The walls close, and he doesn’t look for a way to rip them open.
The walk back to the TARDIS is long and much of the Torchwood tower silent. The Doctor notices neither of these things; instead he notices that his hands are empty and cold as his arms dangle uselessly at his sides, so he keeps his them in his pockets and curls his fingers into fists.
He doesn’t look for a way through, because this is how it was always meant to happen. This is the outcome he’d strove for when he slipped the jumper around her neck, and this is the outcome that has always been inevitable, no matter how many times he’s tried to ignore it. She’s with her family, and this is how it’s meant to be, how it was always meant to be.
Everyone goes home, in the end.
He can’t look because it’s dangerous, because he’s a Time Lord, last of, and risking two universes in favour of very selfish wish-fulfillment is not something he can afford to do. The Void ship, the Cybermen and Torchwood did damage enough on their own, and the last thing the fabric of reality needs is an impatient Time Lord poking holes in it because he misses a single human girl.
There have been times before when making the choice between Rose and the universe has been much harder than he knows it ought to be. The Doctor is quite certain it’s better for Rose and the universe if it’s a decision he never has to make again.
If he believed in any sort of higher power, he might think this is the universe choosing for him.
He won’t look because this is the easy way, because this way he doesn’t have to envision a timeline where years later Rose tires of traveling and steps out of the TARDIS by choice. This way “forever” is a promise that never has to be tested; when it breaks he can blame it on Torchwood and Cybermen and Daleks (always the Daleks) and ignore the irrefutable incompatibility of a human and a Time Lord.
Mostly, though, it’s because he knows if he let himself look for ways to get her back, he’d never stop.
So it’s accident — accident and a very persistent mental nudge from the TARDIS — that leads him to find the last tiny crack between his world and Pete’s. It's accident, because he isn't looking, because he can't.
As he finds a supernova to orbit, he tries not to calculate how many suns he’d need to incinerate in order to pry the crack further apart.
The walk back to the TARDIS is short, and Donna’s waiting behind him as he fumbles for his key. The Doctor barely notices; instead he’s distracted by the fleeting comfort of two Time Lord minds mingling with his, and by the rancid jealousy broiling in his veins.
This is the easy way, he knows. Everyone goes home, in the end. The universe -- or maybe Dalek Caan (always the Daleks) -- has chosen for him.
He cannot ignore the irrefutable compatibility of two humans.
The walls will close, and he won’t look for a way to rip them open.