Hikaru's Theory
Two Hikarus were standing face to face but were not the same.
One was wearing a school uniform, whereas the other was wearing a lab coat with a black turtleneck underneath.
Nothing needed to be explained anymore.
At first, Hikaru thought that this was his future self who had time-traveled to the past, but then the memories of his doppelgänger came.
The other Hikaru was from the future, but not from his timeline. He had come from a parallel Earth (Earth-9) which had its timeline slightly shifted forward compared to Earth-21.
Hence, Hikaru’s Earth-9 doppelgänger would have a future that would be like his, but not the same. The multi-verse has one thing in common, nothing is exactly the same.
On Earth-9, their events would take place about three months in Earth-21’s future.
A month after the invasion, the second wave came.
The first wave were the various species of parasites created by the Architects, whom Hikaru would later discover was a Gray.