After Timescape
Lucifer's Hammer by Larry Niven and Jerry Pournelle
If Timescape's tachyon physics and ecological collapse got under your skin, Lucifer's Hammer turns comet trajectory math into civilization-ending dread. Niven and Pournelle deliver the same academic feuds, interdisciplinary chaos, and Golden Age rigor—but this time, the scientists aren't trying to save the world with time travel. They're watching it burn and doing the brutal calculus of who survives.