Chapter 5: The Recession of Distant Galaxies

American Astronomer, Vesto Slipher, attempted to determine the distance to faraway objects, specifically galaxies. Slipher’s observations were of spectral lines corresponding to specific galaxies. It is known that each element has a highly distinctive set of spectral lines. Each line represents the transition between energy levels of an atom or molecules.

Slipher’s biggest observation was the Andromeda Galaxy, and he observed its spectrum. Its spectrum had elements well known in the laboratory. Although the positions of the lines were familiar, he observed that the spectral lines were situated at shorter wavelengths than what had been measured in the laboratory. When light moves towards us or away from us, a phenomenon takes place, which changes our perception of the light we are receiving. The wavelength of light appears to be shorter than its actual length. The speed of light remains the same, but the separation between the crests and the troughs of the waves are shorter. We then perceive the effect as a colour change in the light source, and it appears bluer. Technically speaking, the light has been blue-shifted.

There is another phenomenon, which we must consider. This time, suppose that light is being emitted from a farther distance. As the light rays travels towards us, the expansion of the Universe will cause their wavelengths to be stretched. Based on the time that each light ray has been traveling, it will dictate how the wavelengths have changed. Slipher had documented red shifts, which corresponds to distant light sources. The farther away a light source is, the longer their light rays will take to reach the earth, therefore, the more stretched their wavelengths will be. The cosmological red shift refers to distant objects and the fact that they have a larger change in wavelength than closer objects do because of the distance they have to travel.

Slipher’s measurement of Andromeda’s spectra indicated that it is moving towards the earth. Obviously, Andromeda’s spectra was blue-shifted, which meant that it was moving towards us at a speed of 300 kilometres per second, the largest velocity recorded at that time. Slipher’s observation of other galaxies showed that most other galaxies were moving away from Earth. In essence, their spectra were red-shifted. Based on his observations, Slipher believed that the galaxy was drifting with respect to other galaxies because he thought that there were larger velocities on one side of the galaxy. Continued measurements showed that the recession of distant galaxies was the same in all directions. Slipher’s method for obtaining atomis spectra was time consuming- it took him two years to observe and record 15 spectra. 13 of these spectra were red-shifted and receding from Earth at speeds over 1000 kilometres per second. By the end of the next 10 years, Slipher had recorded data for more than 30 red shifts. His observations allowed him to conclude that brighter galaxies have smaller red shifts, and fainter galaxies have larger red shifts. The implications of Slipher’s results were that the further galaxies were receding at speeds greater than the closer ones, lending evidence to the theory of an expanding universe.