Age of the universe: 13.8 billion years
Observable universe
- Radius = 46.5 billion light years
- Light from more distant places hasn’t had the time to reach us
- Space is expanding
- 10^11 galaxies, each with 10^11 planets
- Near certainty that many will be habitable
Cosmic event horizon: 16 billion light years
- Because of accelerating expansion
- Approaches asymptotically
the orbits of Oort Cloud comets extend halfway to the nearest stars.
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