Going Further 13.1: Edwin Hubble’s Data
Edwin Hubble’s early published data are shown in Figure B.13.1. The first data (top), from 1929, were the basis for the initial announcement of the Hubble law, and correspondingly, the expansion of the Universe. In the figure, the solid circles and the blue line are for the galaxies treated individually, while the open circles and the red line combine the galaxies into groups. There were few galaxies in the sample and a wide scatter in the data, due largely to measurement errors. Hubble also mislabeled the y-axis ‘km’ rather than ‘km/s’. The 1931 data (bottom), collected with his assistant Milton Humason, demonstrate the Hubble Law far more convincingly, mainly by extending measurements to much more distant galaxies; most of the 1929 data are contained within the small box at the extreme lower left of the 1931 plot.
The values for the Hubble constant derived from the slopes of the lines in these plots were wrong by a large factor (about 10) due to several unrecognized systematic errors. Unknown at the time, there are different classes of Cepheid variables, and Hubble applied the period-luminosity law of one class to objects of the other. Hubble did not know about the cosmic distance ladder, so he had to improvise. For instance, in distant galaxies Cepheids were too faint for him to detect, forcing him to find (or invent) alternative ways of measuring the distances to those galaxies. Even now, when we look at the most distant galaxies visible in our most powerful telescopes, we cannot see the Cepheids in them because they are too faint. Hubble had no chance of seeing them in the 1920s.
This error was not corrected for many years and resulted in serious overestimates of the speed of expansion of the Universe—and correspondingly short age estimates for the Universe, as we will see.
