Astronomy 275 Lecture Notes, Spring 2011 c Edward L. Wright, 2011 Cosmology has long been a fairly speculative field of study, short on data and long on theory. This has inspired some interesting aphorisms: • Cosmologist are often in error but never in doubt - Landau. • There are only two and a half facts in cosmology: 1. The sky is dark at night. 2. The galaxies are receding from each other as expected in a uniform expansion. 3. The contents of the Universe have probably changed as the Universe grows older. Peter Scheuer in 1963 as reported by Malcolm Longair (1993, QJRAS, 34, 157). But since 1992 a large number of facts have been collected and cosmology is becoming an empirical field solidly based on observations.
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Cosmological Observations 1.1.
Recession velocities
Modern cosmology has been driven by observations made in the 20th century. While there were many speculations about the nature of the Universe, little progress was made until data were obtained on distant objects. The first of these observations was the discovery of the expansion of the Universe. In the paper “THE LARGE RADIAL VELOCITY OF NGC 7619” by Milton L. Humason (1929) we read that “About a year ago Mr. Hubble suggested that a selected list of fainter and more distant extra-galactic nebulae, especially those occurring in groups, be observed to determine, if possible, whether the absorption lines in these objects show large displacements toward longer wave-lengths, as might be expected on de Sitter’s theory of curved space-time. During the past year two spectrograms of NGC 7619 were obtained with Cassegrain spectrograph VI attached to the 100-inch telescope. This spectrograph has a 24-inch collimating lens, two prisms, and a 3-inch camera, and gives a dispersion of 183 Angstroms per millimeter at 4500. The exposure times for the spectrograms were 33 hours and 45 hours, respectively. The radial velocity from these plates has been measured by Miss MacCormack, of the computing division, and by myself, the weighted mean value being +3779 km./sec.”