Understand
In the early-mid 20$^{\rm th}$ century Dirac and Zel'dovich were among the first scientists to suggest an intimate connection between cosmology and atomic physics.
- Though a revolutionary proposal for its time, Dirac's Large Number Hypothesis (1937) adopted a standard assumption of the day, namely, the non-existence of the cosmological constant term ($\Lambda = 0$).
- As a result, its implementation necessitated extreme violence to the theory of general relativity -- something few physicists were prepared to sacrifice in favour of `numerology' -- requiring a time-dependent gravitational coupling of the form $G(t) \sim 1/t$.
- Zel'dovich's insight (1968) was to realise that a small but nonzero cosmological term ($\Lambda > 0$) allowed the present day radius of the Universe to be identified with the de Sitter radius, $r_{\rm U} \simeq l_{\rm dS} \simeq 1/{\sqrt{\Lambda}}$, which removed the need for time-dependence in the fundamental couplings.