A Survey of Big Bang Cosmology, Part I: Cosmic Geography

n November 1915, Albert Einstein finished his decade-long struggle to develop the General Theory of Relativity (GR), in which he envisioned gravitation geometrically as the “curvature of spacetime.” Immediate observational confirmation for the theory was significant but slim. In 1915 this consisted of the successful accounting of the anomalous precession of Mercury’s orbit. Tests of other early predictions, all on the solar system scale, would have to wait. Measurement of the deflection of light rays grazing the sun occurred with the 1919 solar total eclipse; measuring gravitational redshift and radar echo delay required high-speed electronics that appeared about 1960. Some measurements, such as gyroscope precession and gravitational waves, are only now being carried out for the first time.[1] Despite limited early opportunities to test the theory, in 1917 Einstein hitched his wagon to the stars and brazenly launched modern cosmology by applying GR to the entire universe. In so doing he started a discussion that, over the next dozen years, blossomed into the genesis of big bang cosmology. The years between 1900 and 1930 saw a confluence of observation and theory that matured cosmology into a science. On the theoretical side was the advent of GR. Newtonian cosmology was spectacularly inconsistent with Newtonian gravitation concerning the boundary conditions at infinity, and in 1917 GR offered a way out of the impasse. On the observational side, the realization that the spiral nebulae lay far outside the Milky Way galaxy was a necessary insight before modern cosmology could be possible. We take up a synopsis of the latter story here, and return to the former in subsequent installments of this series.