Weighing the Invisible
Section 1 of 11
Concept Throughline
By the end of this reading, you will be able to:
Concept Throughline
Gravity is the universe’s scale. If something moves in an orbit, its motion tells us what mass is pulling on it. When the motion does not match the light, the universe is telling us that light is not the whole mass budget.
This reading is about dynamics: how motion reveals mass. You already know the core idea from binaries and orbits. In Module 2, binary stars let us measure stellar masses. In Module 3, gravity battled pressure inside stars. Now the same gravitational reasoning moves outward to galaxies, clusters, and the cosmic web.
The method is still observe → model → infer. We observe positions, velocities, redshifts, or lensing patterns. We model them with gravity. Then we infer masses, dark matter, compact objects, and the architecture of large-scale structure.
Motion and lensing
Orbital speeds at a given radius — S-stars whipping around the Galactic center, gas and stars circling a galaxy’s disk — and the bending of background light around clusters.
Gravity sets the motion
A circular orbit obeys : the speed at radius fixes the total mass enclosed inside it. Lensing weighs all the mass along the line of sight, luminous or not.
The total gravitating mass — including what emits no light
The mass the motion demands exceeds the mass we can see. The excess is dark matter: gravitating material that emits, absorbs, and scatters no light. We weigh it precisely because we cannot see it.
