“The history of astronomy is a history of receding horizons.” -Edwin Hubble
As you look to greater and greater distances, you’re looking back in time in the Universe. But thanks to dark energy, what we can see and access today isn’t always going to be accessible. As galaxies grow more distant with the accelerated expansion of the Universe, they eventually recede faster than the speed of light. At present, 97% of the galaxies in the Universe aren’t reachable by us, even at the speed of light.
The observable (yellow) and reachable (magenta) portions of the Universe, which are what they are thanks to the expansion of space and the energy components of the Universe. Image credit: E. Siegel, based on work by Wikimedia Commons users Azcolvin 429 and Frédéric MICHEL.
But that isn’t the same as losing information. As a galaxy crosses over the horizon, its information never disappears from the Universe connected to us entirely. Instead, it gets imprinted on the cosmic horizon, the same way that information falling into a black hole gets imprinted on its event horizon. But there’s a fundamental difference between a black hole’s decaying horizon to the cosmic horizon’s eternal persistence, and that makes all the difference.
When something falls into a black hole, the information is preserved on the surface of the event horizon. That’s analogous to what happens to a galaxy pushed over the cosmic horizon, and everything is still okay. Image credit: ESO, ESA/Hubble, M. Kornmesser.