The text discusses the boundaries of our universe's visibility, known as horizons. These horizons are determined by the speed of light and the expansion of the universe.
The observable universe is currently 46.5 billion light years in radius. The Cosmic Microwave Background (CMB) is the furthest light we can see, having traveled for nearly the entire age of the universe (13.7 billion years). However, the CMB is not the absolute limit of our view because the universe was opaque at earlier times and greater distances.
The particle horizon is the absolute limit of the observable universe. It represents a finite portion of the compacted universe at the beginning of time. However, this horizon expands over time, and there's a limit to the amount of universe we'll be able to see, even if we wait infinite time.
The Hubble horizon is a sphere that surrounds us where the recession velocity equals the speed of light. It's currently 14.5 billion light years away. Light from beyond the Hubble horizon can reach us because for the first several billion years after the big bang, the Hubble horizon was expanding.
The cosmological event horizon is the boundary beyond which no signal can ever be received. It started out much larger than the Hubble horizon—63 billion light years in comoving distance—but it shrinks over time. At some point—in 10 billion years—the cosmological event horizon will merge with the collapsing Hubble horizon.
After this point, we'll never be able to see new events from beyond this horizon. However, the universe won't suddenly go dark. Light from the most distant points that we will ever see will cross the shrinking Hubble horizon just in the nick of time. This will become the final size of our particle horizon, allowing us to see things that are currently 63 billion light years away.
This is the extent of humanity's potential to see the universe. There is one way we can see further—and that's by traveling as fast as possible to escape our collapsing Hubble horizon.
1. The observable universe is approximately 100 billion light years wide, a tiny fraction of the total extent of space [Document 1].
2. The universe has absolute limits to what we can explore or send signals to, and a different limit to what we can witness [Document 1].
3. Light doesn't travel instantaneously, meaning when we look into the distance, we're seeing objects as they were in the past [Document 1].
4. The most distant light we can see is from the cosmic microwave background (CMB), which has been travelling to us for almost the entire age of the universe—13.7 billion years [Document 1].
5. The CMB is the practical limit of our view because at earlier times and greater distances, the universe was opaque [Document 1].
6. The observable universe is 46.5 billion light years in radius because if you froze the expansion now, that would be the current distance of the furthest objects we can see [Document 1].
7. The particle horizon is the absolute limit of the observable universe [Document 1].
8. The universe has been expanding, so at earlier times, those regions were smaller. This expansion started just a few billion years ago [Document 1].
9. The lightcone, or the boundary of light that has had time to reach us, converges to a point at the particle horizon [Document 1].
10. The fact that the expansion of our universe is accelerating means there’s an absolute upper limit on how much of the universe we can ultimately see [Document 1].
11. In an expanding universe, more distant points seem to be moving away from us more quickly. A point that’s far enough away will be moving away from us at the speed of light [Document 1].
12. There’s a sphere that surrounds us where the recession velocity equals the speed of light. It’s called the Hubble horizon, and it’s currently 14 and a half billion light years away [Document 1].
13. Light from beyond the Hubble horizon can reach us because for the first several billion years after the big bang, the Hubble horizon was expanding [Document 1].
14. The cosmological event horizon is the boundary beyond which no signal can ever be received, no matter how long we wait [Document 1].
15. The cosmological event horizon started out much larger than the Hubble horizon—63 billion light years in comoving distance, but it shrinks over time, because light released at later times has less time to get to us [Document 1].
16. At some point—in 10 billion years give or take—the cosmological event horizon will merge with the collapsing Hubble horizon [Document 1].
17. At that point, we’ll be able to see things that are currently 63 billion light years away [Document 1].
18. When we first get close to our full panorama of the universe in about 10 billion years, the galaxies will shine mostly in infrared light [Document 1].
19. After a few hundred billion years, even the most energetic photons will be stretched to the point that we’d need a radio antenna larger than our contracted Hubble horizon to see them [Document 1].
20. The sky would finally be dark, marking the extent of humanity’s potential to see the universe [Document 1].