우주론은 매우 심각하고 어려운 철학적 난점들을 안고 있습니다. 자연철학의 주제로서 더더욱 해결하기 어려운 문제들이 산재해 있습니다.

남아프리카공화국의 천체물리학자 조지 일리스(G.F.R. Ellis)의 논문이 매우 의미심장합니다.

George F. R. Ellis (2006)  Issues in the Philosophy of Cosmology. In: J Butterfield and J Earman (eds.) Handbook in Philosophy of Physics. Elsevier.
(https://doi.org/10.48550/arXiv.astro-ph/0602280)

일리스가 정리하고 있는 우주론 철학의 주요 쟁점들은 다음과 같이 분류되어 있습니다.

A: The uniqueness of the universe (우주의 유일성)
B: The large scale of the universe in space and time (공간과 시간 속의 우주의 거대규모)
C: The unbound energies in the early universe (초기 우주의 무한한 에너지)
D: Explaining the universe - the question of origins (우주를 설명하기: 기원에 대한 질문)
E: The universe as the background for existence (존재의 배경으로서의 우주)
F: The explicit philosophical basis (철학적 토대)
G: The Anthropic question: fine tuning for life (인간원리: 생명이 생겨나기 위한 정교한 조정)
H: The possible existence of multiverses (다중우주의 존재 가능성)
I: The natures of existence (존재의 본성)

더 세부적인 논제들을 모으면 다음과 같습니다. 우선 영어로 서술된 것을 그대로 가져오지만, 언젠가 틈이 날 때 한국어로 번역하여 올릴 날이 오기를 바라고 있습니다. 더 상세한 것은 위의 링크를 따라 가서 논문을 직접 살펴볼 수 있습니다.

A: The uniqueness of the universe (우주의 유일성)

Thesis A1: The universe itself cannot be subjected to physical experimentation. We cannot re-run the universe with the same or altered conditions to see what would happen if they were different, so we cannot carry out scientific experiments on the universe itself.

Thesis A2: The universe cannot be observationally compared with other universes. We cannot compare the universe with any similar object, nor can we test our hypotheses about it by observations determining statistical properties of a known class of physically existing universes.

Thesis A3: The concept of ‘Laws of Physics’ that apply to only one object is questionable. We cannot scientifically establish ‘laws of the universe’ that might apply to the class of all such objects, for we cannot test any such proposed law except in terms of being consistent with one object (the observed universe).

Thesis A4: The concept of probability is problematic in the context of existence of only one object. Problems arise in applying the idea of probability to cosmology as a whole — it is not clear that this makes much sense in the context of the existence of a single object which cannot be compared with any other existing object.

B: The large scale of the universe in space and time (공간과 시간 속의 우주의 거대규모)

Thesis B1: Astronomical observations are confined to the past null cone, fading with distance. We can effectively only observe the universe, considered on a cosmological scale, from one space-time event. Visual observations are possible only on our past light cone, so we are inevitably looking back into the past as we observe to greater distances. Uncertainty grows with distance and time.

Thesis B2: ‘Geological’ type observations can probe the distant past of our past world line. Physical and astrophysical observations tell us about conditions near matter world-lines in the far distant past. They can be used also to investigate the far distant past of more distant objects.

Thesis B3: Establishing a Robertson-Walker geometry for the universe relies on plausible philosophical assumptions. The deduction of spatial homogeneity follows not directly from astronomical data, but because we add to the observations a philosophical principle that is plausible but untestable.

Thesis B4: Interpreting cosmological observations depends on astrophysical understanding. Observational analysis depends on assessing a variety of auxiliary functions characterizing the sources observed and the observations made. These introduce further parameters that have to be observationally or theoretically determined, allowing considerable freedom in fitting specific models to the observations. Physical cosmology aims to characterize perturbed FL models (which
account for structure formation) rather than just the background exactly smooth FL models; this introduces further parameters to be determined.

Thesis B5: A crucial observational test for cosmology is that the age of the universe must be greater than the ages of stars. The tension between the age of the universe and ages of stars is one area where the standard models are vulnerable to being shown to be inconsistent, hence the vital need to establish reliable distance scales, basic to estimates of both H0 and the ages of stars, and good limits on Λ. Other consistency tests help confirm the standard model and consolidate cosmology’s standing as an empirical science.

