5 brilliant, glow-in-the-dark stars! 
 Ok -- mind blown! This was a good one. It combined what I love about contemporary YAs: a sprinkling of mystery, a generous doze of quirkiness (or geekiness), a chunk of sorrow, and a gigantic dollop of romance. 
 I can't write too much about it, because it won't do the story justice. Aside from the quantum physics, wormholes, theorems, and time travel, there was a lot of character and world building; it was brilliant use Gottie's voice and story to experience the book. 
 My one recommendation: Either read very carefully or go back and re-read. When I re-read certain sections a second time, I was able to understand it a lot more, (view spoiler)[and in the process, discovered the truth about what Thomas really knows. (hide spoiler)] 
 Take risks. Live boldly. Say yes. Like a comet, I know: that's how you stop a wormhole, that's the opposite of grief -- love.