Quick reference for rotation curves, the evidence for dark matter, accelerating expansion, and the cosmic recipe.
🕳️ Plot Summary
In the 1970s, Vera Rubin measured how fast stars orbit at different distances from the centers of galaxies. Stars far out at the edges should have been orbiting slowly, the way Neptune crawls around the Sun compared with Mercury. They weren't — they were moving just as fast as stars closer in. The only explanation that works is extra mass we can't see: dark matter, outweighing all the stars and gas by about five to one. Then, in 1998, two teams studying exploding stars found that the expansion of the universe is speeding up, driven by something even stranger, called dark energy. This activity is about how scientists read evidence for things they can't see directly.
What you'll do in this activity
- Predict how fast stars should orbit at the edge of a galaxy — then see what Vera Rubin actually measured.
- Weigh a galaxy by its spin, and discover that most of its mass is invisible.
- Line up the other witnesses: speeding galaxies in clusters, bent light, and two colliding clusters that pulled apart.
- Follow the 1998 surprise that the universe's expansion isn't slowing down — it's speeding up.
- Face the humbling bottom line: everything we can see is only about 5% of the universe.
Why it matters
Problem 5.3.1, The Galaxy That Doesn't Add Up, hands you a real-style rotation curve and asks you to explain why a galaxy's outer stars move faster than its visible matter can account for. The reasoning you practice here is exactly what that argument needs.
✅ Self-Check Before You Roll On
Check off each item as you get there. These aren't grades — they're your own signal.