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Galactic empires are a common trope used in science fantasy and science fiction, particularly in space opera. Many authors have either used a galaxy-spanning empire as background, or written about the growth or decline of such an empire. The capital of a galactic empire is frequently a core world or home world.
Most of these galaxy-spanning domains depend on some form of transportation capable of quickly or instantly crossing vast cosmic distances (usually measured in light-years), many times faster than could photons at light speed. These invariably require some type of propulsion or displacement technology forbidden by Einstein's Theory of Relativity, or that otherwise relies on theories that circumvent or supersede relativity. (See: warp drive; hyperspace; Alcubierre drive.)
The term "galactic empire" has, no doubt because of association with the Empire from Star Wars, gained an unfavorable reputation. However, the Galactic Empires from Foundation and the CoDominium universe are relatively benign organizations; indeed, much of the plot of the Foundation series revolves around the issue of who can best and most quickly revive the fallen galactic empire, it being taken for granted that this is a positive and worthy aim.
In many cases, the term "galactic empire" is misleading as it suggests a galaxywide empire. This is likely due to the once common tendency for fiction to either confuse galaxy and star system or to simply underestimate the size of the galaxy. While some of the noted fictional empires tend to encompass a large portion of the galaxy, many other empires may be classified as interplanetary or interstellar empires since they encompass only a local group of star systems.
Poul Anderson makes the point that the declining Empire depicted in his Dominic Flandry series does not span the entire Galaxy but only a fraction of one of its Spiral Arms - and still, it is vast beyond humans' ability to truly comprehend, and is in the process of collapsing under its own weight.
Galactic Empires are in some cases consciously modeled on historical Earth-bound empires. Isaac Asimov stated explicitly that the Galactic Empire whose fall is depicted in his Foundation Series is modeled on the Roman Empire. Also Poul Anderson's Dominic Flandry consciously compares the space empire which he serves with the Roman Empire, even to tracing out the space equivalents of the Roman Principate and Dominate phases.
Frank Herbert's Dune recalls the late medieval Holy Roman Empire, but also the Byzantine and Islamic empires, especially given the role of hitherto disregarded desert-dwellers, who, due to a powerful new religion, expand to topple an old empire and build a new one. For example, the Egyptian Canadian commentator Khalid M. Baheyeldin enumerated the obviously Islamic concepts and references appearing in "Dune" and the similarities between the career of Herbert's Paul Atreides and that of the Islamic prophet, Muhammad.
Another notable example of a galactic empire would be the Imperium of Man from the Warhammer 40,000 universe, which is a theocratic industrial and militaristic totalitarian regime that does in fact span almost the entirety of the Milky Way Galaxy, but is losing territories due to unending conflict with various alien races and rebel factions.
In Ursula K. Le Guin's Hainish Cycle, the interstellar entity known as "The League of All Worlds" and later as "The Ekumen" is in possesion of the ansible, making possible instantaneous interstellar communications, and can send instantaneous unmanned ships carrying bombs to another planet - but since living beings can't survive such travel, humans are limited to relativistic speeds. Correspondingly, this organization - though on occasion waging war across interstellar distances - is more loose than a true empire.