How Hubble destroyed the universe

In 1920, a discussion took place during the American National Academy of Science’s Annual Meeting. It quickly became known as the Great Debate and centred around the scale of the universe.

Through observations, astronomers had discovered objects that they called spiral nebulae. We now know these objects as galaxies but, at the time, their nature was unclear. Harlow Shapley, of the Harvard College Observatory, argued that spiral nebulae were located within the Milky Way and were simply giant clouds of dust and gas. On the other side of the debate, Heber D. Curtis put forward the case that these objects exist outside the Milky Way and that the universe is made up of many spiral nebulae, each an ‘island universe’ composed of billions of stars.

The debate was settled, a few years later, by Edwin Hubble (see page 4). He used Henrietta Swan Leavitt’s (see page 15) variable star period-luminosity relationship to calculate the distance to the Andromeda ‘nebula’ and hence showed it existed outside of the Milky Way. This led to the realisation that ‘spiral nebulae’ were galaxies just like our own.

Shapley and Hubble had long been rivals. When Shapley received the news of Hubble’s data, he is reported to have commented: “Here is the letter that has destroyed my universe.”