Outer Limits

Emissaries from the cosmic deep

EDIT: THIS ESSAY IS WRONG. Thanks to @noncompleteness for pointing out the lightspeed logicfail. I’m dumb. Also, never use an LLM as your only editor.

Each year, a few of the two trillion galaxies in our observable universe vanish beyond the cosmological horizon, the reach of their light cut off by the accelerating expansion of space. They pass into the unobservable universe, a place of chillingly vast—maybe infinite—size.

Since we can’t see the unobservable universe, we know almost nothing about it. But almost nothing isn’t nothing. Photons from the early universe swarm all around us; when we look at them closely, they tell us the universe everywhere has a samey structure, on large scales. So we have reason to think the unobservable universe is likewise samey.

More controversially, cosmologists in 2008 published research showing that galactic clusters seem drawn to a specific region outside the visible universe. This research has been heavily challenged, but if true it suggests that structures much larger than any we’ve seen might lie beyond our causal horizon.

But there’s a much more valuable source of knowledge about the unobservable universe than ancient photons or gravitational effects: aliens. Any explorers we meet from other civilizations will hail from a different observable universe, one that overlaps in part but not entirely with ours. The farther away their origins, the more knowledge they’ll have about the regions beyond our observable universe.

When these explorers share photos of the regions beyond our ken, what will we see? Voids of untold enormity? Supercluster filaments of migraine-inducing immensity? Relic islands of primordial antimatter, surrounded by violent seas of matter-antimatter annihilation? Bizarrely linear patterns of galaxies formed by cosmic strings—or even the strings themselves, their one-dimensional yet hyperdense nature twisting light and making double-images of the stellar background? Or how about domain walls, those two-dimensional defects in spacetime that might have been created during inflation, and which might paint the distant sky with surreal shimmers and lensing effects?

But that’s not the half of it. The civilizations from which these explorers come may have learned things from civilizations beyond their causal horizons. So, through a game of telephone that connects partially overlapping volumes of spacetime, we might be able to see much more of the universe than physics would otherwise allow. If we’re lucky, our greatly expanded observable universe might have some fun surprises.