Thesis B6: Observational horizons limit our ability to observationally determine the very large scale geometry of the universe. We can only see back to the time of decoupling of matter and radiation, and so have no direct information about earlier times; and unless we live in a ‘small universe’, most of the matter in the universe is hidden behind the visual horizon. Conjectures as to its geometry on larger scales cannot be observationally tested. The situation is completely different in the small universe case: then we can see everything there is in the universe, including our own galaxy at earlier times.

Thesis B7: We have made great progress towards observational completeness. We have already seen most of the part of the universe that is observable by electromagnetic radiation. It is plausible that not many new astronomical phenomena remain to be discovered by us observationally; we will determine more details (so understanding more about what we have seen) and see more objects, but not discover many new kinds of things.

C: The unbound energies in the early universe (초기 우주의 무한한 에너지)

Thesis C1: The Physics Horizon limits our knowledge of physics relevant to the very early universe. We cannot experimentally test much of the physics that is important in the very early universe because we cannot attain the required energies in accelerators on Earth. We have to extrapolate from known physics to the unknown and then test the implications; to do this, we assume some specific features of known lower energy physics are the true key to how things are at higher energies. We cannot experimentally test if we have got it right.

Thesis C2: The unknown nature of the inflaton means that inflationary universe proposals are incomplete. The promise of inflationary theory in terms of relating cosmology to particle physics has not been realized. This will only be the case when the nature of the inflaton has been pinned down to a specific field that experiment confirms or particle physics requires to exist.

D: Explaining the universe - the question of origins (우주를 설명하기: 기원에 대한 질문)

Thesis D1: An initial singularity may or may not have occurred. A start to the universe may have occurred a finite time ago, but a variety of alternatives are conceivable: eternal universes, or universes where time as we know it came into existence in one or another way. We do not know which actually happened, although quantum gravity ideas suggest a singularity might be avoided.

Thesis D2: Testable physics cannot explain the initial state and hence specific nature of the universe. A choice between different contingent possibilities has somehow occurred; the fundamental issue is what underlies this choice. Why does the universe have one specific form rather than another, when other forms consistent with physical laws seem perfectly possible? The reasons underlying the choice between different contingent possibilities for the universe (why one occurred rather than another) cannot be explored scientifically. It is an issue to be examined through philosophy or metaphysics.

Thesis D3: The initial state of the universe may have been special or general. Whether there was generality or speciality of geometrical initial conditions for the universe is a key question. It seems likely that the initial state of the observed part of the universe was not generic.

E: The universe as the background for existence (존재의 배경으로서의 우주)

Thesis E1: Physical laws may depend on the nature of the universe. We have an essential difficulty in distinguishing between laws of physics and boundary conditions in the cosmological context of the origin of the universe. Effective physical laws may depend on the boundary conditions of the universe, and may even vary in different spatial and/or temporal locations in the cosmos.

Thesis E2: We cannot take the nature of the laws of physics for granted. Cosmology is interested in investigating hypothetical universes where the laws of physics are different from those that obtain in the real universe in which we live — for this may help us understand why the laws of physics are as they are (a fundamental feature of the real physical universe).

Thesis E3: Physical novelty emerges in the expanding universe. New kinds of physical existence come into being in the universe as it evolves, that did not exist previously. Their existence is allowed by the boundary conditions provided by the universe for local systems, together with the possibility space generated by the underlying physics. While their physical existence is novel, every new thing that comes into being is foreshadowed in possibility structures that precede their existence.

F: The explicit philosophical basis (철학적 토대)

Thesis F1: Philosophical choices necessarily underly cosmological theory. Unavoidable metaphysical issues inevitably arise in both observational and physical cosmology. Philosophical choices are needed in order to shape the theory.

Thesis F2: Criteria of satisfactoriness for theories cannot be scientifically chosen or validated. Criteria of satisfactoriness are necessary for choosing good cosmological theories; these criteria have to be chosen on the basis of philosophical considerations. They should include criteria for satisfactory structure of the theory, intrinsic explanatory power, extrinsic explanatory power, and observational and experimental support.

Thesis F3: Conflicts will inevitably arise in applying criteria for satisfactory cosmological theories. Philosophical criteria for satisfactory cosmological theories will in general come into conflict with each other, so that one will have to choose between them to some degree; this choice will shape the resulting theory.

