This boundary is the cosmological horizon, effectively the edge of the observable universe. It represents the maximum distance from which light has had time to travel to Earth since the universe began, approximately 13.8 billion years ago. Because the speed of light is finite, looking out into space means looking back in time. However, there is a physical limit to how far observers can see, dictated by the age of the universe and the speed of light.
What makes this boundary dynamic—and fatal for observing distant galaxies—is the expansion of the universe. The universe is not just expanding; its expansion is actively accelerating, driven by a phenomenon physicists call dark energy. As the fabric of space stretches, it pushes galaxies farther apart. For galaxies that are already at immense distances, the sheer volume of space expanding between them and Earth means they are receding at a rate faster than the speed of light.
This does not violate Einstein's theory of relativity. The galaxies themselves are not traveling through space faster than light; rather, the space between the galaxies is expanding.
When a galaxy is pushed far enough away, it crosses what is known as the cosmological event horizon. Any light that the galaxy emits after crossing this boundary will never be able to cross the rapidly expanding gulf of space to reach Earth. The expansion of the universe outpaces the light trying to traverse it, much like a swimmer trying to swim upstream against a current that is moving faster than they can paddle.
Observers on Earth can still see the light these galaxies emitted before they crossed the horizon, but even this light is severely affected. The expansion of space stretches the light waves, shifting them toward the red end of the spectrum. Over time, this "redshift" stretches the light into infrared, then microwaves, and eventually faint radio waves, until the galaxy fades into complete undetectability.
Because of this accelerating expansion, the night sky is slowly emptying out. Trillions of years from now, any beings looking up at the cosmos will see only the stars within their own gravitationally bound local group of galaxies. The rest of the universe will have slipped beyond the cosmological horizon, leaving behind an incredibly dark and isolated sky.