Cosmology

The universe's origin, expansion, structure, and composition

What is cosmology?

Cosmology is the study of the universe as a whole — its overall structure, its origin, and how it has changed and continues to change over the vastest timescales imaginable. Where the rest of astronomy studies individual stars, galaxies, and planets, cosmology studies everything at once.

The universe is expanding

As covered in the Astrophysics article, the universe is expanding — space itself is stretching, carrying galaxies apart from each other over time. Tracing this expansion backward in time, the universe was once far smaller, hotter, and denser than it is today, in an event called the Big Bang, which occurred roughly 13.8 billion years ago.

The large-scale structure of the universe

On the very largest scales, galaxies aren't scattered randomly through the universe — they're arranged into a vast, web-like structure, with long filaments and dense clusters of galaxies separated by enormous, relatively empty voids. This large-scale structure formed gradually, over billions of years, as gravity slowly pulled matter together from the nearly (but not perfectly) uniform early universe.

What the universe is made of

Cosmologists have determined that ordinary matter — the atoms making up stars, planets, and everything we can directly see — makes up only a small fraction of the universe's total content. The rest consists of dark matter (introduced in the Astrophysics and Galactic Astronomy articles) and dark energy, a mysterious component whose confirmed effect is causing the universe's expansion to accelerate over time.

Why cosmology matters

Cosmology addresses some of the biggest questions people have ever asked about the universe: how it began, what it's made of, how it has evolved, and what its long-term future might look like — questions this branch approaches through careful measurement and mathematics rather than speculation alone.