The Discworld novels of Sir Terry Pratchett are a very widely read and popular series of works in fantasy, and latterly steampunk sci-fi. It is implausible that the Caltech gang would not be aware of these books and would not have read at least one, or indeed the graphic novel versions. Coming out of the United Kingdom, Pratchett's books are an absurdist deconstruction of the premises of fantasy fiction and latterly of steampunk imaginings in mainstream sci-fi. The humour is rooted in British comedy and satire and owes something to Douglas Adams' H2G2 reimagining of hi-tech science fiction.

It is possible Pratchett's Discworld has been  homaged at least twice, in the form of deeply embedded in-jokes for those who will get the references.

The scene where Leonard fires up hologramatic images of the earth and the solar system for Penny's delight evokes the first of the Science of Discworld novels, where Terry is assisted by co-writers Professor Jack Cohen and Professor Ian Stewart in using the fantasy world to illustrate and explain deep concepts in Earth's science. Cohen and Stewart hold residences at various American universities and would be well-known in academic circles worldwide. In the book, the wizards of Unseen University are faced with a crisis where rogue magic is building up to universe-shattering prooprtions. In desperation, they divert the runaway magic into potential space, so as to avert a seriously Big Bang in their own universe. They succeed in creating what we later came to know as our Universe. And in the fullness of time, Earth and its people. Indeed, If Sheldon were to look too closely, he might well find out exactly  who triggered the Big Bang.

The "Roundworld" universe in all its infinite glory is about the size of a football and sits gathering dust on a desk at UU. Periodically the wizards hook it up to scanners to observe what's going on.

Meanwhile Leonard Hofstadter, a man who would not be out of place as a UU wizard (in fact he has a kindred spirit there, Professor Ponder Stibbons, geeky, bespectacled and favouring shapless parkas) calls up a hologram of the Earth about the shape of a football, and speculates to Penny it might all turn out to be one enormous recording program being read by intelligences outside time and space...

Seaparated at birth

From about the end of Season Seven, Amy Farrah-Fowler abruptly begins to take more care about experimental animals called monkeys, as opposed to true apes.  In all the prior seven seasons she has never bothered before and everything has been just "a monkey". Suddenly, she distinguishes gorillas and especially orang-utans as being apes.

After an unfortunate magical accident, the Librarian of Unseen University - a place very like Caltech for oddball academics and seething rivalries - is a sentient orang-utan. Who takes direct and drastic action against anyone who cannot get the terminology right and calls him a "monkey". Now somebody in the Caltech gang (or the TBBT production crew) must have read the Discworld books, or at least the graphic novels. And they'd know that the surefire way of pressing an ape's Berserk Button - to make it go, perhaps, Killer Gorilla - is to call one a mere "monkey". Another shout-out to a widely-read and influential fantasy series?

Jack Cohen, co-writer of the Science of Discworld series, is an evolutionary biologist well-known and respected in US academic circles. He ruefully describes the night on a lecture tour where he was savaged by Creationists who thought he was Satan's anointed Antichrist. his attempts to explain the essential truth of evolution to Creationists did not go down well. He makes a point of explaining he was, on the night, in East Texas.

A flat earth, riding on the back of four elephants standing on a turtle. totally implausible - unless you're a Hindu...