“Look at that point, that’s here, that's us. In it, every human being who has ever existed lived their life. The sum of all our joys and sufferings, every king and peasant, every hero and coward, every creator and destroyer of civilisations, every saint and sinner in the history of our species lived there. On a speck of dust suspended in a ray of sunlight”.
These were Carl Sagan's first words on the program Cosmos, and these were precisely the words that come to mind when I think about the universe.
🌌A temporal Frontier
When I think about the universe, I imagine something so vast that it feels infinite. Yet only a few centuries ago, many believed the end of the world lay just beyond the Atlantic Ocean, somewhere past the legendary Cabo de Finisterre (curiously close to my city). Then, America was discovered, revealing the New World. And now, centuries have flown by, but we still face another kind of frontier, invisible but very real.
The observable universe extends roughly 46 billion light-years in every direction. Beyond that limit, the cosmos continues, yet becomes forever unreachable. This border is not made of matter or energy; it is made by time.
The reason lies in the universe's accelerated expansion. From the very beginning, the cosmos has not been expanding into something. Instead, space itself stretches. The galaxies are not travelling through space; they are carried by it.
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Imagine dots painted on a balloon. As the balloon inflates, the dots move apart, not because they travel, but because the very fabric beneath them grows.
Some galaxies are already so distant that their light will never reach us. The space between them and us expands faster than light can travel across it. This is why astronomers refer to a cosmic bubble. Everything we see, from galaxies and clusters to the faint background radiation, belongs to the portion of the universe whose light has had enough time to reach us since the Big Bang.
🔚The Edges of the Known World
Beyond that bubble, the universe continues, perhaps endlessly, but disconnected from our reality. Even if we waited for eternity, we would never receive a single photon from those distant places. They are, quite literally, beyond existence as we can define it.
“What does it mean to know the universe if all we can ever hold is a fragment of its light?”
However, this bubble is not the same for everyone. If an observer were positioned elsewhere in the cosmos, their own observable universe would be centred on them, enclosing regions we could never see from here.
Every point in space lives inside its own horizon of the visible. If we could instantly travel to a galaxy twenty billion light-years away, we would observe new regions of the universe but also lose others. Knowledge would not grow; it would simply change perspective.
And this horizon keeps changing. With every passing second, the cosmos grows a little more, and some galaxies quietly slip beyond the edge of visibility. They do not disappear; they simply become unreachable, like ships fading into an ocean that grows faster than they can sail.
In the far future, even the closest galaxies will vanish from sight. The night sky will empty, the traces of our origin will fade, and the universe will finally turn silent, infinite, and invisible.
So the only question left is, does curiosity itself have a horizon?
