Poul Anderson's Tau Zero is, for good and bad, a true sf classic. The good is that it has a compelling big science question, spins an interesting story about it, and has a setting that makes very good use of the implications of said science. The bad is that it has bland characters and a rather silly plot.
In summary, a colonization vessel (the Leonora Christine) is dispatched from Earth to another star system. It's powered by a fusion drive and a ramjet scoop collecting hydrogen and various other particles, and travels strictly at sublight speeds - this is not a universe in which faster-than-light travel is possible. Unfortunately, a technical accident prevents the Leonora Christine from decelerating - in fact, it is prevented from stopping its acceleration. Due to relativistic time dilation, the crew of the ship ends up perceiving millions of years in Earth's frame of reference as mere seconds in theirs. We follow the desperation and onboard intrigues of the crew as they ﬁrst must cope with the realization that the rest of the human race is likely billions of years extinct - and eventually must cope with living in a desolate universe with no more star formation.
I found the narrative about humans trying to survive in such an environment compelling, and the miniature sociology of the crew very well thought out. I also liked the speculation on how extreme time dilation might actually feel for humans. How a brief thud in the ship is actually them passing through a galaxy - a distance unimaginably vast to humans, gone by in the blink of an eye. Their reflections on whether their ramscoop-driven relativistic vessel has become a monster, a huge wandering destroyer of worlds.
The Earth political situation as they take off - where Sweden has become the dominant world superpower - made me chuckle.
The characters are highly stereotyped and very, very dull. Not as bad as Ringworld or Heart of the Comet - but it's quite obvious that Anderson prioritized getting a story about a science concept, not getting a story with compelling characters. This isn't exactly unknown in science ﬁction - the same criticism can be levelled at many masters of the genre. Arthur C. Clarke wasn't exactly known for his deep character studies either.
On the subject of characters, I absolutely despised the Charles Reymont character. Essentially he's the square-jawed alpha-male hero who ﬁxes all the problems, and (believably) he's also a colossal jerk, and a bully to boot. I kept hoping that he'd meet an unfortunate end, or otherwise get what was coming to him. Unfortunately, I think I was supposed to sympathize with him. To the story's credit, he's not the main protagonist (that would be Ingrid Lindgren, the politically-savvy ﬁrst officer). An annoying little piece of sexism that shows the book's date (it's a contemporary of Ringworld - which is, admittedly, much worse), is how we end up knowing the jobs of nearly all the men on the ship, but only a tiny minority of the women. It seems as if Anderson had trouble imagining which jobs women could do on a spaceship, but at least he didn't simply assume that obviously they'd all be tasked with sexually servicing the male crew members, like Larry Niven did in Ringworld.
I am not a theoretical physicist, but the ending seemed to make little sense. How can a spaceship survive a Big Crunch event or a Big Bang event? If the entire universe collapses to a singularity, then it seems to me that the ship must either also collapse with it, or forever be trapped outside the newly formed universe. To me, it seemed like an upbeat ending for the sake of having an upbeat ending, and ultimately made the universe out to be friendlier than I think it is. The ending was Anti-Lovecraftian, if you will.
There's good, there's bad, but in my opinion there's more good than bad. Recommended.