Time's seemingly inexorable march has always provoked interest in, and speculation about, the far future of the cosmos. The usual picture is grim. Five billion years from now the sun will puff itself into a red giant star and swallow the inner solar system before slowly fading to black. But this temporal frame captures only a tiny portion—in fact, an infinitesimal one—of the entire future. As astronomers look ahead, say, “five hundred and seventy-six thousand million years,” as humorist Douglas Adams did in 1980 in The Restaurant at the End of the Universe, they meet a cosmos replete with myriad slow fades to oblivion. By then the accelerating expansion of space will have already carried everything outside our galaxy beyond our view, leaving the night sky ever emptier. Lord Byron captured the prospect of such a celestial wasteland in his 1816 poem “Darkness”: “The bright sun was extinguish'd, and the stars/Did wander darkling in the eternal space.”