the Copernican principle.

A reasonable interpretation of this somewhat vague principle is to understand it as implying that, when viewed on a suitable scale, the universe is approximately spatially homogeneous. ”
—"The Large-Scale Structure of Space-Time," Stephen Hawking, p.134

“ To date, there has been no general way of determining [that] we live at a typical position in the Universe ”
—Chris Clarkson et al. in Physical Review Letters in 2008

“ People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that. ”
—George F. R. Ellis, Scientific American, October 1995, Vol. 273, No.4, p. 55.

“ The Copernican principle states that humans are not privileged observers of the universe and provides our philosophical basis for assuming that on the largest scales the universe is spatially homogeneous. While it is one of the foundational aspects of modern cosmology, this assumption remains untested outside of the standard paradigm. Though it may seem pedantic to test something so obvious, the standard paradigm itself is built on shaky foundations, relying on an unexplained, gravitationally repulsive, dark-energy component for observations to fit the model. The implications of this cannot be overstated. Assuming that the laws of physics do apply equally everywhere in the universe, the only non- copernican configuration possible is one in which we live in a place that originates from special initial conditions. ”
—"Testing the Copernican principle by constraining spatial homogeneity" : Wessel Valkenburg, Valerio Marra, Chris Clarkson, Instituut-Lorentz for Theoretical Physics, Universiteit Leiden (Archive)

“ Astronomers will find it hard to settle that troubling sensation in the pit of their stomachs. The truth is that when it comes to swallowing uncomfortable ideas, dark energy may turn out to be a sugar-coated doughnut compared to a rejection of the Copernican principle. ”
—"Dark Energy and the Bitterest Pill,” July 14, 2008 at the Discover Magazine associated Physics arXiv blog.

“ Dark Energy is problematic. No one really knows what it is. We can make an educated guess, and use quantum theory to estimate how much of it there might be, but then we overshoot by an astounding factor of 10¹²⁰. That is grounds enough, says George Ellis…to take a hard look at our assumptions about the universe and our place in it. “If we analyse the supernova data by assuming the Copernican principle is correct and get out something unphysical, I think we should start questioning the Copernican principle…. Whatever our theoretical predilections, they will in the end have to give way to the observational evidence.”

So what would it mean if…the outcome were that the Copernican principle is wrong? It would certainly require a seismic reassessment of what we know about the universe….If the Copernican Principle fails, all that goes [with] that goes out the window too….Cosmology would be back at the drawing board. If we are in a void, answering how we came to be in such a privileged spot in the universe would be even trickier. ”
—Marcus Chown, "Is the Earth at the Heart of a Giant Cosmic Void?," New Scientist, Nov. 12, 2008, pp. 32‐35.

@@ Line 13: / Line 13: @@
 ==Cosmological Redshift==
-The galaxies we see are redshifted wherever we look, in a way that doesn't really make sense, and the implication is that we are the center of the universe. The cosmological redshift is known as Hubble’s law, and postulated that the known universe is expanding. Hypothetical mechanisms were put in place to change the observation and its implications from redshift and blueshit of light into the metric expansion of space and time, otherwise known as the Expanding Universe.
+The galaxies we see are redshifted wherever we look and in a way that doesn't really make sense. The implication is that we are the center of the universe. The cosmological redshift is known as Hubble’s law, and postulated that the known universe is expanding. Hypothetical mechanisms were put in place to change the observation and its implications from redshift of light due to velicity into the metric expansion of space and time, otherwise known as the Expanding Universe.
 See this quote from Edwin Hubble: