The Importance of Cosmological Principles for Research in Cosmology
| Scientific Paper | |
|---|---|
| Title | The Importance of Cosmological Principles for Research in Cosmology |
| Read in full | Link to paper |
| Author(s) | Konrad Rudnicki |
| Keywords | Cosmology, Cosmological Principle |
| Published | 1989 |
| Journal | Apeiron |
| Volume | 1 |
| Number | 4 |
| No. of pages | 13 |
| Pages | 1-7 |
Read the full paper here
Abstract
Any cosmological consideration must be based on one or more cosmological principles. Six of them are described namely: The Ancient Hindu, the Ancient Greek, the Genuine Copernican, the Generalized Copernican, the Perfect and the Anthropic Ones. Some cosmological principles have common properties, some are contradictory; some are logically independent, some dependent. Cosmological principles influence not only interpretation of results but they influence the results of cosmology as such. A consciousness of problems connected with cosmological principles is necessary when getting in touch with cosmology.
Overview
Konrad Rudnicki wrote this paper in Apeiron No. 4 (February 1989) while on leave from the Jagiellonian University Observatory at Rice University. It is not a technical cosmology paper but a methodological audit, and its target is an assumption so deeply embedded in modern cosmology that most working astronomers no longer notice they are making it. Rudnicki's opening image is precise and damning: "The cosmological principle intruded itself to the scene of modern science through the back door and remained there many years anonymously and even imperceptibly." What entered as a mathematical convenience — a simplification adopted by Einstein, Friedmann and others to make the field equations tractable — was later reinterpreted as a substantive statement about the world, and is now routinely treated as though it were an observational finding.
Rudnicki's central claim is that a cosmological principle is unavoidable, is not a law of nature, and is not observationally testable — and that all three of these facts must be held simultaneously. He surveys six such principles, three historical and three modern, showing that they differ in what they permit (a centre, finiteness, an evolving universe) and in what they force (the Hubble law, the Big Bang, the steady state). The point of the survey is comparative: once one sees that the ancient Hindu, ancient Greek and Copernican principles are structurally the same kind of assumption as the one modern cosmology uses, the modern one loses its air of neutrality. It becomes visible as a choice, made by people, in a particular epoch, reflecting that epoch's philosophical temper.
The argument
Rudnicki first separates cosmology from astronomy. Astronomy observes what is observable and explains it in terms of physics and chemistry; "cosmology differs from astronomy in that it wants to give scientific statements about the Universe as a whole." If every region and epoch were accessible to observation, cosmology would reduce to astronomy and no principle would be needed. But if — as the prevailing view holds — parts of the universe lie beyond a cosmological horizon or behind evolutionary epochs "impenetrable neither for observations nor for reliable physical theories based on actual experiments", then the map has blanks. Cosmological principles, in his formulation, "are just the general assumptions serving for filling up blanks in the general 'map' of the spacetime." A principle that reaches into an inaccessible epoch can then be used in turn to license laws of physics that cannot be tested in any laboratory — a chain of inference Rudnicki wants readers to see whole.
The six principles
- The Ancient Hindu Principle (reconstructed by Rudnicki, 1982): the universe is infinitely heterogeneous; the Earth is neither exceptional nor typical. Because no mathematics yet exists that can describe infinite heterogeneity, no definite model can be calculated from it — "it is impossible to obtain any mean, any average value out of infinitely dispersed parameters."
- The Ancient Greek Principle (Heller and Rudnicki, 1973): the Earth is the natural centre or quasi-centre of the universe and is unique in space and time. Many models followed from it: Hipparchus, Ptolemy, Tycho Brahe.
- The Genuine Copernican Principle: the universe looks roughly the same observed from every planet. This underlay Copernicus' own model and Tycho's.
- The Generalized Copernican Principle (also Ordinary, Narrow or Weak): the universe looks roughly the same from every point in every direction — homogeneous and isotropic. Rudnicki stresses that despite the labels "weak" and "narrow" this is "a powerful cosmological principle carrying binding consequences", because it "literally dictates how to fill the picture of the Universe in all its blanks ... just by duplicating the picture which is seen in the observable areas."
- The Perfect Cosmological Principle (Bondi and Gold, 1948): the same from every point, every direction and every time. Its only known solution consistent with observation in the observable region is the steady state model, requiring continuous creation of matter.
- The Anthropic Principle (e.g. Carr, 1982): there exists an intelligent being striving after knowledge, and from this the physical laws and constants can be deduced within narrow intervals.
The Hubble law as a consequence of an assumption
The sharpest technical point in the paper concerns provenance. The Generalized Copernican Principle, Rudnicki argues, produces the linear relation Vr = Hr between separation and recession velocity — and it does so independently of any theory of gravitation. He cites Bondi (1952, ch. IX) for the derivation and Robertson (1935, 1936) and Walker (1936) as cosmologies reaching it without gravitational input. Conversely, dropping the principle allows relativistic models in which the Hubble law does not hold (Taub 1951). Historically, he notes, the result was so unexpected that many cosmologists in the early twentieth century, "including Einstein himself, took at first the Hubble Law as a consequence of a general relativity rather than of accepted cosmological principle" — and "even today there appear in bad popular literature statements like the Hubble Law follows theoretically from the general relativity." Since the Big Bang hypothesis, with its exotic near-singularity states, follows from the Hubble law, the whole edifice traces back to a filling-in convention.
