Zwicky’s Dark Matter Hypothesis

Introduction

The concept of dark matter was first proposed by Swiss astrophysicist Fritz Zwicky in 1933. Observing the Coma galaxy cluster, Zwicky noted that the visible mass of the galaxies within the cluster was insufficient to account for the cluster’s observed dynamics. He suggested that much of the mass must be in some unseen form, which he termed “dunkle Materie,” or “dark matter.”

The Missing Mass Problem

Zwicky’s observation involved applying the virial theorem, a principle from mechanics, to calculate the mass of the Coma cluster. This theorem, in the context of a galaxy cluster, can be stated as:

where is the total kinetic energy of the galaxies, and is the total gravitational potential energy. Zwicky found that the mass inferred from the galaxies’ velocities (and hence their kinetic energy) was much larger than the mass inferred from the number of visible galaxies. This discrepancy is known as the “missing mass problem.”

The Dark Matter Hypothesis

To resolve this problem, Zwicky hypothesized the existence of a large amount of non-luminous matter, which he named “dark matter.” This invisible matter would contribute to the total gravitational mass, thus accounting for the observed dynamics.

Evidence and Acceptance

Zwicky’s hypothesis did not gain much attention initially, as it was based on a single observation. It was not until the 1970s, with the work of Vera Rubin and others, that the existence of dark matter became widely accepted. Rubin’s observations of galaxy rotation curves provided strong evidence for a significant amount of matter not accounted for by visible material. This reinforced Zwicky’s dark matter hypothesis.

Dark Matter Today

Today, dark matter is believed to make up approximately 27% of the universe’s mass-energy content. However, despite being a crucial component of the universe and the subject of extensive research, the nature of dark matter remains one of the most significant unsolved mysteries in cosmology. Various candidates have been proposed, including weakly interacting massive particles (WIMPs), axions, and massive compact halo objects (MACHOs), among others.

Zwicky’s dark matter hypothesis thus marked the beginning of a new chapter in our understanding of the universe, one where unseen forces govern the motion of galaxies and shape the large-scale structure of the cosmos.

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