[Paper Review] A Short History of the Missing Mass and Dark Energy Paradigms

Sidney van den Bergh|arXiv (Cornell University)|May 15, 2000

History and Developments in Astronomy1 references4 citations

TL;DR

This paper traces the historical evolution of cosmological paradigms concerning missing mass and dark energy from the early 20th century to 2000, highlighting the delayed recognition of Zwicky’s 1933 discovery of mass discrepancies in galaxy clusters and the subsequent development of cold dark matter and dark energy concepts. It establishes that by 2000, observations indicated dark energy constitutes ~66% of the universe’s mass-energy, cold dark matter ~30%, and baryons ~4%, with the remainder in hot gas, forming a flat universe consistent with Ωm + ΩΛ = 1.00 ± 0.12.

ABSTRACT

In 1900 it was believed that almost 100% of the mass of the Universe resided in stars. Now, in the year 2000, such stars (and cold gas) are known to account for only ~1% its mass. The remaining mass of the Universe is thought to reside in hot baryons (~3%), cold dark matter (~30%) and dark energy (~66%). The present paper traces the evolution of our thinking about the density of the Universe during the Twentieth Century, with special emphasis on the of the discovery of cold dark matter.

Motivation & Objective

• To document the historical development of the missing mass and dark energy paradigms in 20th-century cosmology.
• To explain the delayed impact of Zwicky’s 1933 discovery of mass discrepancies in galaxy clusters despite its significance.
• To analyze the emergence of cold dark matter and dark energy as central components of the modern cosmological model.
• To contextualize the shift from a baryon-dominated to a dark-energy-dominated universe, emphasizing observational constraints.
• To highlight the paradigm shift in which visible matter accounts for only ~1% of the universe’s mass, with the rest in invisible forms.

Proposed method

• Tracing the evolution of cosmological thinking through analysis of key observational papers and citations from 1900 to 2000.
• Using citation data (e.g., Table 1) to assess the delayed recognition of Zwicky’s 1933 paper on mass discrepancies in galaxy clusters.
• Applying the virial theorem to radial velocity dispersion data from Coma and Virgo clusters to infer mass-to-light ratios.
• Evaluating observational constraints from supernovae (Perlmutter et al. 1998), cosmic microwave background anisotropies (Melchiorri et al. 1999), and gravitational lensing (Mellier 1999) to infer dark energy.
• Using the equation Ωm + ΩΛ = 1.00 ± 0.12 (95% confidence) to assess the flatness of the universe and derive dark energy density.
• Comparing theoretical models, including the cosmological constant and quintessence, to explain the nature of dark energy.

Experimental results

Research questions

• RQ1 Why was Zwicky’s 1933 discovery of mass discrepancies in galaxy clusters largely ignored for decades despite its significance?
• RQ2 How did the concept of cold dark matter emerge and gain acceptance in the face of initial skepticism and competing explanations like cluster instability?
• RQ3 What observational evidence led to the recognition of dark energy as a dominant component of the universe’s mass-energy budget by 2000?
• RQ4 How do constraints from supernovae, cosmic microwave background anisotropies, and gravitational lensing collectively support a flat universe with Ωm + ΩΛ ≈ 1?
• RQ5 What is the significance of the near-equality of the fractional contributions of baryons (~4%), cold dark matter (~30%), and dark energy (~66%) in the context of cosmological naturalness?

Key findings

• Zwicky’s 1933 paper on mass discrepancies in the Coma cluster, which yielded a mass-to-light ratio of ~50 solar units, received only two citations before 1975, indicating widespread neglect of the discovery.
• Edwin Hubble acknowledged the mass discrepancy in the Virgo cluster in 1936, but the broader astronomical community did not recognize the implications until decades later.
• The virial theorem applied to cluster velocity dispersions revealed that visible matter alone could not account for the observed gravitational binding, leading to the inference of unseen mass.
• By 2000, observational constraints indicated that Ωm + ΩΛ = 1.00 ± 0.12 (95% confidence), supporting a flat universe with dark energy contributing ~66% of the mass-energy density.
• The balance of the universe’s mass-energy is now understood as ~0.01 (stars and cold gas), ~0.03 (hot gas), ~0.04 (total baryons), ~0.30 (cold dark matter), and ~0.66 (dark energy), with dark energy dominating.
• The paper concludes that the universe is dominated by invisible forms—dark energy and cold dark matter—while visible matter constitutes only ~1% of the total mass, a paradigm shift comparable in impact to the quantum revolution.