cosmos

Cosmos refers to the universe as a whole—the totality of space, time, matter, energy, and the laws that govern them. The term derives from the Greek kosmos, meaning order or arrangement, and has been used in philosophy, religion, and science to describe the organized nature of reality.

In modern science, cosmology is the study of the cosmos. The dominant model is the Big Bang theory, which posits that the universe began from a hot, dense state around 13.8 billion years ago and has expanded ever since. The observable universe is estimated to be about 93 billion light-years in diameter. The current composition is inferred to be about 5% ordinary matter, 27% dark matter, and 68% dark energy, with gravity shaping structures from galaxies to galaxy clusters. The cosmic microwave background provides a fossil signal of early conditions, and large-scale structures form a web-like distribution connected by gravitational forces.

Observations show the cosmos is expanding, with galaxies moving apart at speeds described by the Hubble law; the rate of expansion is measured by the Hubble constant, and its precise value remains an active area of study. The study of the cosmos is called cosmology, and it intersects physics, astronomy, philosophy, and, in some traditions, religion.

Cosmos is also used in popular culture as a title for science communication projects. The 1980 television series Cosmos: A Personal Voyage, created and hosted by Carl Sagan, popularized astronomical ideas. Subsequent series include Cosmos: A Spacetime Odyssey (2014) and Cosmos: Possible Worlds (2020), produced to explain cosmology and science to broad audiences.