As David Gerrold puts it, when it comes to interstellar empires Control depends upon communication.
If you are keeping your novel constrained to realistic physics, it is not fair to exceed the speed of light. No FTL starships and no Ansibles. Which will more or less limit your empire to the bounds of the Solar System because of timelag. Travel and communication with with other stars will take years to decades to hundreds of thousands of years to arrive. Communication within the orbit of Neptune will be at a maximum travel time of about 8 hours (though that can increase to up to two years if you decide to include the Oort cloud).
No way around it: a galactic empire is going to need FTL starships and/or FTL communication.
As a vague rule of thumb, figure that the maximum time allowed to send a message from the central capital to a colony on the rim of the empire should be no more than about 15 weeks. This is about the lag-time of the old Mongol Empire. So the radius of the empire and the speed of the courier starships or superluminal radio should be adjusted so the results is 15 weeks or less.
The three main ways communication methods used in various science fiction universes are:
- Communication by faster-than-light radio
- Communication by faster-than-light courier starships
- Faster-than-light radar detection of hostile star fleets
If the science fiction author is postulating the existence of either FTL travel and/or FTL communications things get more complicated. Does FTL travel exist yes/no? Does FTL communication exist yes/no? And if both FTL travel and FTL communication exists, which if either is faster? And faster by how many orders of magnitude?
The answer to these questions will have major implications to your science fiction universe in general and your galactic empire in specific.
If you have both and both have the same speed, which one you will use will depend upon whether you want to move matter or move information.
As I mentioned before, the maximum speed of starships and the maximum speed of communications limits the maximum size of the empire. If the travel time of the intel and the travel time of the armed response is too high a total, the Empire will not be able to prevent a rebellious planet on the rim from leaving the empire. This is assuming that the travel time increases with the distance, all bets are off if you have something weird like instantaneous communication.
Bandwidth is important as well. The Capital will have the data to make a reasoned policy if you can transmit to them several terabytes of situational reports, not so much if all you can send is a 140 character tweet.
If ansibles are much faster that starships, the situation will be much like modern-day Navy vessels talking to home port by radio. You will be not be required to send the ship's engines along with the message. Note that "much faster" does not necessarily mean "instantaneous." In classic Star Trek, when the Starship Enterprise was in deep space, a Subspace Radio message might take a couple of weeks to travel to Starfleet Command. But the Enterprise would take months to make the same trip. Subspace radio was only "real time" if you were closer than a few tens of light-years or so.
If there are ansibles but no FTL starships, you will have a lively interstellar Internet, but the massive overhead of slower-than-light starships will restrict interstellar travel to only such people and material objects that absolutely positively must be transported.
And it is possible to have ansibles that provide instantaneous communication, regardless of distance.
In Charles Stross' IRON SUNRISE, the most valuable things are packages of entangled quantum dots (called a "causal channel"), used for FTL communication via Bell's Inequality (yes, I know that is impossible, it's science-fiction OK?). Each dot can instantaneously transmit one bit of information, and then is worthless.
They also have the fascinating twist that the dots must be transported slower than light or they are ruined. They are shipped by Starwisp, taking years to transport between stars.
In the Renegade Legion universe, the Terran Overlord Government (TOG) alone has the technology for "Very Large Communication Relays" (VLCA). These are titanic FTL communication installations (meaning they will not fit inside a standard starship, you need one that is outrageously huge) that have a range of pretty much anywhere inside the entire Milky Way galaxy.
Their rivals the Renegades have to make do with pathetic P-Comm FTL communication. P-Comms are lucky if it can reach a neighboring star system. VLCAs have a range that is about 100,000 times greater.
Such is the overwhelming advantage of TOG having VLCAs, that for the Renegades such installations are pretty much automatically the primary military target in a given star system. If the Renegades can destroy the VLCA, the TOG loses its unfair communication advantage and the military odds become more equal. As long as the VLCA exists the TOG has it all their own way.
Also not that since the VLCAs are such a powerful strategic asset, they are more or less a government monopoly not a private corporation.
In the BattleTech universe, ansibles are a monopoly of a private corporation called ComStar. The corporation arose out of the ashes of the Star League Department of Communications, when the Star League died in the Successor wars. ComStar is transnational, beholden to none of the Successor States. They jealously guard their monopoly and the independence of their FTL relay stations. States that threaten ComStar could have their ansible links cut off, no long able to engage in FTL communication or trade. ComStar also has their own military.
