It is the second decade of the 20th century and on a small planet orbiting a run-of-the-mill G-class star in the Orion spiral arm of the Milky Way Galaxy, another rising star is putting final touches on his General Theory of Relativity.  He is of course Albert Einstein and he has a problem.  It is this: in this universe which everyone then “knew” was static, by which was essentially meant “of fixed volume”, his theory indicated that such a state was not a viable solution of the equations that describe the theory.  According to his theory the universe should be either expanding or contracting.  In short, his theory predicted that the universe, as it was observed to be, was impossible. 

Needless to say the universe did not dutifully comply with Einstein by suddenly allowing itself to be observed expanding or contracting, as his influence and reputation in the physics community had not yet and would never arrive at that level.  So poor old Albert had little choice but to “fix up” his theory by arbitrarily inserting into the equations what he called a “cosmological constant”, that permitted a solution of his equations that was in agreement with the observed behavior of the then known universe; and the whole embarrassing matter was discretely and politely forgotten.

Some 10 years later, and an ocean and a continent away, Edwin Hubble has become interested in determining the distance from the earth of remote celestial objects.  Up until then, distances were only well established for nearby objects by parallax, a method which becomes increasingly insensitive, and therefore inaccurate and eventually useless, as distance increases.  What Hubble needed was a “standard candle”, an object of known brightness, or as it is known, absolute magnitude.  The distance from earth of such a standard candle and, with it, that of whatever structure contained it could be worked out from its apparent magnitude, that is, its apparent brightness as observed from earth.  Hubble found his standard candle in a type of star called a Cepheid variable.  These giant yellow stars pulse at a rate very tightly correlated to their absolute magnitude, a relationship discovered at Harvard College Observatory in the first decade of the 20th Century by Henrietta Swan Leavitt in a period during which she worked there as a lowly “computer”, which meant she counted objects on astrophotographic plates.  I know another similar story involving a Swiss patent office.

Anyway, Hubble had his standard candle, and working at the newly opened Mt. Wilson Observatory in Pasedena, California, identified Cepheid variables in many of the spiral nebulae which, at the time, were all thought to be inside of the Milky Way.  You see, as hard as it is for us to imagine, back then, the contents of the Milky Way were thought to be the entirety of the universe.  All visible objects were assumed to be within the Milky Way.  This was not unlike the medieval belief that the fixed stars were attached to the inside of a large sphere centered on the earth.  Objects too far away to be measured by parallax were not assumed to be very far away as parallax fails at relatively modest distances.  However when Hubble performed his first calculations using the pulsation rate and apparent magnitude of Cepheids in spiral nebulae he was flabbergasted to discover that these objects were tens to thousands of times more distant than the most generous estimate of the distance across the Milky Way. 

This discovery instantaneously increased the size of the universe, as we then understood it by a stupendous factor, seriously introducing the possibility that it might be infinite in extent.  It also vindicated the idea that had earlier occurred to some astronomers, but was much ridiculed by others, that the view form inside any of these spiral nebulae would be much the same as our view of the Milky Way thus suggesting that these nebulae were in fact “island universes” as was indeed the Milky Way.  Soon after the term ‘galaxy’ replaced ‘island universe’.  Despite its now much expanded size the universe was still considered to be static.  Our certainty in that in that belief was however about to be severely challenged.