Thesis F4: The physical reason for believing in inflation is its explanatory power as regards structure growth in the universe. Inflation predicts the existence of Gaussian scale-free perturbations in the early universe thereby (given the presence of cold dark matter) explaining bottom-up structure formation in a satisfactory way. This theory has been vindicated spectacularly through observations of the CBR and matter power spectra. It is this explanatory power that makes it so acceptable to physicists, even though the underlying physics is neither well-defined nor tested, and its major large-scale observational predictions are untestable.

Thesis F5: Cosmological theory can have a wide or narrow scope of enquiry. The scope we envisage for our cosmological theory shapes the questions we seek to answer. The cosmological philosophical base becomes more or less dominant in shaping our theory according to the degree that we pursue a theory with more or less ambitious explanatory aims in terms of all of physics, geometry, and underlying fundamental causation.

Thesis F6: Reality is not fully reflected in either observations or theoretical models. Problems arise from confusion of epistemology (the theory of knowledge) with ontology (the nature of existence): existence is not always manifest clearly in the available evidence. The theories and models of reality we use as our basis for understanding are necessarily partial and incomplete reflections of the true nature of reality, helpful in many ways but also inevitably misleading in others. They should not be confused with reality itself!

G: The Anthropic question: fine tuning for life (인간원리: 생명이 생겨나기 위한 정교한 조정)

Thesis G1: Life is possible because both the laws of physics and the boundary conditions for the universe have a very special nature. Only particular laws of physics, and particular initial conditions in the Universe, allow the existence of intelligent life of the kind we know. No evolutionary process whatever is possible for any kind of life if these laws and conditions do not have this restricted form.

Thesis G2: Metaphysical uncertainty remains about ultimate causation in cosmology. We cannot attain certainty on the underlying metaphysical cosmological issues through either science or philosophy.

H: The possible existence of multiverses (다중우주의 존재 가능성)

Thesis H1: Multiverse proposals are unprovable by observation or experiment, but some self-consistency tests are possible. Direct observations cannot prove or disprove that a multiverse exists, for the necessary causal relations allowing observation or testing of their existence are absent. Their existence cannot be predicted from known physics, because the supposed causal or pre-causal processes are either unproven or indeed untestable. However some self-consistency conditions for specific multiverse models can be tested.

Thesis H2: Probability-based arguments cannot demonstrate the existence of multiverses. Probability arguments cannot be used to prove the existence of a multiverse, for they are only applicable if a multiverse exists. Furthermore probability arguments can never prove anything for certain, as it is not possible to violate any probability predictions, and this is a fortiori so when there is only one case to consider, so that no statistical observations are possible.

Thesis H3: Multiverses are a philosophical rather than scientific proposal. The idea of a multiverse provides a possible route for the explanation of fine tuning. But it is not uniquely defined, is not scientifically testable apart from some possible consistency tests, and in the end simply postpones the ultimate metaphysical questions.

Thesis H4: The underlying physics paradigm of cosmology could be extended to include biological insights. The dominant paradigm in cosmology is that of theoretical physics. It may be that it will attain deeper explanatory power by embracing biological insights, and specifically that of Darwinian evolution. The Smolin proposal for evolution of populations of expanding universe domains [Smolin, 1992] is an example of this kind of thinking.

I: The natures of existence (존재의 본성)

Thesis I1: We do not understand the dominant dynamical matter components of the universe at early or late times. A key goal for physical cosmology is determining the nature of the inflaton, of dark matter, and of dark energy. Until this is done, the causal understanding of cosmology is incomplete, and in particular the far future fate of the universe is unknown.

Thesis I2: The often claimed physical existence of infinities is questionable. The claimed existence of physically realized infinities in cosmology or multiverses raises problematic issues. One can suggest they are unphysical; in any case such claims are certainly unverifiable.

Thesis I3: A deep issue underlying the nature of cosmology is the nature of the laws of physics. The nature of the possibility space for physical existence is characterized by the laws of physics. However it is unclear if these laws are prescriptive or descriptive; whether they come into being with space-time and matter, or pre-exist them.

Thesis of Uncertainty: Ultimate uncertainty is a key aspect of cosmology. Scientific exploration can tell us much about the universe but not about its ultimate nature, or even much about some of its major geometrical and physical characteristics. Some of this uncertainty may be resolved, but much will remain. Cosmological theory should acknowledge this uncertainty.