As it turns out, the events playing out in this sequence are the memories of Tetsuo, and we know that Tetsuo is being consumed by Akira, as Kei puts it. There’s two ways to look at this, neither of which imply Tetsuo could create a big bang. Akira is recreating everything we see here. This would mean that Akira is powerful enough to do this, but also clearly vastly superior to Tetsuo, since he’s consuming Tetsuo at the same time. More likely, these are all projections and not literal. Kei later calls all of these sequences “illusions”, which makes sense when considering the wording used to describe “the memory of an entire universe”. These are espers, psychics, who deal with mental projection and readings. It’s far more likely that these are indeed just illusions and mental images, not literal feats on a universal scale.