The cosmological principle is a part-philosophical, part-physical assertion that proclaims a uniformness in the universe. In other words, physical laws and actions observed on Earth are not unique, but rather are representative of a standard modus operandi for the entire known universe. Additionally, any type of observer at any point will observe the same characteristics of the universe, provided the observation is taking place on a grand scale.
This set of assumptions laid the foundation for physical cosmology to emerge as a true science. In modern times, the study of space, or astronomy, has several subdivisions, and cosmology is a major component. Specifically, physical cosmology encompasses the study of the structure, formation, and function of the universe on a large scale. The belief — called the Copernican Principle or background independence — that earthly objects and celestial objects answer to basically the same laws of physics serves as a primary inspiration for cosmology's emergence as a scientific discipline.
The term "observer" in the cosmological principle refers to any being that bears witness to an existence and related forces within this existence. This observer could be a human being on Earth or a human being at another point in the universe. A theoretical observer may also be non-human or even non-Earthly, so long as it has an awareness of its surroundings.
Further, the observable conformity is not exclusive to physically apparent objects. Rather, the laws of physics, such as the equations of motion, are believed to be the same in all points of the universe. In terms of physical similarity, the cosmological principle primarily considers the sameness of objects when viewed on a large scale, such as the observed distribution of galaxies or the metric expansion of space.
Several renowned individuals and theories have promoted the idea of the cosmological principle, perhaps beginning with Nicolaus Copernicus and his assertion that the Earth does have a favored status in the center of the universe. In addition, scientist Isaac Newton owed much of his work in gravitation to the belief in a universal force that kept objects stable. Albert Einstein used the assumptions of the cosmological principle as a lynchpin for his theory of relativity as well. Even the big bang theory for the origin of the universe somewhat rests on the notion that all points in the universe have a similar and common path of development. An Englishman named Edward Milner actually gave the cosmological principle its title.