This series of posts goes through the elements of the concordance cosmology: general relativity, inflation, the Big Bang and the expansion of the universe, cold dark matter (CDM) and a positive cosmological constant. I will start with a review of the historical, theoretical, and observational advances that led to each model’s widespread acceptance. The posts are excerpts from my Ph.D. thesis, lightly modified for blog format. No mathematical equations are written, with the intent of making the introductory material accessible to the non-specialist. A full bibliography of references in posts in this series is available.
What is today considered cosmology was, for most of the history of humanity, relegated to the realm of religion and philosophy. As humans were aided by the naked eye alone, the majority of the universe was outside of observational and, consequently, theoretical reach. Nevertheless, the universe and humanity’s place in it has been a universal trait of humanity’s civilizations, with people wondering, mythologizing, and trying to make sense of the night sky throughout the ages (Krupp, 2012).
Modern cosmology is defined as “the science or theory of the universe as an ordered whole, and of the general laws which govern it. Also, a particular account or system of the universe and its laws” (The Oxford English Dictionary (Simpson et al. , 1989)), began with the invention of instrumental optical astronomy. The optical telescope was first invented in the 1600s (King, 1955). Fraunhofer invented the optical spectroscope in 1814 (Berry, 1910). With optical telescope technology steadily improving, astronomers realized wavelengths outside of the optical range were extremely informative. Starting in the 1930s with radio (Jansky, 1933), astronomers progressed to observations throughout the spectrum from those in the infrared (Rieke, 2009) to those in the gamma-ray (Ramaty et al. , 1979). Each of these advances spawned greater theoretical and practical understanding of these fundamental questions long pondered, increasing the knowledge of the universe from far beyond the solar system in the last century.
With advances in observational techniques occurring together with the gradual development of a robust consensus theoretical framework, cosmology came into its own right as a scientific field in the 1900s. The 2000s, with the maturity of the concordance cosmological framework, ushered in the era of the so-called precision cosmology (Pecker & Narlikar, 2006). This concordance model was made possible not only by the incredible observational strides made in the past century, but also by theoretical advances. In the mid 1910s, Einstein developed the modern theory of gravitation, General Relativity (GR), (Einstein, 1914, 1916, 1920) and by 1917 Einstein had published his work on the cosmological consequences of GR (Einstein, 1917). The advent of GR paved the way for a rigorous understanding of the Cosmos from a theoretical perspective.
Observations and advances in theory in the mid 20th century put the Big Bang at the core of cosmology and the concordance cosmology now incorporates three additional elements: a positive cosmological constant Λ, cold dark matter (CDM) and inflation. Λ and CDM are two phenomena that behave in a manner outside the realm of ordinary matter. CDM interacts only gravitationally and Λ drives the observed late-time accelerated expansion of the Universe. The phenomenon known as inflation was theorized in the 1980s to explain the emergence of large scale structures of the Universe by events in the first micro-yocto seconds after the Big Bang. GR, the Big Bang, inflation, CDM, and Λ form the concordance cosmology, which developed through the interplay between theory and observation in the last century.
Cosmological N-Body Simulations as Probes of Gravity, pp 1-2, Christine Corbett Moran, University of Zurich, Ph.D. Thesis, 2014.