Module 3: Galaxies & Cosmology
Weeks 14–16 | Zooming out from our galaxy to the whole universe
Why this module matters
Module 2 took us inside individual stars. Module 3 pulls the camera back to the largest scales: the Milky Way we live in, the hundreds of billions of galaxies we live among, and the 13.8-Gyr-old universe that contains them. Along the way we meet the three hidden things that define modern cosmology — dark matter, dark energy, and the origin of the elements — and we climb the cosmic distance ladder from parallax out to the cosmic microwave background.
Every rung of the ladder, and every inference we make about dark matter and dark energy, is read from the light of stars. This module is the payoff of Module 2: the stellar physics you learned is what lets you measure the universe.
Learning objectives
By the end of this module, you will be able to:
- Describe the structure of the Milky Way and explain how flat rotation curves imply dark matter
- Use stellar orbits around Sgr A* to weigh a supermassive black hole with Kepler’s third law
- Classify galaxies on the Hubble sequence and explain active galactic nuclei as SMBH-accretion signatures, bounded by the Eddington limit
- Climb the cosmic distance ladder — parallax → main-sequence fitting → Cepheid variables → Type Ia supernovae → Hubble’s law — and state which kind of star anchors each rung
- Apply Hubble’s law \(v = H_0 \, d\) to convert redshifts into distances and estimate the Hubble time
- Explain how the 1998 Type Ia supernova results established cosmic acceleration and the existence of dark energy
- Describe the three pillars of Big Bang evidence (Hubble expansion, the CMB, and primordial H/He/Li abundances from BBN) and state the cosmic Ω-budget (~5% baryons, ~27% dark matter, ~68% dark energy)
- Identify the astrophysical origin of every element in your body
Lecture Readings
These five lecture readings are the primary required materials for Module 3. Each one corresponds to one class session (Mon or Wed); dates are in the course schedule.
Lecture 22: The Milky Way and the First Hidden Thing
April 22, 2026How we map a galaxy we live inside, and why its rotation demands that most of its mass is invisible.
Lecture 23: The Galaxy Zoo
April 24, 2026The Milky Way is one galaxy in a universe of hundreds of billions. This reading classifies what we see, meets the supermassive black holes that turn galactic centers into lighthouses, and previews the structure the distance ladder will unlock.
Lecture 24: The Distance Ladder and Hubble’s Law
April 27, 2026The full cosmic distance ladder — parallax, Cepheids, Type Ia supernovae — and the velocity-distance relation that launched modern cosmology. Ends with the Hubble tension, one of the field’s hottest open problems.
Lecture 25: Type Ia Supernovae and Dark Energy
April 29, 2026The full stellar-physics story of Type Ia supernovae, why they are standard candles, and how in 1998 they forced us to add a mysterious ~70% of the universe — dark energy.
Lecture 26: The Big Bang, the CMB, and the First Three Minutes
May 4, 2026The cosmic microwave background, Big Bang nucleosynthesis, and the complete cosmic recycling loop that ties the origin of the elements to the distance ladder and the life cycles of stars.
Module 3 Schedule at a Glance
| Date | Reading | Topic |
|---|---|---|
| Wed Apr 22 | L22 | The Milky Way and the First Hidden Thing (dark matter) |
| Fri Apr 24 | L23 | The Galaxy Zoo, quasars, and the cosmic web |
| Mon Apr 27 | L24 | The distance ladder and Hubble’s law |
| Wed Apr 29 | L25 | Type Ia supernovae and dark energy |
| Mon May 4 | L26 | Big Bang, CMB, BBN, and the first three minutes |
| Wed May 6 | — | Final-exam review (no new reading) |
Mon Apr 20 is the Module 2 exam. Fri Apr 24 is a lecture day this week (exception to the usual Mon/Wed pattern) so that Wed May 6 can be reserved for final-exam review. Fri May 8 is the comprehensive final exam (10:30 AM – 12:30 PM).
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 and Quasars; The Evolution of Galaxies
- Chapter 29: The Big Bang
- What is dark matter? It makes up ~27% of the universe, but no direct detection has confirmed a candidate particle.
- What is dark energy? ~68% of the universe by energy density. Cosmological constant, quintessence, or modified gravity? DESI/Euclid/Roman/LSST aim to pin it down.
- Is the Hubble tension real? Local and CMB-derived values of \(H_0\) disagree by ~5σ. Systematic error or new physics?
- Why is there more matter than antimatter? The Big Bang should have made equal amounts — where did the antimatter go?