**Summary**
The video explores how far humanity could spread if we mastered interstellar and intergalactic travel.
- Because the universe’s expansion is accelerating, there is a **cosmological event horizon** that limits how far we can ever reach. Light‑speed ships launched today could only reach galaxies that are currently about **16.5 billion light‑years away** (the “affectable universe”).
- This sphere contains roughly **20 billion galaxies** and a **sextillion stars**. If every Sun‑like star hosted an Earth‑like world, we could potentially occupy ~10²⁰ planets and support up to ~10³⁰ human descendants at any given time, assuming maximal proliferation.
- The actual size of our future domain scales with our travel speed and departure delay: at half light‑speed the radius halves (volume ≈ 1/8), and each year of delay loses about three galaxies from our reach. Even at 20 % light‑speed we could still access about a billion galaxies.
- Over cosmic time the observable universe will grow to ~64 billion light‑years radius, but the **ultimately observable universe**—the union of all past light‑cones of possible light‑speed explorers—could span ~160 billion light‑years.
- However, once we pass the event horizon, our settlements become **causally disconnected**: far‑flung colonies will lie beyond each other’s horizons, making communication impossible and leading to cultural divergence.
- The analysis suggests we have a generous window (≈ a billion years) to launch before losing a significant fraction of the reachable universe, but the sooner we go and the faster we travel, the larger our eventual cosmic footprint.
In short, physics caps humanity’s expandable reach at a sphere ~16.5 billion light‑years in radius (≈ 20 billion galaxies), but the exact number of worlds we could inhabit depends on when we leave, how fast we travel, and whether we can maintain contact across the growing distances.
1. The maximum extent of our potential view of the universe from the Milky Way is about 63 billion light‑years.
2. The cosmological event horizon (the limit of what we can ever reach) is currently ≈16.5 billion light‑years in radius.
3. Toby Ord defines the “affectable universe” as a sphere of 16.5 billion light‑years radius in comoving distance.
4. The affectable universe contains roughly 20 billion galaxies and about a sextillion (10²¹) stars.
5. If every Earth‑like planet around Sun‑like stars were occupied, there would be ~10²⁰ inhabited planets and up to ~10³⁰ human descendants at any one time.
6. Each year of delay in launching intergalactic settlement causes an average loss of about 3 galaxies from our reachable volume.
7. After a billion years of delay, only ~20 % of the affectable universe would be lost.
8. The era of isolation—when only the local galaxy cluster remains reachable—would begin in roughly a hundred billion years.
9. Traveling at half the speed of light reduces the affectable universe’s radius to half and its volume to about one‑eighth of that achievable at light speed.
10. Maintaining an average speed of at least 20 % of light speed grants access to on the order of a billion galaxies or more.
11. The “ultimately observable universe”—the combined past light‑cones of all possible light‑speed explorers—spans nearly 160 billion light‑years across.
12. A signal sent today can only reach points inside our future light‑cone; regions outside are forever inaccessible because nothing exceeds light speed.
13. The cosmological event horizon can be crossed outward but not inward, analogous to a reverse black‑hole horizon.
14. To reach a destination, an explorer must lie within that destination’s cosmological event horizon, which is also ≈16.5 billion light‑years in the modern universe.
15. Galaxies recede due to cosmic expansion; at a certain distance their recession speed equals light speed, defining the Hubble horizon.
16. In our accelerating universe the Hubble horizon is currently moving toward us, allowing us to eventually cross our own Hubble horizon.
17. Using comoving coordinates and a conformal spacetime diagram makes it easy to determine which light‑speed signals can reach us at any future time.
18. The past light‑cone expands as we age, while the future light‑cone shifts upward, shrinking the region we can still access.
19. The cosmic expansion was initially slowed by gravity and later accelerated by dark energy.
20. Dark energy’s acceleration causes the boundary of superluminal expansion (the Hubble horizon) to approach us over time.