The Hubble Constant

When we hear the word Hubble, NASA’s wonderful space based telescope immediately springs to mind. You will probably be aware that its named after the great astronomer Edwin Hubble, but what was it that Hubble did to have his name attached to the most iconic telescope in NASA’s history?

Born in Missouri in 1889, Edwin Hubble was a gifted athlete in his early life, not really concentrating on his academic studies. Despite the athletic focus he later achieved a degree in Science at the University of Chicago, but being the dutiful son he promised his father he’d go on to study law for his masters. However he lacked motivation when it came to law and after his fathers death Hubble decided to start over, eventually receiving his masters in astronomy at the age of 28. After serving his country in the war Hubble took on a position at Mount Wilson Observatory in California, which conveniently coincided with the completion of the 2.5 metre Hooker telescope, the largest in the world at the time.

Initially Hubble used the Hooker telescope to study a type of variable star in order to calculate the distances of these objects. When these objects appeared to be well outside of the Milky Way, it was then that he introduced the notion of other possible galaxies in the universe. Hubble began to compare these galaxy distances to their red-shifts and discovered that they were both roughly in proportion.

Redshift describes how objects in space move further away from us by the viewing the nature of the light’s wavelength. This concept is key to understanding that the universe is expanding. In a similar way to the Doppler effect the constant speed of light travelling through the expanding space will shift the wavelength to the red part of the spectrum. The greater the shift the faster the speed of the object in question. Similarly, if an object is moving towards us the light will in effect be squashed and the wavelength will become shorter. This indicates a shift to the blue end of the spectrum and is said to be a blueshift.

When Hubble plotted the redshift of the galaxies against their distances, he was able to determine that space was expanding at a rate of 500 kilometres per second per Megaparsec (A parsec is a measure of distance at around 3.2 light years). This rate became known as the Hubble Constant, and effectively means that for every 3 million light years increase in distance, the expansion rate of space increases by 500 kilometres per second.

Two years later Hubble determined a more accurate value for his constant, and placed it in an equation to derive Hubble’s Law. It was so significant that Einstein abandoned his work on his own cosmological constant, stating his belief of a static universe as his “biggest mistake”. In 1931 Einstein made a trip to California to personally thank Hubble for finding the observational proof that the universe was in fact expanding.

The Hubble constant has since been re-evaluated further, and due to the increased precision of our measurements it has now been placed at a value of around 67 km/s/Mpc.

It was eventually possible to use Hubble’s law to correctly estimate the age of the universe. If it proved that space was expanding then it followed that there must of been a beginning and as such an age.