Warning:
This post contains spoilers for Outer Wilds.
It may spoil not just the narrative, but also solutions to important puzzles.
Please play the game first, it's quite good.
Here is a quick rundown on real Black Holes, for the unfamiliar. Black Holes (BH) are a phenomena predicted by General Relativity (GR) where lots of energy is condensed to a small region, such that its escape velocity is faster than light. They appear as dark (black) spheres, because no light can exit past a certain radius, and the space around them appears warped, as the path of nearby light is dramatically bent. Anything that falls inside is stuck inside forever (and torn apart by an extreme gravity differential). It is provable from GR that Black Holes store very little information, only energy/momentum, angular momentum, and charge. GR's description of BHs has features which contradict important principles of Quantum Mechanics, a currently unsolved problem, so we don't have a good theory for what's going on inside them. I'm not very well-versed in all the exotic BH-interior theories, sorry.
Outer Wilds' BHs are different. They come in Black Hole - White Hole pairs, and anything falling into the BH is spit out of the WH (with a negative time interval).
Here is a quick rundown on real White Holes... just kidding, there are no real White Holes. A WH is (definitionally) a time-reversed BH; it has all the same properties, but flipped backwards in time. WHs are speculated to be possible because the laws of physics are generally symmetric on time-reversion. Even if they are physcially possible, though, they are effectively impossible for entropic reasons.
Size
Outer Wilds' BH-WH dynamics raised some questions, for me. First: How are the BHs getting so large? Real BHs grow because their gravity gets stronger, because their energy increases, because something fell in. But in Outer Wilds, everything that falls in is immediately ejected from the corresponding WH, rather than being trapped inside. So where does the BH get its energy?We're told that the negative time interval between entering and exiting is determined by the energy, as demonstrated in the High Energy Lab, which is why the Ash Twin Project requires the energy of a Supernova for a 22-minute interval. But, the BHs with large intervals (and thus, high energy) in the Lab and Ash Twin are quite small, and the much larger BH inside Brittle Hollow has a negligible interval. So, it seems that the size of a BH is not actually derivable from its energy, contrary to real BHs.
So... What determines the size of a BH, in Outer Wilds? What is a BH, in Outer Wilds? It isn't clear. A couple of possibly relevant phenomena I've noticed: 1: Large rocks falling into the BH release flashes of light on "impact", and 2: Objects emerging from the WH quickly slow down, inheriting the WH's velocity, suggesting that the BH is absorbing the momentum of infalling objects. Though, it's worth considering that the BH and WH are moving at different velocities, which creates an A vs B "Portal problem" that I will not be discussing further.
Black Hole collisions
Real BHs simply merge on impact. But in Outer Wilds, anything falling into the BH comes out of the WH. So, if you threw one BH into another, it should come out the other side, just like everything else. You can actually do this experiment in-game by carrying a Warp Core though the BH in Brittle Hollow.But wait, BH collisions are symmetric, in the sense that "A falling into B" is the same as "B falling into A". So it seems that both BHs in the collision should come out the other's WH.
Applying this to the aforementioned experiment, it'd imply that Brittle Hollow's BH warps through the BH in the Warp Core you're holding, and comes out of the core's WH. With the Ash Twin Warp Core, this arguably matches what we see, since the core's BH and WH are right next to each other, possible creating very little effect. But the cores on Ember Twin only have one BH or WH, not a pair... which means Hollow's BH should warp into Ember Twin?
This obviously does not happen in-game. (Though they should definitely patch it in...) It requires a bit of speculation regarding the Warp Cores. It's completely plausible, for example, that the BH/WHs in the cores are kept "inert" somehow, and are not able to warp anything, evidenced by the fact that any BH core can be paired with any WH core at the Lab. It's also possible that the cores all secretly have a BH/WH pair inside them, evidenced by the fact that receiving warp pads are able to send you back to the sender after being used. It's also possible that, due to the size difference, only a small portion of the Brittle Hollow BH should be warped, so the effect is negligible. It's also possible that the situation is simply not symmetric, with the larger BH acting as the de-facto "consumer" which warps the other.
Okay, okay... What if the WH falls into its own BH? I imagine they would annihilate, just like particle-antiparticle annihilations. Though, it's possible it simply comes out the other side, or passes through unaffected. Obviously, this can't be tested in-game.
Rambling about time paradoxes
I don't think the Ash Twin time paradox is very well thought through...In the High Energy Lab, you can use the cores to create a small BH-WH setup with a noticeable negative interval, and send your scout slightly back in time. There is a short moment after the scout has emerged from the WH and before it's entered the BH where you can remove the cores, preventing the scout from entering the BH. Since it's already emerged from the WH, there are now 2 scouts, and the game ends with the text "You destroyed the fabric of spacetime". The thinking is: Since the scout came out of the WH without entering the BH, this scout has no origin, and there's a paradox.
Ash Twin Project has a similar easter egg, where you can jump into the warp to send yourself back in time, and then meet up with the time-travelling version of yourself on the next loop. Here too you can create a paradox, simply by not jumping into the warp again. The thinking is: If you don't jump into the warp, then your double (just like the scout) has no origin, and there's a paradox.
Except... no there isn't? With the scout, there was no actual time travel: The game spawned a "future" scout because it predicted that you were about to send one back in time. If you stop it from sending, then the game has spawned an object that shouldn't be there, and there is clearly a contradiction. But in Ash Twin, your double does have an origin, they came from the previous loop. There's no contradiction.
It's argued that everything must have an origin within that loop (the previous loops are overwritten and "don't exist"). This would open a huge can of worms, ultimately implying that all time travel should be impossible. The memories you receive from Ash Twin every loop have no origin within that loop, they come from the previous loop. Okay, well, it can be argued that those old memories do have an origin within the loop, because they are eventually sent back in time again (along with the new memories from the current loop). But this would mean that it's impossible to shut off the Ash Twin Project without creating a paradox. And we can see in-game that this is not the case (it would mess up the Nomai's plans, anyway).
Some say that only objects can cause paradoxes, and all the Ash Twin Project sends is information. I'm guessing this was the developers' intention, but I don't think it actually works... The distinction between "information" and "objects" is artificial at best, and incoherent at worst. All objects carry information, and all information is carried by objects (depending on how "object" is defined). It's not about objects, it's about "effects with no cause". But all "rewriting the past" is inherently an effect without a cause, if we restrict ourselves to that loop.
Jumping through the warp twice shouldn't stop the paradox, anyway, because you're different from the version of you who jumped in previously. The game acknowledges this explicitely, because your twice-warped double has different dialogue. Ultimately, this is just a plothole, it seems. I can't think of any reasonable way around it.
(Side note, it's possible to enter Ash Twin Project without your suit. If you do this, and then warp back in time to double yourself, will your double be suitless as well?)