You’d have an up quark-antiquark annihilation, and a down quark-antiquark annihilation, leaving behind an up quark and a down antiquark. These have charges of +2/3 e and +1/3 e, respectively, so they can combine to form a meson with a +1 charge (I forget what the specific name would be, probably a pi meson?). So, the proton-antineutron annihilation is totally fine in terms of charge conservation and in terms of not leaving solo quarks.

Space is a real thing that can expand. If you’ve heard phrases like “the fabric of spacetime” or “the spacetime continuum”, these are actually real, not just some sci-fi mumbo jumbo. You can imagine a big rubber sheet, on which all the planets and stars and everything are sitting. If you label this sheet with a grid and stretch it out, you’ll see that stuff gets further apart, but it doesn’t change position on the grid. That’s how space expands: it doesn’t move things, it just makes the distance between them bigger. (Note: don’t take this analogy too far: unlike rubber, space can stretch infinitely, and it doesn’t “snap back” into place).

So space expanding makes distances bigger, but it doesn’t make objects move any faster. Nothing ever moves faster than light, even when space expands. It just travels a shorter distance, so it can get places earlier.

Also, there is no “center” of the universe. No matter where you are, whether on Earth or on Jupiter or floating somewhere in the middle of the Andromeda Galaxy, if you take the measurements and do the calculations, you’ll find that you are at the center. Every single point in the universe can be treated as the “center”, and every single one of those points would be perfectly accurate for any tests or measurements or calculations you could think of. So, either everything is the center, or nothing is, but there’s not one singular point we can look at and say “yeah that’s the literal exact center and nothing else is.”

Ok I see what you mean now. Ordinarily, you would be exactly right: there’s no way we could’ve gotten that far away from the photons that they would take 13 billion years to reach us. However, early universe is anything but ordinary.

Going back to grandma and the mailman, imagine they’re both on the sidewalk, but grandma is on the side closer to the house. At the same time, they both start to move towards the house, but as grandma steps off the sidewalk, it suddenly quadruples in size. Grandma is fine since she already got off the sidewalk, but the mailman suddenly has to walk four times as far to catch up. Then, as he hits the halfway point, it quadruples again. And as he keeps going, it keeps getting bigger and bigger. Grandma already made it to the porch and is knitting a sweater, but the mailman hasn’t even gotten off this piece of sidewalk.

This is how the early universe looked. In the first instants after the Big Bang, some photons were going the same direction but from different places. Because space expanded so rapidly, the photons with a “head start” in their direction got a lot further ahead than the others, and this head start kept growing as space continued to expand. Except instead of doubling or quadrupling, it was expanding by millions and billions. Even though they started so close in the beginning, a gap of a nanometer could expand to a light year faster than the photons could cross it, so they got left in the dust.

Remember that all the stuff we see and interact with started off as photons too, just running a shorter race, so after they got here and started settling down into planets, the other photons were still trying to overcome the vast distance that space itself had made by expanding

Imagine taking a picture of yourself and mailing it to your grandma. That picture shows how you look on the day you take it. It then travels through the postal system for a couple days (or weeks, or months, depending on how bad the system is). In that time, you could change a lot: you could shave your eyebrows, or dye your hair, or get a tattoo, or go tanning. However, the photo still shows the you from a couple days ago, so when your grandma sees it she only knows what you looked like then. The further away she lives, the more stuff can change in the meantime.

In the same way, photons are like pictures of the stuff that emitted them. When the Big Bang happened, a bunch of photons got shot out in all different directions. At the same time, space itself expanded, and it expanded a lot. The distance those photons had to travel went from almost zero to millions of light years faster than they could traverse it. Imagine if the postman was walking the 20 feet to your grandmas mailbox, when suddenly it grew into a 30 mile hike. He’d take a lot longer to bring her the mail. Eventually though, he would make it, and grandma would finally see your lovely face. In the same way, these photons would eventually make it to us, and we could see the early universe: it just takes 13 billion years to cross that gap, since space just keeps expanding while the photons move.

So yes, those photons are from the early early universe, because space itself expanded and made them travel for longer to reach us.

Nope! Let’s say each neutrino is going 51% the speed of light, in opposite directions. If neutrino A were to look at neutrino B, it would only see B traveling at about 81% the speed of light. B would see A going the same speed, but in the other direction.

Now, if you’re on the ground watching these particles fly, you would see them move apart with the gap between them growing at 102% the speed of light. However, the individual objects would only move at 51% C, so nothing is violating physics

The speed of light is the same for every reference frame, and no object with mass can ever go at or above that speed. If you stood on an rocket going at 99% the speed of light relative to the earth and threw a rock at 10% the speed of light relative to you, that rock wouldn’t be moving at 109% the speed of light relative to earth. Instead, the Lorentz equations tell us the rock would move at about 99.2% of the speed of light relative to earth.

Talking about the “fabric of space” isn’t really an accurate way to describe the universe because it implies that there is some sort of universal background that everything takes place against. In reality, it’s more like every single object in the universe has its own “fabric” of space that it sees, and two different objects might completely disagree about what the fabric looks like, and both could be correct. It’s very confusing and not helpful for these scenarios (relativistic speeds).

Tl;dr: very high speeds are not intuitive and don’t work the way you might think. Just remember that nothing can ever go faster than the speed of light in any reference frame, and there’s no known way to “cheat” this. There also is no “absolute” speed: everything is relative to something else, whether it’s the earth or the sun or the CMB or whatever