Rudnicki explicitly declines to adjudicate whether the Hubble law is observationally confirmed, referring the reader instead to the disputants: Halton Arp (1987), Tifft and Cocke (1989), Vera Rubin (1986), Jaakkola (1984) "as well as in papers of their adversaries." His concern is logical genealogy, not the redshift controversy itself.
Comparison and dependence
Section 4 tabulates the relations. Three principles — Ancient Hindu, Generalized Copernican, Perfect — forbid any centre or privileged point. All six allow an infinite universe; only the Ancient Greek, Genuine Copernican, Generalized Copernican and Anthropic also allow a finite one. Some may be held jointly (Tycho Brahe combined Ancient Greek with Genuine Copernican); some are contradictory (Ancient Hindu and Generalized Copernican); some are dependent (the Perfect Principle cannot be accepted without the content of the Generalized Copernican). The consequences differ in strength too: the Perfect Principle yields a single model, the Generalized Copernican a whole class (H may be any function of time, positive, negative or zero), and the Ancient Hindu and Anthropic yield none at all without further assumptions. Rudnicki observes that Anthropic-Principle cosmologists in practice smuggle in the Generalized Copernican Principle to get a model.
"No field of human knowledge can be practiced automatically without adding conscious human decision and interpretation," he writes, and cosmology depends on human outlook more intrinsically than any other science. The extragalactic astronomer therefore faces a genuine fork. Restrict oneself to well-defined nearby spacetime — in which case, "if the decision has to be honest", one must "get rid of all 'generally accepted' results of any cosmological investigations", and physicists must be warned that results about the Hubble law, singularities and primordial nucleosynthesis "are just products of pure human fantasy, not real science." Or adopt a principle consciously and with full awareness of its consequences. What one may not do is pretend the choice was not made. He notes that no attempt to test any cosmological principle observationally has yielded a decisive result either way (Bonnor and Ellis 1979; Stoeger, Ellis and Hellaby 1987), and that the principles track their epochs: the mechanistic age produced the Generalized Copernican and Perfect Principles, and the Anthropic Principle arrives as biological reductionism loses favour.
Assessment
The paper's strength is that its central thesis is essentially unassailable and remains under-acknowledged. Homogeneity and isotropy on the largest scales cannot be observed — we occupy one worldline and see one backward light cone — and every large-scale cosmological result is conditional on an assumption imported to bridge that gap. Rudnicki's genealogy of the Hubble law is correct and useful: the linear velocity-distance relation does follow from homogeneity plus expansion by a purely kinematic argument, with no gravitational theory required, and the popular claim that general relativity predicts it is a genuine confusion. His comparative method is also effective in a way a direct polemic would not be. Setting the modern principle beside the Hindu and Greek ones does not refute it; it simply strips it of the appearance of being a discovery, which is exactly the rhetorical work the paper needs to do. The insistence that the honest anti-cosmologist must abandon all large-scale results, not merely the ones he dislikes, is admirably even-handed.
The weaknesses are of scope and of nuance. The claim that no test of a cosmological principle has given a decisive result was defensible in 1989 but has aged: the survey era has supplied substantial, if not conclusive, evidence. The near-perfect isotropy of the cosmic microwave background at one part in 105, combined with the Copernican assumption, gives homogeneity; and the transition to statistical homogeneity at scales above roughly 100 megaparsecs is now a measured feature of galaxy redshift surveys rather than a postulate. Rudnicki would rightly reply that isotropy about us plus a Copernican assumption is still an assumption — the observational programme cannot escape the circle entirely — but the empirical situation is no longer the blank he describes. In the other direction, the Perfect Principle's steady state model is not merely "not a favorite one among cosmologists": it conflicts with the measured (1+z) stretching of Type Ia supernova light curves and with the observed evolution of the radio source population, both of which require that the universe looked different in the past. Rudnicki sets aside "whether this picture is a realistic one", which is a legitimate move for a methodological survey but leaves the reader with a misleadingly symmetric menu.
There is also an unremarked tension in the closing pages. Having argued that cosmological principles are products of human thinking rather than laws of nature, Rudnicki concludes that one "must trust to the process of thinking" and that "when we think of something we get some information as well, provided our thinking is reliable", referring the reader to Rudolf Steiner's epistemology alongside Zwicky's morphological method. Whatever one makes of that epistemology, invoking it converts a sceptical argument about the limits of observation into a positive claim about a non-observational source of knowledge — a much stronger position, and one the paper's own analysis does not establish. Readers persuaded by the first eleven pages need not follow him into the twelfth. Taken on its own terms, though, this is a lucid, honest and still-relevant piece of methodological hygiene, and its warning about assumptions that enter "through the back door" applies as much to current precision cosmology as to the cosmology of 1989.