Cosmology is the study of the origin, evolution, and structure of the universe. This science grew out of mythology, religion, and simple observations and is now grounded in mathematical theories, technological advances, and space exploration.

Ancient astronomers in Babylon, China, Greece, Italy, India, and Egypt made observations without the assistance of sophisticated instruments. One of their first quests was to determine Earth's place in the universe. In A.D. 100, Alexandrian astronomer Ptolemy suggested that everything in the solar system revolved around Earth. His theory, known as the Ptolemaic system (or geocentric theory), was readily accepted (especially by the Christian Church) and remained largely unchallenged for 1,300 years.

A Sun-centered solar system

In the early 1500s, Polish astronomer Nicolaus Copernicus (1473–1543) rose to challenge the Ptolemaic system. Copernicus countered that the Sun was at the center of the solar system with Earth and the other planets in orbit around it. This sun-centered theory, called the Copernican system (or heliocentric theory), was soon supported with proof by Danish astronomer Tycho Brahe (1546–1601) and German astronomer Johannes Kepler (1571–1630). This proof consisted of careful calculations of the positions of the planets. In the early 1600s, Kepler developed the laws of planetary motion, showing that the planets follow an ellipse, or an oval-shaped path, around the Sun. He also pointed out that the universe was bigger than previously thought, although he still had no idea of its truly massive size.

The first astronomer to use a telescope to study the skies was Italian Galileo Galilei (1564–1642). His observations, beginning in 1609, supported the Copernican system. In the late 1600s, English physicist Isaac Newton (1642–1727) introduced the theories of gravity and mass, explaining how they are both responsible for the planets' motion around the Sun.

Over the next few centuries, astronomers and scientists continued to make additions to people's knowledge of the universe. These included the discoveries of nebulae (interstellar clouds) and asteroids (small, rocky chunks of matter) and the development of spectroscopy (the process of breaking down light into its component parts).

Modern cosmology

During the first two decades of the twentieth century, physicists and astronomers looked beyond our solar system and our Milky Way galaxy, forming ideas about the very nature of the universe. In 1916, German-born American physicist Albert Einstein (1879–1955) developed the general theory of relativity, which states that the speed of light is a constant and that the curvature of space and the passage of time are linked to gravity. A few years later, Dutch astronomer Willem de Sitter (1872–1934) used Einstein's theory to suggest that the universe began as a single point and has continued to expand.

Words to Know

Asteroid: Relatively small, rocky chunk of matter that orbits the Sun.

Copernican system: Theory proposing that the Sun is at the center of the solar system and all planets, including Earth, revolve around it.

Gravity: Force of attraction between objects, the strength of which depends on the mass of each object and the distance between them.

Light-year: Distance light travels in one solar year, roughly 5.9 trillion miles (9.5 trillion kilometers).

Mass: Measure of the total amount of matter in an object.

Nebula: Cloud of interstellar gas and dust.

Ptolemaic system: Theory proposing that Earth is at the center of the solar system and the Sun, the Moon, and all the planets revolve around it.

Radiation: Energy in the form of waves or particles.

Spectroscopy: Process of separating the light of an object (generally, a star) into its component colors so that the various elements present within that object can be identified.

Speed of light: Speed at which light travels in a vacuum: 186,282 miles (299,728 kilometers) per second.

In the 1920s, American astronomer Edwin Hubble (1889–1953) encountered observable proof that other galaxies exist in the universe besides our Milky Way. In 1929, he discovered that all matter in the universe was moving away from all other matter, proving de Sitter's theory that the universe was expanding.

Creation of the universe

Astronomers have long been interested in the question of how the universe was created. The two most popular theories are the big bang theory and the steady-state theory. Belgian astrophysicist Georges-Henri Lemaître (1894–1966) proposed the big bang theory in 1927. He suggested that the universe came into being 10 to 15 billion years ago with a big explosion. Almost immediately, gravity came into being, followed by atoms, stars, and galaxies. Our solar system formed 4.5 billion years ago from a cloud of dust and gas.

In contrast, the steady-state theory claims that all matter in the universe has been created continuously, a little at a time at a constant rate, from the beginning of time. The theory, first elaborated in 1948 by Austrian-American astronomer Thomas Gold, also states that the universe is structurally the same all over and has been forever. In other words, the universe is infinite, unchanging, and will last forever.

Astronomers quickly abandoned the steady-state theory when microwave radiation (energy in the form of waves or particles) filling space throughout the universe was discovered in 1964. The existence of this radiation—called cosmic microwave background—had been predicted by supporters of the big bang theory. In April 1992, NASA (National Aeronautics and Space Administration) announced that its Cosmic Background Explorer (COBE) satellite had detected temperature fluctuations in the cosmic microwave background. These fluctuations indicated that gravitational disturbances existed in the early universe, which allowed matter to clump together to form large stellar bodies such as galaxies and planets. This evidence all but proves that a big bang is responsible for the expansion of the universe.

Continued discoveries

At the end of the twentieth century, astronomers continued to revise their notion of the size of the universe. They repeatedly found that it is larger than they thought. In 1991, astronomers making maps of the universe discovered great "sheets" of galaxies in clusters and super-clusters filling areas hundreds of millions of light-years in diameter. They are separated by huge empty spaces of darkness, up to 400 million light-years across. And in early 1996, the Hubble Space Telescope photographed at least 1,500 new galaxies in various stages of formation.

In the late 1990s, while studying a certain group of supernovas, astronomers discovered that older objects in the group were receding at a speed similar to younger objects. In a "closed" universe, the expansion of the universe should slow down as it ages. Older supernovas should be receding more rapidly than younger ones. This is the theory that astronomers used to put forth: that at some future point the universe would stop expanding and then close back in on itself, an inverted big bang. However, with this recent finding, astronomers tend to believe that the universe is "open," meaning that the universe will continue its outward expansion for billions of years until everything simply burns out.


Creationism is a theory about the origin of the universe and all life in it. Creationism holds that Earth is perhaps less than 10,000 years old, that its physical features (mountains, oceans, etc.) were created as a result of sudden calamities, and that all life on the planet was miraculously created as it exists today. It is based on the account of creation given in the Old Testament of the Bible.

Because Creationism is not based on any presently held scientific principles, members of the scientific community dismiss it as a possible theory on how the universe was created. However, people who strongly believe in Creationism feel that it should be taught as a part of science education. The heated debate between the two sides continues.

[ See also Big bang theory ; Dark matter ; Doppler effect ; Galaxy ; Redshift ; Relativity, theory of ]

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