# Can you communicate what’s beyond the edge of the observable universe?

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Radius of the observable universe is 46 billion light years. Say you have two civilizations 26 billion light years apart. Could they send maps of their observable universe to the other civilization so in 26 billion years they know what was beyond the edge of their observable universe?

In: Physics

Long answer: Also yes, but with a lot of caveats.

The issue: a signal 26 billion lightyears away will take far longer than 26 billion lightyears to arrive. The universe is expanding, so the distance increases while the signal is travelling resulting in farther to travel.

There is something called the particle horizon. This is the distance where the expansion of the universe exceeds the ability of a light speed signal from ever crossing the distance. No action from a particle (even the generation of light) will ever interact with another particle that distance away or farther.

Watch PBS Spacetime. Lots of videos covering this sort of thing. Very well done.

This would not be possible if communication is limited by the speed of light. The universe is expanding and accelerating, and galaxies that are super far away will be out of the visible universe of each other before a light speed communication could make the trip. The information would never make it. However, in some sci fi scenario where communication can happen faster than the speed of light (like the ansible in Ender’s Game) then yes it could happen.

In this specific case, no; universal expansion is too great at such distances. The universe is expanding at ~20km/s per million light years, meaning that two points 26 billion light years from each other would be moving apart from each other at approximately 520,000,000 m/s. Speed of light is just shy of 300,000,000 m/s.

The cosmic horizon is essentially constantly shrinking; with every moment the fraction of the universe that we will ever be able to interact with gets smaller and smaller as expansion continues.

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“Everything that is so far away and moving away from us fast enough such that it is physically impossible for us to ever reach or observe it”