Module 3: Galaxies & Cosmology

Weeks 14–16 | The large-scale universe

Why this module matters

We’ve studied individual stars, but stars are just building blocks. In this final module, we zoom out to the largest scales: the Milky Way, billions of external galaxies, and the universe as a whole. We’ll see how galaxy motions reveal dark matter, and how the expansion of space tells us about cosmic origins.

Learning objectives

By the end of this module, you will be able to:

• Explain the structure of the Milky Way and evidence for dark matter
• Classify galaxies and describe how we measure their distances
• Apply the Hubble law to describe cosmic expansion
• Describe the Big Bang model and evidence from the cosmic microwave background

Lectures

Lecture Readings

Optional Reference (OpenStax Astronomy 2e)

Lecture readings are provided on this website. For additional depth, see the free OpenStax Astronomy 2e:

• Chapters 25–26: The Milky Way Galaxy; Galaxies
• Chapters 27–28: Active Galaxies; The Evolution of Galaxies
• Chapter 29: The Big Bang

• What is dark matter? It makes up ~27% of the universe, but we’ve never detected a dark matter particle directly.
• Why is there more matter than antimatter? The Big Bang should have made equal amounts — where did the antimatter go?
• Is the Hubble tension real? Local and CMB measurements of H₀ disagree by ~5σ. Systematic error or new physics?