Module 4: Galaxies & Cosmology
Weeks 13–15 | The large-scale universe
Why this module matters
We have studied stars as physical systems. In this final module, we zoom out: stars live inside galaxies, galaxies move inside dark matter halos, and the largest maps we can build reveal an expanding universe with a history.
The throughline is observe → model → infer. We observe gas, dust, motions, redshifts, deep fields, and the cosmic microwave background. We model those signals with gravity, radiation, nuclear physics, and expanding spacetime. Then we infer galaxy histories, hidden mass, large-scale structure, and the origin of the first nuclei.
Learning objectives
By the end of this module, you will be able to:
- Interpret galaxies as evolving ecosystems of gas, dust, stars, remnants, feedback, and gravity
- Use orbital motion, cluster collisions, and redshift surveys to infer hidden mass and gravitational structure
- Explain how redshift, expansion, the cosmic microwave background, and light elements support the hot Big Bang model
- Connect stellar evolution to galactic chemical enrichment and Big Bang nucleosynthesis to the origin of hydrogen and helium
Reading arc
- Galaxies as Ecosystems — gas, dust, stars, remnants, and feedback cycle through galaxies.
- The Dynamical Universe — gravity turns motions into mass measurements, revealing dark matter and the cosmic web.
- The Universe as an Evolving Spacetime — redshift, expansion, deep fields, the CMB, and light elements reveal cosmic history.
Lectures
Lecture Readings
Lecture 1: Galaxies as Ecosystems
April 23, 2026Galaxies are evolving ecosystems: gas becomes stars, stars return energy and enriched material, and feedback reshapes the next generation.
Lecture 2: The Dynamical Universe
April 28, 2026Milky Way structure, external galaxies, dark matter, supermassive black holes, clusters, and large-scale structure as gravitational inferences from motion.
Lecture 3: The Universe as an Evolving Spacetime
April 30, 2026Redshift as a record of expansion, the distance ladder, deep fields, the hot Big Bang, and Big Bang nucleosynthesis as the origin of the first nuclei.
Required Textbook Reading
- Chapter 21 + Chapter 22 + Chapter 12.3: Interstellar gas, dust, and star formation
- Chapter 26 + Chapter 27.1–27.3: The Milky Way, external galaxies, and Hubble’s law
- Chapter 30 + Chapter 32.2–32.4: The Big Bang and the expanding universe
- What is dark matter? It makes up ~27% of the universe. 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?
- What happened before the Big Bang? Our physics breaks down at t = 0.
- Is the Hubble tension real? Local and CMB measurements of H₀ disagree by ~5σ. Systematic error or new physics?
- Why do galaxies spin faster than expected? Dark matter or modified gravity (MOND)?