Loading module…
What you will learn in Cosmology, Dark Matter and Dark Energy
1. Space is expanding
Not galaxies flying apart through space. The grid stretches and the galaxies sit still on it, and almost everything strange about cosmology follows from getting that the right way round.
2. Redshift
The light did not leave red. It left the colour it left, and the space it crossed grew while it was in transit — which is why cosmological redshift is not a Doppler shift and does not obey a speed limit.
3. Hubble's law, and the tension
The expansion rate has been measured twice, carefully, by methods that share nothing. The answers disagree by five sigma, and averaging them would be the one thing you must not do.
4. How far is far?
Ask how far away a distant galaxy is and there are five correct answers, disagreeing by more than a factor of ten. One of them decreases with redshift.
5. The observable universe
It is 13.8 billion years old and about 46 billion light-years in radius. Both numbers are right, nothing moved faster than light, and the gap between them is the whole of cosmology in one ratio.
6. The oldest light
For 380 000 years the universe was an opaque plasma. The moment it cleared, the light that was there streamed off in every direction, and it is still arriving.
7. The missing mass
Four independent gravitational detections; zero non-gravitational ones. Dark matter is an inference with strong evidence and no direct detection, and both halves of that sentence matter.
8. The accelerating universe
Two teams set out in the 1990s to measure how fast the expansion was slowing down. Both found it speeding up, and neither believed it first.