This year, we celebrate 30 years of the history of the Hubble Space Telescope Here’s the HST itself, and one of its most famous images, taken in 1995.
The extent of the universe
The HST is named after the American astronomer, Edwin P Hubble, whose observations in the early 20th Century, lead to two, profound discoveries. Looking into these we will also meet several other important characters.
Hubble was physically large and imposing. he was a US Army boxing champion, serving at the closing stages of WW1, although his unit did not go into combat. He affected an English accent despite being very much an American.
For the first twenty or so years of the twentieth Century, there was great scientific debate about the extent of the universe. Many scientists believed at the time that the whole of the universe consisted of our Milky Way galaxy, and that what were then called “spiral nebulae” were some kind of structure within the Milky Way. Following painstaking observations at the 100 inch Hooker telescope at the Mount Wilson Observatory in California, Hubble and his assistant Humasson established in 1924 that spiral nebulae are in fact remote galaxies in their own right; they are now called spiral galaxies.
Hubble’s discovery was made possible by way of an earlier crucial discovery made by Henrietta Swan Leavitt, who worked at the Harvard College Observatory. Leavitt had the task of examining photographic plates to measure and catalog the brightness of stars.
This work led Leavitt to discover the so-called 'period-luminosity relationship' of Cepheid variable stars. Probably the best known Cepheid variable star is Polaris, the current pole star. Leavitt’s discovery was that the rate at which these stars appeared to vary in brightness was directly related to their intrinsic luminosity. This meant that measuring the period of change provided astronomers with the first "standard candle" with which to measure the distance to remote astronomical objects. Hubble used this technique to show that Cepheids in the Andromeda galaxy, M31, was too far distant to be part of the Milky Way Galaxy. It was later discovered that there different types of Cepheid variables, and this meant that M31 is actually twice as far distant as Hubble first calculated.
The expansion of the universe
Hubble’s second observational discovery was to prove equally profound. It was in fact preceded by a theoretical discovery by Georges Lemaître, a Belgian Catholic priest and professor of physics at the Catholic University of Louvain. Lemaître applied Einstein’s general relativity (GR) to cosmology deriving solutions to Einsteins field equations, giving results that implied an expanding universe.
Extrapolating back in time, Lemaître postulated an origin of the universe in what he called a 'primeval atom' – in effect, the “big bang”. This was in 1927, two years before Hubble's publication of his observational findings of expansion of the universe.
An advanced mathematician, Lemaître could hold his corner in intellectual argument with Einstein (no less!). The two met on several occasions, including at the Solvay Conference in 1931.
Albert Einstein of course needs no introduction. Einstein published his theory of General Relativity in 1917. Developed from his theory of Special Relativity (published in 1905), GR included an explanation of the phenomenon of gravity. Among other things, GR successfully accounted for variations in the precession of the orbit of Mercury which Newtonian gravitational theory was unable to explain. Einstein had believed that the universe was static, although others (including Alexander Friedman and Georges Lemaître) provided solutions to his equations that indicated that the universe must be either expanding or contracting.
In January 1931, Einstein visited Hubble at the Mount Wilson Observatory where the 100 inch Hooker telescope is located.
Einstein, perhaps rather reluctantly, conceded that the expansion predicted by general relativity must be real, added a term called the 'cosmological constant' to his field equations. In later life, he said that this was "his biggest blunder", although today the cosmological constant is now thought by many cosmologists to account for the role of dark energy.
Confirming Hubble’s discovery using modern data
Hubble's observations, published in 1929, established that the spectra of majority of galaxies exhibit a redshift, showing they are moving away from us, and that the further away they are, the faster they appear to be receding. This became what is now called Hubble's Law and is a cornerstone of modern cosmology.
The data plot below shows the plot published in Hubble's 1929 paper.
The slope of the trendline indicates the value of what is called the Hubble parameter, H₀, a measure of the velocity of recession of galaxies vs. distance. Hubble's early estimate was that H₀ ~500 km s¯¹ Mpc¯¹ . This was quickly realized to be much too high, as it implied an age of the universe of less than 2 million years, whereas it was known that Earth was much older than this.
The plot below has been constructed from modern data in the NASA Extragalactic Database (NED).
As in Hubble’s work, the plot shows recessional velocities against distances. The red trendline represents H₀. Observations since Hubble's time have refined and reduced the value of H₀ and today the value is thought to be in the range 60-90 km s¯¹ Mpc¯¹ - the exact value is still highly debated in the community. The slope indicates on this plot for this sample of 44 galaxies, H₀ ~64 km s¯¹ Mpc¯¹ .
For Hubble’s confirmation of the extent of the universe and for Hubble’s Law, the Hubble Space Telescope, which has made so many discoveries of its own in its 30 year operation, is named in his honour.