[SPOILERS WARNING: IF YOU HAVEN'T WATCH THE LATEST 2-3 EPISODES, DON'T READ FURTHER. THE TIME TO TURN BACK IS NOW.]
Okay. So I was just Googling whether it's actually possible to time travel (yes, you may call me delusional) and I came across this thing called Grandfather Paradox. The Grandfather Paradox is a complex thing to explain in detail, but here's the main idea: A time traveler travels back in time to kill his grandfather, therefore his grandfather and grandmother never meet. If his grandfather and grandmother never meet, he is never born, but if he's never born, his grandfather doesn't die, therefore, he is born. This got me relating real-life physics notations to The Flash. If you're wondering why, that's because The Flash has used real-life physics notations before. Don't remember? Tachyons. That's right, tachyons are real-life theoretical particles which supposedly travel faster than the speed of light. But enough physics, let's get to the main theory.
The Main Theory (Part 1) :-
The first part of the theory goes a little something like this: Eobard Thawne is from the 25th century, A.K.A, the future.
His ancestor is Eddie Thawne, who lives in Barry's time period from the 21st Century, A.K.A, the past.
Eobard goes back in time, initially, to kill Barry Allen as a child after learning the Flash's name and what time period he's from. He had to go back to his time but couldn't as he used up of all his Speed Force. So he kills Nora Allen, creating a Time Constant (an event which occurs at each instant when intended in time, no matter which universe. Although it may differ in different universes, as long as the concept of time is present, the event WILL occur.)
This ultimately leads to Barry becoming from a forensic scientist for the CCPD to the Scarlet Speedster. Fast forwarding a little bit (no pun intended there), Eobard Thawne is about to go home and our hero comes and starts fighting with Eobard. Eobard is just about to kill Barry when Eddie (the guy up there) shoots himself. Since Eddie is Eobard's ancestor, if Eddie dies, then this Eobard Thawne doesn't exist.
But if Eobard Thawne doesn't exist, then Nora Allen does cannot die (In this universe's Time Constant, Eobard Thawne is the one to kill Nora Allen and ignite Barry's destiny to be the Flash) and like the butterfly effect, Barry does not become the Flash, Henry Allen doesn't go to jail, Barry lives happily with his real parents. But obviously, Nora's death is a constant for Barry to become the Flash, so Eobard Thawne HAS to exist. This is explained in a much simpler way in the show (Eobard was protected by the Speed Force). They said he was protected by the Speed Force, but actually, he was protected by the laws of time travel. Ultimately, he lives.
Not related to the theory, but if you're wondering why Eobard didn't remember anything, it's because that was the first time he traveled back in time, after his "rebirth".
Main Theory (Part 2) :-
Now, as we know, in Earth 2, everything is quite the opposite. Everyone of Earth 2 is in no way the same as Earth 1. Now, getting back to physics (boring, I know, I apologize), there's another complex thing called Parallel Universes. It's sort of a theory which goes a little something like this. It tells us that there's this thing called the Multiverse in which, not only our universe, but many other universes exist right next to each other, and are, in some ways, connected to each other.The Flash for each parallel universe.
This connection is theorized to be in the form of a portal (like the breaches in Flash which lead to Earth 2) but in actuality, nobody knows for sure. Anyway, the Parallel Universes theory has 4 levels or parts to it, but I'll be talking about only one, which relates to the main theory.
If you go far enough, you'll get back home.
As we know, our universe is expanding exponentially every instant. Therefore, if we were to travel far enough into the cosmos, we would find a planet exactly to that of Earth, with the physical features in the exact places. This planet would (according to the theory) have a connection between all the planets of its type across the Multiverse. ( That's why Earth 2 is almost exactly like Earth 1 and Harrison Wells of Earth 2 found a direct portal to S.T.A.R Labs).
Now we come to the exciting part. As I have told you before, Nora Allen HAS to die (Time Constant, remember?) but there's really no telling WHO killed Nora Allen (Time Constant may differ a tiny bit in other universes. For example, if someone dies due to a knife and it's a time constant, then in another universe, the same person could die due to a gun or a robbery but ultimately that person would indeed die.) If it was Eobard who killed Nora, Henry Allen (Barry's father) would be in jail, Barry would be the Flash, and so on and so forth. But the story on Earth 2 is quite different. There's only one (in this case, good) speedster on Earth 2. Who, you ask? Our very own Jay Garrick!
Now, because of this, Barry of Earth 2 may have an entirely different backstory. He's obviously not a speedster, his mother died because either due to an accident or a criminal might have killed her. Basically any way not involving Eobard Thawne. Henry Allen father went away because he couldn't handle his wife's death. Barry had to survive on his own and become the great forensic scientist in CCPD.
There, he meets Iris West and gets married to her (Iris West-Allen).
As we all know, Harry (Harrison Wells of Earth 2) really, really hates Jay Garrick. He tries to make the perfect "super soldier" (shoutout to Pollux. If you don't know Pollux, he was a clone of Barry Allen shown in the 1990 TV Series. Barry's DNA was taken so that they can create a super soldier, but it seems highly unlikely as Barry is not a speedster in Earth 2) so that people don't need Garrick to be their hero.
Harry tries extremely hard but, alas, he fails (therefore creating the Meta Humans of Earth 2). This is where he acquires Henry Allen. Henry Allen (of Earth 2) seems to be the only person alive who can bare the pain and endurance to become the super soldier the Central City of Earth 2 needs. Harry promises Henry that people will look up to him as a super soldier and begins doing his thing. When Harry starts the electrocution something goes terribly wrong. The (most likely, in opinion, blue) suit ends up melting onto Henry's skin, giving it that stitched look and Harry thinks Henry is dead. But, Henry wakes up and looks at himself. He asks Harry what happened to him and Harry tells him that the experiment failed. Henry becomes overcome with rage and vows to take revenge on Harry. He finds out that he finally does get super speed like Garrick, but not he's not fast enough. So he sets out to become the fastest speedster alive (he was initially faster than Garrick) by killing all the other ones. Few months later, Barry opens the Breach, Henry Allen (now known as Zoom) fights with Garrick, stealing his speed, leading Garrick to Earth 1.
FYI, in the Barry-Zoom fight, Henry doesn't know that's Barry because his mom died when he was young, and Henry left just about then. He didn't know what Barry looks like when he's grown up. I don't think Earth 2 Barry saw his father after he left, because Henry became Zoom.
If my theory does turn out to be correct, I would be the happiest person. Also, it would have been a really cool job to see the show make John Wesley Shipp (Henry Allen) a speedster again. (He was Barry Allen/Pollux in the original 1990's T.V show)