Assuming that dark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside the Local Supercluster will pass behind the cosmological horizon. It will then be impossible for events in the Local Group to affect other galaxies. Similarly it will be impossible for events after 150 billion years, as seen by observers in distant galaxies, to affect events in the Local Group. (More)

My last two posts suggested that the average spacing between independently-originating aggressive alien civilizations is roughly 1-4 billion light years. If we can eventually get light signals from galaxies that are roughly 100 billion light years away today, this suggests that we’ll be able to dimly see the first billion or so years of the history of a few tens to hundreds of such alien civilizations. But just seeing them dimly won’t really tell us that much about them, and we may be terribly curious to know much more.

Aliens who aspire to win in the great universal meme evolution contest should seek to take advantage of this curiosity, by sending out messages about themselves and their memes. There are two obvious ways to do this. They can either sent data in light (or other fast particle) signal messages, or they can send physical emissaries that carry lots of data with them.

Over these distances, data sent by physical emissaries goes slower, and is sent directly to fewer locations, but its quantity can be far more. However, it will be harder to believe that the emissary data you receive is actually the data that was originally sent. Especially if it is passed on via several intermediary civilizations. In contrast, while less data can be sent in light signals, not only does it go faster, but one can have stronger confidence that the signal received was actually the signal sent.

The possibility that history may be rewritten is a problem not only for emissaries, but also for ourselves. In fact, the most trustworthy data on our own history might be the signals that we sent out long ago to others, which they then simply reflect back to us. By mixing up the signals that you send out with the signals you reflect back to others, you give them a modestly stronger incentive to read what you send.

To believe our reflected signals, we’d need to encrypt what we send our outgoing signals in some way to make it very hard for them to change them without corrupting them. However, if there are cryptographic hash scheme that can’t be cracked over billions of years by civilizations eager to change history, we could use this not only to trust our reflected signals, but also to let distant aliens verify that the large data they get via emissaries was actually the data that we sent out long long ago.

As with all cosmic beacons, there’s be an advantage to coordinating on where to look when to see them. Such as sending a signal right after seeing a gamma ray burst, and in the exact opposite direction so your signal follows the burst. Then listeners look for your message right after seeing a burst, and in that same direction.

Added 24Dec: As I’ve discussed before, humans cultures had separated diversity for ~1Myr, and now have much stronger  integration, but will again diversify in 1Kyr+ as we spread out among the stars. It seems a similar pattern will play out among alien civs later. They go from separated diversity for 1st ~1Byr, to much stronger integration at ~1-100Byr, but then they diversify again as they lose contact w/ each other after that.