The time when our fundamental understanding of the universe changed

A few centuries ago, many believed our planet was the centre of creation and everything we see in the sky orbits around us.

Then, we found our world is just one of a number of planets orbiting the sun. When, in 1785, William Herschel made the first ever map of the Milky Way, he assumed we live in the centre. However, by the time of the “Great Debate,” we were further demoted to the suburbs of the Milky Way, well out from the centre, with our sun just one star out of billions.

Our next demotions started to come together in 1920. Before then most astronomers believed the Milky Way was alone in the universe. As telescopes improved, astronomers found an increasing number of fuzzy, spiral things. It was widely accepted this was the spiralling in of material in the process of forming new stars and planetary systems, and was located within the Milky Way. However, by 1920, a substantial minority of astronomers suggested those spirals were other galaxies, lying well outside ours.

The discussion of the two ideas culminated in what became known as the "Great Debate", when, on April 28, 1920, the two factions got together to argue it out. Some, including Harlow Shapley, hung onto the idea those spirals lay within our galaxy. Others, such as Heber Curtis, maintained the spirals were other galaxies, "island universes" in their own right.

Over the following decade, the evidence of the "other galaxies idea" proved correct and became conclusive. The universe was filled with galaxies, some larger than ours, some smaller, extending out into space as far as we could see at the time. Our concept of the universe had fundamentally changed. We now orbited a star that was one of billions, in a galaxy that is just one of billions. The picture changed again in the early 1930s, when evidence started to appear that the matter making up us, and the observable universe, is actually a very minor ingredient. The main ingredient is invisible.

Astronomer Fritz Zwicky found himself a puzzle. It was 1933, and he was observing a distant cluster of galaxies known as the Coma Supercluster. The cluster lies about 321 million light years away, far outside our galaxy. Zwicky measured the speeds the galaxies were orbiting around each other, and how far apart they were. He estimated the numbers of stars and from these he obtained a value for the masses of the galaxies.

Newton's theory of gravitation ties masses, separations and orbital speeds together. If we have two of these quantities, we can calculate the other. So Zwicky used the orbital speeds and separation distances to calculate the masses. In measurements and calculations like this we expect there to be errors or discrepancies. However, the estimated mass of the cluster was about ten times larger than he estimated from what he could see.

The galaxies were moving so fast that the gravitational attraction of the visible matter could not hold the cluster together. This discrepancy was far too big to be an error. There was something fundamentally wrong. He concluded there was a lot of matter - some 90% of the matter in the cluster - that was invisible. Being a native German speaker, he called this unseen matter "Dunkle Materie" , which in English means "Dark Matter".

Since the 1930s, we have found countless galaxies extending out into space and back in time to around a billion years after the Big Bang, which happened just under 14 billion years ago. Dark matter is a major ingredient in galaxies. Moreover, it seems that without this massive fraction of dark matter, the galaxies would not even have formed.

That decade between the early 1920s and 1930s, between the two World Wars, fundamentally changed our view of the universe.

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• Venus and Mars lie close together in the west after sunset.

• Saturn rises in the early hours, and Jupiter appears low in the sky before dawn.

• Mercury lies very low and is hard to see in the dawn glow.

• The Moon will reach its last quarter on June 10.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.