Cosmos is another name for the universe. The cosmos, and our understanding of the reasons for its existence and significance, are studied in cosmology. The scientific study of the large-scale properties of the universe as a whole.
Anaximander studied the universe in 600 BCE in ancient Greece, which the Greeks believed could be defined in geometric or mathematical terms. Based on the work of Plato and Aristotle, Ptolemy depicted the universe as spherical, with the Earth at its center and all stars and planets revolving around it in circles by the second century CE. Each planet's motion was explained by a distinct mathematical formula, and the motion of the planets appeared to be rather complicated.
The work of Aristotle was the basis of European beliefs about the Earth’s place in the universe. Islamic societies in Europe continued to advance science, astronomy, and cosmology during the Dark Ages. In Europe, it wasn't until the 1500s that progress was achieved beyond Ptolemy's work, thanks to Nicolaus Copernicus. Copernicus aspired to develop a single universe hypothesis in which everything operated on the same principle. He was uneasy with models that needed each planet to be described by its own set of sophisticated equations. He proved that the Sun, not the Earth, was at the center of the Solar System, substantially simplifying the equations required to describe it. This development, however, did not totally simplify the motion of the planets. He left his writings on this topic to be published after his death because he didn’t want to face the backlash he knew his ideas would provoke.
Johannes Kepler was the first to solve the mystery of the planets' unusual motions a century later. He and Tycho Brahe discovered that the orbits of the planets are elliptical based on reliable astronomical measurements.
Newton and other early scientists believed that the universe must be infinitely large, endlessly old, and evenly dispersed. They reasoned that if gravity didn't exist, the universe would have collapsed on itself long ago. The elliptical orbits defined by Kepler were shown to be the inevitable result of a universal law of gravitation by Newton. Newton thought that the cosmos was infinite and operated as a vast machine, obeying laws established by an all-powerful God.
For the next 200 years, Newton's model dominated scientific thought, despite the fact that some questioned the plausibility of an infinite universe, and scientists' growing understanding of thermodynamics led them to theorize that a machine like a universe can't go on forever without running out of energy.
Heinrich Olbers, a German astronomer, wondered in 1823 why, if the world is limitless, the sky is dark at night. If the universe is infinitely huge, old, and uniformly dispersed, then a star should appear in every direction in the night sky. If that were the case, the sky should be as bright as the sun at all times of the day and night. Scientists realized that the universe couldn't possibly be limitless.
In his General Theory of Relativity, Albert Einstein advocated the unification of space and time. The advent of the twentieth century offered new insights into the vastness of the universe.