On a 2D sphere expanding uniformly, no point is the true center. Galaxies are not moving through space; the space between them is stretching.

| Concept | Key Physics | Typical Observation | |---------|-------------|----------------------| | Rotation curve | Newtonian gravity + DM halo | HI 21 cm line | | Tully-Fisher | L ~ v⁴ | Optical + Hα rotation | | Oort limit | Poisson eqn + vertical motions | Stellar number counts + proper motions | | M/L variation | IMF + star formation history | Colors + spectral indices | | AGN feedback | Jet heating vs. cooling | X-ray cavities + radio lobes | | Lensing | GR deflection | Multiple images, Einstein rings |

The Law of Conservation of Energy states that energy cannot be created or destroyed, only converted from one form to another.

Solution