By matt Marsden...
I think you may find the problem here is in the question. Asking "How can something happen when time does not exist?" - implies that 'Time exists' and that 'Time is needed' for things to be able to happen.
Many people seem to 'assume' that time exists, and is 'needed' or 'passes' as things happen, but every apparent 'proof' of this I have read can be seriously questioned, and IMO, seen to be unfounded.
Direct observation of the world around us shows that 'things' exist, and that they can move, change, and interact etc where there is energy.
IMO careful consideration shows that direct observation of how the world around us exists and changes, does not actually prove or suggest, that extra to what we see, 'a past' or 'a future' or a thing called 'time' also exist. Although if we over extrapolate what we do observe, it can give a very subtle false impression, or 'illusion' that 'time' and in particular 'the past' exist.
As we walk around the world, we constantly change the contents of our minds.
That is to say, as matter exists and interacts around us, we see things moving and changing, and this changes the contents of our minds in a highly organised way.
And, we may 'CALL' some of these changed contents 'the past', and we may 'CALL' some of our imaginings 'The' Future, and from this we may (wrongly) assume that we have proof that 'THE' past and 'THE' future, and thus 'Time' exist.
And we may then assume that 'time is needed' for things to be able to happen.
But in fact, I think you will find things just exist and interact, or as Einstein suggested, but could not explain how, 'time... is a persistent illusion'.
In other words, while our 'memories' seem to point to or prove the existence of 'another' record of events called 'the past', it is not in fact obvious, or proven, that 'THE' 'temporal past' actually exists.
thus, for example, timelessly at great speeds (SR) or near a black hole (GR), things may indeed 'change more slowly' - but this doesn't prove that things happen more slowly because a thing called 'time' exists, and passes more slowly from an invisible place called the 'future' into an invisible place called 'the past'.
And note, we can't claim that...
A - 'everyone knows 'time is just a word everyone uses to describe all the motion we see',
and then also claim
B - The past and the future really exist, and 'time is also a real thing, connecting the 'past' and 'future' and Time is needed for things to be able to happen.
(And worse still, claim that 'everyone knows time is just a word', and then ask if we can 'warp', or 'travel through' Time etc).
Thus IMO it is invalid to say there 'would be no 'time' before the big bang', unless Hawking, or someone else has produced both 'a good reason to initially suspect Time exists', and 'an experiment that proves time's existence'.
(Author of 'A Brief History of TIMELESSNESS')
(Timelessness at the Greenwich Observatory)