So it appears that the Renagade Legion Terran Overlord Government has an ansible monopoly because TOG is close to being a galactic empire, while the BattleTech ComStar independent corporation has an ansible monopoly because there is no galactic empire.
But whether government or private monopoly, anybody stupid enough to deliberately attack an ansible relay station had better be prepared for the prompt arrival of a monopoly battle fleet who will hunt you down like an animal. The monopoly takes such attacks very personally, since you are more or less threatening their very existence. Especially so if the relay is in an remote area, since in such places there is little or no redundancy.
If there are starships but no ansibles, or if the starships are much faster than ansibles, you will see the creation of ultra-fast manned message-courier FTL starships or unmanned FTL message drones. Much like the Express Boat Network in the Traveller role playing game or the unmanned Courier Drones from Starfire.
Remember that historically the postal service and telegraph coexisted. The telegraph was much faster but the mail was much cheaper (with telegraph the cost was per word of message). Physical mail didn't start to die until telegraph messages basically became free, when the telegraph morphed into Internet email.
For some ideas about how a planetary and interstellar postal service can help a fallen civilization bootstrap its way out of a dark ages, read David Brin's novel The Postman.
Ever since postcards went out of style people have wanted their mail to be sort of private, only to be read by the intended recipient. But they don't bother with extraordinary privacy methods unless the missive contains something about money, ultra classified documents, or something you'd rather your spouse didn't find out.
Electronically the current state of the art is email using public-key encryption. With this, the contents of the are public, but in theory only the intended recipient can decrypt and read the message. But this is vulnerable to advances in cryptanalysis and Man-in-the-middle attacks.
Sometimes the old inconvenient ways are more secure. A courier can deliver a physical message, sometimes inside a briefcase handcuffed to the wrist (though a sufficiently determined enemy can circumvent that). The message container can be rigged to destroy the message if tampered with, or to also destroy the tamperer as well with a booby-trap if the message sender is not playing around. In some science fiction stories intelligence agencies and governments keep top secret information stored only in the form of written paper, since there is no such thing as an unhackable computer.
Sometimes the message is gimmicked to self-destruct after reading. Venerable people may remember the TV show Mission: Impossible where the IMF leader would covertly receive an audio tape from a faceless voice giving the mission briefing, ending with "This message will self-destruct in five seconds" followed by the tape turning into ash. This was a running gag in the cartoon Inspector Gadget. In Isaac Asimov's FOUNDATION series, the message enclosed in a Personal Capsule was written on paper that would vaporize ninety seconds after it was exposed to air.
But sometimes something more durable is desired. In Asimov's THE END ETERNITY they write messages on metal foil instead of paper. Paper wears out too soon, when you have to store it for millennia.
There is another "communication-like" consideration. If you have FTL starships, and you want your universe to contain FTL starship battles and also have interstellar empires, you need Strategic FTL Sensors. This means the communication-esque function known as "detection" must be capable of spotting hostile incoming starships early enough for the defenders to muster their defenses. Otherwise FTL starship battles will only occur by mutual agreement. And there will be no interstellar empires because enemy FTL starships will be able to drop a planet-buster on all the empire's planets before the empire knows what is happening. Meanwhile the empire's FTL starships will be returning the favor.
As discussed in the Strategic Sensor link, the two traditional methods are Jump Points "choke points" at some distance from habitable planets and FTL "radar" that can detect enemy ships in time to send your interceptors.
Propagating data via physical books (perhaps printed on bamboo paper), USB flash drives containing eBooks, or other material non-cloud non-broadcasty item is much slower than distribution by the local equivalent of the internet.
But it has advantages:
- It does not require the existence of an FTL internet (in case the science fiction author does not want their empire having that high a tech level, or FTL Wifi is forbidden by the laws of physics)
- It does not require the local infrastructure of the unindustrialized newly colonized planet to be high enough to support an FTL internet. A tech level equivalent to 2000s USA will do for USB flash Drives. Or a tech level of 1400s Europe for printed books.
- It can be (slowly) propagating by unorganized random starships carrying the books, after the galactic empire declines and falls
In the Dark Ages scenario, isolated planets will have their local languages drift and change over the centuries. The language used on the books may become sort of a Lingua franca, much like the Roman Empire language Latin did during the medieval era after Rome fell.