Discovery of the Expanding Universe

Vesto Melvin Slipher
Source: Warwick Centre for Fusion, Space and Astrophysics

The astronomer Vesto M. Slipher at the Lowell Observatory in Arizona, was the first to measure the spectrum of the Andromeda galaxy in 1912. This was no simple feat in those days. He continued these measurements, and by around 1925 he had measured 45 of these spectra. At that time galaxies were called “spiral nebulae” but their nature was actually unknown. Were they galaxies like our own Milky Way and outside our galaxy or were they nebulae with a spiral shape, inside our Milky Way? At that time it was not even known what our Milky Way galaxy itself looked like and how big that was.


Slipher discovered something very intriguing in these spectra, when he measured the red shift to find radial velocities. With the exception of the Andromeda Galaxy they were all moving away from us. This was a puzzle, also because there were not yet accurate distances to these “spiral nebulae”, but this red shift discovery looked significant for the astronomers of the day.

 

Edwin Hubble
Source: The Franklin Institute

Edwin Hubble, at the Carnegie Observatories in Pasadena, California, made observations of Cepheid variable stars in various spiral nebulae in 1922 and 1923 and proved that these objects were outside the Milky Way galaxy, using Henrietta Swan Leavitt's period-luminosity relationship for Cepheids. This technique, described earlier in this EBook, was decisive to end the discussion among astronomers about whether there was more to the Universe than just the Milky Way galaxy.

 

Milton L Humason
Source Library www.ira.cnr.it

 

The breakthrough came when Hubble and Milton L. Humason combined these distance measurements with Slipher’s red shift measurements. This revealed a rough proportionality of the distance of the object with their redshift. The larger the distance, the larger the receding velocity.

In 1929 Hubble and Humason formulated the empirical Red shift Distance Law of galaxies, nowadays simply termed Hubble's law. This law dictates that an object twice as far away as another object, has a velocity that is also twice as much as the velocity of the closer object. This suggestion was astounding because this conflicted with the general consensus at the time that the Universe was static.

 Hubble's Law.
Source: ircamera.as.arizona.edu

Hubble and Humason were able to plot a trend line from the 46 galaxies they studied. Though there was considerable scatter in the data, the conclusion was made that recession velocity and distance are proportional, and they obtained a value for the Hubble constant of 500 km/s/Mpc (this constant determines the slope of the line in the graph).

 

Hubble estimated a value of about 500 km/s/Mpc (red line). This original value is far from the modern value, primarily because of the difficulty of acquiring accurate large distances in space, and the fact that they used nearby galaxies, that also have their own peculiar velocity with respect to the Milky Way. As an example, the Andromeda galaxy is now known to have a velocity towards us (blue shift) of 300 km/s. The expansion of space can only clearly be demonstrated with objects that are much further away.

Einstein, Hubble and Adams in Mt. Wilson Observatory.
Source Astronomy-links.net

When Einstein heard of Hubble's discovery he realised that the cosmological constant that he had added to his equations of General Relativity to “make” the Universe static, was a big mistake. He was quoted as saying: "this is the biggest blunder of my life". Einstein visited the Mt. Wilson observatory to discuss the discovery with Hubble and others. This cosmological constant is now an important parameter in modern cosmology.