Understanding BIG BANG , from the law of physics of today, is not achievable.
The laws of physics , just before the big bang , were different . We have evidence of speed , greater than light In the universe . These speed were attained immediately after big bang & lasted for a very short period . There after the laws were concreted , which the nature follows today.
To understand BIG Bang we have to look & understand , the laws prevailing, before the big bang.
Probably there was no third dimension, or gravity, before big bang , & the introduction of the third dimension or the gravity caused the big bang .
Time also came into being after the big bang. As time is understood as the " Distance travelled , divide by speed. The light, which, does not need any medium for travel, was it self not there. Hence There was nothing that travelled . NO travel , no distance covered , no speed hence no time