According to this question's answers, the difficulty is entirely a result of previous info on a blockchain:
So why can't someone simply fork an earlier state of Bitcoin's blockhain, back when the difficulty was far less, and simply pretend it always took a long time to solve an easy (by now) hash problem, ie simulate a bitcoin world where no ASIC were released etc.
According to that chain, the hash rate would be low and thus a perfectly valid chain could be made with fake transactions, mined in secret many years into the future.
If Bitcoin modes really simply consider the longest chain the valid one, wouldn't this totally bork the whole Bitcoin ecosystem? I hope that "longestt chain" actually means "most proof of work" rather than "most blocks"!