The Milky Way, as our galaxy is known, has two major and several minor spiral arms, as shown in the image below.
A reference to Charles Darwin's theory of evolution, which posits that humans are descended from apes.
Adams refers to Jesus of Nazareth, the central Christian religious figure whose message was somewhat more complicated than everyone just being nicer to each other. Adams himself was not religious, describing himself as a “radical atheist”, although he was fascinated by religion due to its huge influence on human history.
Rickmansworth is an unremarkable home counties town just outside London. Describing a character suddenly discovering the meaning of life, the universe and everything in an unremarkable café in an unexceptional town is a juxtaposition of the prosaic with the extraordinary that is typical of Adam's writing style.
In 1992 Douglas Adams founded the h2g2 website as a kind of online version of the Hitchhiker's Guide, "an unconventional guide to life, the universe and everything". Of course this could also be a definition of Wikipedia, making us all Ford Prefects, travelling the world (if not the universe) to update the 'guide'.
Eoin Colfer, author of the Artemis Fowl series and the official The Hitchhiker's Guide sequel And Another Thing..., commented “I think the Hitchhiker’s Guide basically was the iPhone. Adams even described the guide as a small black gadget with a touch button."
Genghis Khan was the founder and ruler of the Mongol empire in the twelfth and early thirteen centuries, and was proverbially a despotic ruler and bloodthirsty warrior.
Comparing the very ordinary town of Guildford with a planet somewhere near Betelgeuse is classic Adams technique. Guildford is a large town in Surrey, a county in southern England. Betelgeuse is one of the brightest stars visible from earth, located around 640 lightyears away.
This is a reference to Arthur Frommer’s seminal 1957 travel guide Europe on 5 Dollars a Day. The book was highly influential in changing attitudes to foreign travel among Americans. It laid the foundations for Frommer’s guide book business as well as backpacker orientated guide books like the Rough Guide and Lonely Planet series.
A variation on George Berkeley's theory of immaterialism or subjective idealism: "esse est percipi" (to be is to be perceived).
In his work A Treatise Concerning the Principles of Human Knowledge Berkeley posits the idea that "surely there is nothing easier than for me to imagine trees, for instance, in a park and nobody by to perceive them. The objects of sense exist only when they are perceived; the trees therefore are in the garden no longer than while there is somebody by to perceive them."
This is more often rendered as the hypothetical question: "if a tree falls in an empty forest with no one present to witness it, did it really make a sound?"