The question behind the largest maps
Galaxies are the course’s first systems large enough to be both laboratories and tracers. Their light reveals stars, dust, cold gas, hot plasma, and feedback; their motions reveal gravitating mass; their redshifts locate them in an expanding spacetime. A redshift survey can therefore map the cosmic web, but the map is not the explanation. The task is to connect each measurement to a physical model and ask what history follows.
The module keeps the same grammar used for stars:
observable → model → inference
The observables become more indirect as the scale grows. Multiwavelength light can identify a galaxy’s ingredients and current activity. Orbital motion and lensing can weigh mass that emits no light. Standard-candle brightnesses and spectral redshifts can constrain distance and the changing scale factor. The microwave sky and primordial element abundances preserve evidence from a past that no telescope can observe directly.
Build outward, one model at a time
Galaxies as Ecosystems treats a galaxy as a changing baryon cycle. Gas falls in, cools, forms stars, and is reheated, enriched, or expelled by feedback. Different wavelength bands isolate different components, so a galaxy image becomes evidence for its present state and past history rather than a portrait of a fixed object.
Weighing the Invisible uses gravity as the translator. For an approximately circular orbit, connects speed and radius to enclosed mass. Flat rotation curves, cluster lensing, and the cosmic web show that luminous matter is not the full gravitating mass budget. The agreement among these independent measurements makes dark matter a convergent inference.
The Expanding Universe completes the distance ladder and uses redshift as a record of the scale factor. Hubble’s law organizes the low-redshift relation between distance and recession, while deep fields turn distance into lookback time. The model is expansion of space, not ordinary motion through a static stage; at high redshift, is no longer the interpretation.
What the Universe Is Made Of asks what governs that expansion. The Friedmann equation relates the expansion rate to energy density and spatial curvature. The cosmic microwave background and distant Type Ia supernovae constrain the resulting budget: ordinary matter, dark matter, dark energy, and the near-flat geometry of space.
The First Three Minutes runs the expansion backward. A hot, dense early universe predicts primordial light-element abundances and a relic microwave glow. That closes the scale ladder: galaxy structure records gravity acting over cosmic time, while helium and the microwave background preserve the conditions from which that history began.
Keep the limits visible
Each inference has a domain. A galaxy’s classification is not an evolutionary sequence; a circular-orbit mass estimate is an approximation to a real disk; the local motions of bound galaxies do not simply follow the Hubble flow; and the low-redshift form of Hubble’s law is not a universal velocity rule. The point is not to avoid inference. It is to state the model and its conditions before treating a cosmic map or number as a physical claim.