Galaxies, those enormous accumulations of stars, dust, gas, and other stuff, are a bit like people – they tend to congregate in big groups.
Many galaxies are found in galaxy clusters, collections of hundreds to thousands of galaxies that are gravitationally connected. Gravity and the closeness of these galaxies to one another have allowed galaxy clusters to resist the expansion of the Universe to some extent. Aggregations of galaxy clusters form galaxy superclusters, arguably the largest structures in the Universe. Galaxy superclusters tend to be connected to other superclusters by filaments of galaxies, forming a complex, sponge-like framework that constitutes the Universe that we observe.
On dark spring evenings, the light-absorbing disk of our own Milky Way Galaxy is below our horizon, giving us a clear view into the Universe beyond. After twilight ends, the central part of the Virgo Galaxy Cluster, to which our Milky Way belongs as an outlying member, is now high in the south. We can view the brightest Virgo Cluster members with small telescopes and even large binoculars. And on spring nights, astronomers with larger telescopes can study more distant galaxy clusters, like the Coma, Hydra, and Hercules Clusters.
Astronomers have learned much about the composition and history of our Universe from galaxy clusters. In 1933 Fritz Zwicky noticed that motions of galaxies in the Coma Cluster were too rapid for that Cluster to have remained intact. Zwicky concluded that most of the Coma Cluster is composed of non-luminous “dark matter.” Zwicky’s suggestion was not taken seriously in 1933. But, in recent decades, astronomers have discovered that galaxy clusters contain so much mass that their enormous gravity distorts and lenses light from more distant galaxies. Such gravitational lensing indicates that more than 80% of the mass of galaxy clusters is comprised of dark matter, the nature of which is still poorly understood.
Space probes that measure the Cosmic Microwave Background (or CMB) have detected small temperature and density variations that are theorized to have been present just 370,000 years after the Big Bang, 13.8 billion years ago. These ancient variations in the CMB may have evolved into the galaxy clusters and superclusters that we see today.
Western Slope Skies is produced by the Black Canyon Astronomical Society. This episode was written and recorded by Art Trevena.