What Is Cosmology?

You have looked at the night sky and seen stars. Cosmology asks a different question than astronomy: not “what is that star?” but “why is there a sky at all?”

Astronomy studies individual objects – stars, planets, nebulae. Astrophysics studies the physical processes inside those objects – how stars burn, how planets form, how supernovae explode. Cosmology studies the universe as a single physical system. It asks questions that cannot be answered by examining any individual object, no matter how carefully:

Is the universe finite or infinite? Has it always existed? Is it expanding, and if so, how fast? What is the universe made of? What will happen to it?

Answering these questions requires three things. First, a theory of gravity that applies to the universe as a whole. That theory is general relativity, which describes how spacetime curves in response to matter and energy. Second, observations of the distant universe – the cosmic microwave background, galaxy surveys, supernova distances – that constrain which models are consistent with reality. Third, models that connect the theory to the observations.

The current standard model is Lambda-CDM. It describes a universe that began in a hot, dense state approximately 13.8 billion years ago, has been expanding ever since, and is composed of roughly 5% ordinary matter, 27% dark matter, and 68% dark energy. This model fits all major observations with six free parameters. It is remarkably successful and remarkably incomplete – we do not know what 95% of the universe is made of.

This library is organized into four domains. Structure covers what the universe contains. Evolution covers how the universe has changed over time. Observation covers how we know what we know. Black holes covers the extreme objects where general relativity, thermodynamics, and information theory meet.