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The year is 2012. Steve sees the arc reactor in Tony's chest for the first time as Tony lets Steve undress him, lets Steve lay him down on the workshop couch and kiss every inch of his body, and Steve can't believe his luck.
He's known about the arc reactor since he read Tony's file, since he found out how Tony survived in Afghanistan. He can't say he really understands it, and as interested as he is in it, he never wanted to risk making Tony uncomfortable by asking. All he knows — and all he needs to know — is that it keeps Tony alive, and honestly, that's all that matters. That's a good enough reason for Steve to love it.
Steve knows Tony feels self-conscious about the scars surrounding it, so he makes sure to cover Tony's chest in kisses while muttering praise into his skin, and when the arc reactor lights up his face in the dimly lit workshop, he hopes Tony can see all the love in his eyes.
Later that week, when Steve wakes up shaking from another nightmare about the ice, the first thing he sees is the pale blue light glowing brightly right next to him. He's in Tony's bed, in the Avengers Tower. He's safe. He pulls Tony into his arms, snuggles against his warm body, watches as Tony smiles in his sleep. The arc reactor light gives him something to focus on. It soothes his anxiety. Grounds him.
The shaking subsides. He doesn't feel cold anymore.
He has no reason to be afraid.
The year is 2013 and they're in the workshop in Tony's Malibu house, and Tony has a couch here too, and Steve thinks he likes this workshop even better than the one in New York. Steve's naked on Tony's lap, rolling his hips as Tony's hands settle on his waist, and he gasps and moans against Tony's mouth because somehow Tony manages to touch all the right places inside him.
The arc reactor glows in Tony's bare chest, bathing Steve's face in blue light, and it's beautiful, it's beautiful like everything Tony creates, it's beautiful like Tony himself.
Steve kisses him, moans into his mouth as Tony's fingers dig into his skin, as Tony's hips thrust up into him, and he begs Tony to come inside him while he repeats, over and over against Tony's ear, just how much he loves him.
When they fall asleep on that same couch afterwards, Steve rests his hand on top of the arc reactor, its light glowing bright through the spaces between Steve's fingers, and Tony lets out a happy sigh, cuddling even closer to him.
The year is 2015. The arc reactor isn't there anymore when Steve wakes up from his nightmares about the ice. But Tony is.
Steve wraps his arms around the man he loves and reminds himself that he's safe — he's with Tony in the Avengers Compound, in their room, their bed. Tony had the arc reactor removed two years ago, and even though Steve misses its soft, calming blue light whenever he wakes up shaking from the cold he sometimes experiences in his dreams, he's happy that he no longer needs to worry about Tony having shrapnel in his chest.
Arc reactor or not, it's Tony who lights up Steve's world. He's Steve's sun — bright and warm and wonderful, and Steve doesn't think that the world would make sense without him.
The year is 2017 and Steve wakes up alone.
He doesn't sleep much these days. Whenever he does, he either dreams about the ice or about the last time he saw Tony. About burying the shield in Tony's armor. About the fear in Tony's eyes.
He never meant for any of that to happen.
He doesn't know how to go back to Tony now, how to apologize, how to fix everything that was ruined between them. He doesn't think that Tony wants him to. He's afraid that Tony might be happier now, without him.
He doesn't know how to explain to Tony that he didn't have a choice.
All he knows is that living without Tony feels like living in a world without light.
The year is 2018 and the world has ended. At least, that's what it feels like.
Steve sleeps even less now, plagued by grief and regrets and nightmares about his friends disintegrating into ash in front of him.
He holds Tony in his arms and tries not to think of all the times he's dreamt about Tony turning into ash. He holds Tony in his arms because this is the only way Tony can sleep these days, and so Steve cuddles him and runs his fingers through his hair and hopes that Tony's dreaming about something beautiful instead of all the horrors they've seen.
On the nightstand, Tony's arc reactor glows, a guiding light for when Steve's thoughts get too dark. It's the one Tony detached from his own chest and put in Steve's hand on the same night Carol rescued him from space, and Steve loves it — loves it like he loves everything that Tony creates, loves it because it's part of Tony. Loves it because it's familiar. Comforting, much like Tony himself.
He stares into the blue light until morning comes, until Tony stirs and yawns and stretches, slowly opening his eyes to fix his gaze on Steve and ask him if he's okay, even though they both know that nothing will ever be okay again.
Steve holds him tighter and kisses his forehead instead of answering. He doesn't tell Tony he stayed up all night, doesn't tell him he's terrified of what he'll see in his dreams. He doesn't need to — Tony already knows.
Sunlight is shining through the windows. The arc reactor is glowing on the nightstand. Tony is warm and real and alive in his arms.
After everything, they still have each other. They'll survive this, somehow.
He thinks he could survive anything, with Tony by his side.