Is it correct to write out the sound of a heart beat as lub dub or are there other variations that are also acceptable?
There are different versions in different languages. In english I have seen thump thump, ba boom, ba bump and lub-dub. In India, it is "dhakdhak." In Italian, it's "tu tump."
Likewise, there are many others. Here is a link:
open "human sounds" and then "heartbeat."