Three discoveries revolutionized cosmology, which is the study of the structure and history of the universe: (1) the structure of the solar system, (2) the structure and formation of galaxies, and (3) the Big Bang model of the expansion of the universe. Copernicus derived his model of the solar system without a telescope. The invention of the telescope and the science of optics then led to the discovery of galaxies and the expanding universe. Georges Lemaitre discovered the Big Bang model in 1931 after he visited Hubble and realized that galaxies are moving away from us at speeds proportional to their distance.
At the end of his life (1540 AD), Copernicus published On the Revolutions of the Celestial Spheres and began the Copernican Revolution (Section 1-2). In this paradigm shift, Earth began as the stable center of the universe in which God and angels pushed the sun, planets, and stars around Earth. Earth ended up as one small planet orbiting one of countless stars. The Copernican model remained controversial, but Brahe's improved maps of the paths of the planets, Kepler's equations of planetary motion, and Newton's discovery of the cause of planetary rotation around the sun finally led to general acceptance.
The science of optics (light) led to the development of the first telescopes, and advances in optics led to improvements in image quality and resolution (lenses and mirrors), detection of all the wavelengths of electromagnetic radiation (spectrographs), and detection of the extremely faint light of distant stars and galaxies (photographic plates and CCD cameras). Modern telescopes are a window to the universe and the foundation of cosmology.
The discovery of galaxies began in the sixth century BC when Persian astronomers wondered whether the white light of the Milky Way (Figure 1‑20) might be the combined light of many stars. Galileo's telescope confirmed that the Milky way is composed of stars. Little did people realize when they were arguing about the Copernican model of the solar system that we are a tiny part of a galaxy with 100 billion stars, which is a tiny part of a universe with 100 billion galaxies.
Observational astronomers George Slipher from the University of Arizona and Edwin Hubble from Cal Tech attached spectrophotometers to telescopes and calculated the speeds of galaxies based on the redshift of light. They discovered that that most galaxies are moving away from us. Hubble used the brightness of Henrietta Leavitt's Cepheid variable stars as a standard candle and calculated the distances of those galaxies. Hubble was an observational astronomer and did not realize that the entire universe is expanding or that the general law of relativity described the expansion of the universe; however, he was kind enough to meet with Georges Lemaitre.
Georges Lemaitre was an interesting person. He was a priest, mathematician, and astronomer. After meeting with Hubble in California and Slipher in Arizona, Lemaitre realized that galaxies were receding from us at speeds proportional to their distance (Hubble's Law), which implied to Lemaitre that the entire universe is expanding. Lemaitre then combined Einstein’s general theory of relativity (mathematics) with Hubble’s Law (observation of receding galaxies) to derive the Big Bang model of the universe. An expanding universe also explained by the universe does not collapse on itself due to gravity.
By observing rotation rates of galaxies and clusters of galaxies, Fritz Zwicky in the 1930s and then Vera Rubin in the 1970s discovered that most of the matter in the universe is invisible dark matter. Shortly thereafter, three Nobel Laureates used the brightness of type 1a supernovae as a standard candle to discover that the expansion rate of the universe is slightly increasing, and, thus, most of the energy in the universe is dark energy. With these standard candles that determine the distances to galaxies and red shifts that determine the velocities of galaxies, they are able to prove the steady expansion of the universe back from 7 billion years ago to the present.
Just after the beginning of the universe, the period of inflation lasted for 10-34 seconds (a trillion, trillion, trillionth of a second). During inflation, the universe doubled in size 100 times and expanded at a rate that was much faster than the speed of light. Although light cannot travel faster than the speed of light, this does not mean that the universe cannot grow faster than the speed of light. During inflation, the universe grew from the size of a proton to the size of a grapefruit. After inflation, the combination of three expanding forces counteracted the inward pull of gravity during the entire history of the universe. The precise balance between expansion and contraction caused the universe to steadily expand for 14 billion years.
The transition to the heliocentric model took so long that some historians refer to it as the Copernican nonrevolution. It was an extremely traumatic event in the history of science and religion, and it led to some fundamental paradigm shifts.