[Paper Review] Is cosmology just a plausibility argument?

David W. Hogg|ArXiv.org|Oct 18, 2009

Cosmology and Gravitation Theories25 references3 citations

TL;DR

This paper argues that cosmology relies fundamentally on plausibility reasoning due to the impossibility of exhaustive hypothesis sets, yet maintains scientific rigor through pragmatic confidence in well-supported models. It defends Bayesian inference as the proper framework for uncertain reasoning, applies it to key cosmological debates—such as large extra dimensions and fractal structures—and concludes that while extra dimensions are plausible, a fractal universe is untenable.

ABSTRACT

I review the basis and limitations of plausible inference in cosmology, in particular the limitation that it can only provide fundamentally true inferences when the hypotheses under consideration form a set that is exhaustive. They never do; this recommends abandoning realism. Despite this, we can adopt a scientifically correct pragmatism and understand aspects of the cosmological model with enormous confidence. I illustrate these points with discussion of one certainty--expansion--and two current controversies--the existence of large extra dimensions and the possibility that the matter distribution forms a fractal on large scales. I argue that the existence of large extra dimensions is certainly plausible, but a fractal universe is untenable.

Motivation & Objective

• To examine the role of plausibility reasoning in cosmology, where controlled experiments are impossible and hypotheses are rarely exhaustive.
• To argue that despite the limitations of plausibility arguments, cosmology can still achieve high-confidence conclusions through pragmatic scientific reasoning.
• To evaluate the scientific status of two controversial cosmological ideas—large extra dimensions and a fractal matter distribution—using plausibility and Bayesian inference.
• To address the tension between radical theoretical exploration and scientific orthodoxy, particularly in the context of career incentives and decision-making in science.
• To advocate for a scientifically correct pragmatism that acknowledges uncertainty while maintaining confidence in well-supported models like cosmic expansion.

Proposed method

• Applies Bayesian inference as a formal framework for reasoning under uncertainty, where degrees of plausibility are represented by real numbers satisfying consistency and common-sense desiderata.
• Uses the multi-armed bandit analogy to model scientific decision-making, where researchers must choose between exploring radical ideas or refining established models under limited time and resources.
• Analyzes the structure of plausibility arguments in cosmology by comparing competing explanations for observed phenomena—e.g., galaxy clustering—when direct observational discrimination is impossible.
• Evaluates the viability of alternative gravity theories by assessing their effective cosmological predictions under observational constraints, even when such theories are not yet falsified.
• Draws on historical and contemporary examples (e.g., CMB fluctuations, high-redshift galaxies, weak lensing) to illustrate how plausibility evolves into confidence through accumulating evidence.
• Integrates insights from decision theory and utility functions to model how age and career stage affect risk tolerance in scientific inquiry, particularly in testing speculative models.

Experimental results

Research questions

• RQ1 Can scientific confidence in cosmology be justified when hypotheses are not exhaustive and inference is based on plausibility?
• RQ2 How should scientists balance exploration of radical models against refinement of established theories, given limited time and resources?
• RQ3 To what extent can plausibility arguments in cosmology lead to reliable conclusions, especially when direct observational discrimination between models is impossible?
• RQ4 Why do younger scientists tend to favor radical ideas more than older scientists, and how does this affect scientific progress?
• RQ5 What is the scientific status of two controversial ideas—large extra dimensions and a fractal distribution of matter—when judged through the lens of plausibility and Bayesian reasoning?

Key findings

• Plausibility reasoning is unavoidable in cosmology due to the impossibility of exhaustive hypothesis sets, but it can still yield scientifically reliable conclusions when grounded in consistent Bayesian inference.
• The existence of large extra dimensions is considered plausible, though not yet confirmed, because it remains consistent with current observational constraints and offers testable predictions.
• A fractal distribution of matter on large scales is deemed untenable because it contradicts the observed isotropy and homogeneity of the cosmic microwave background and large-scale structure data.
• Cosmic expansion is treated as a certain conclusion, supported by overwhelming and consistent observational evidence across multiple independent datasets.
• The multi-armed bandit framework explains why younger scientists are more inclined to explore radical ideas: they face higher risk tolerance and shorter time horizons, making them more likely to pursue high-impact, uncertain paths.
• Despite the dominance of the ΛCDM model, the scientific community has not abandoned speculative inquiry—many young researchers continue to explore ideas like multiverse collisions and modified gravity, indicating a partial recovery from Blandford’s critique of scientific stagnation.