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There is a fairly comprehensive exposition of the standard distance measurement methods—and how they mesh together—in section 1.3 of Weinberg’s Cosmology. There’s something like a dozen of them, with intersecting ranges of applicability, some need others to bootstrap at smaller scales, etc. What you said sounds like too much of a simplification to be of any use when judging how well-founded the whole thing is. (I don’t know how well that reflects the video—an hour reading or writing is perfectly fine, but an hour watching a video I really can’t stand.) (It’s not like the subject experts don’t realize the dangers of one or two observations or approaches holding up a theory, especially in a field like astronomy where you’re stuck with the experiments nature deigned to perform for you. That’s why the Bullet Cluster is not the knock-out argument for dark matter that it appears to be—it’s a single object, maybe it’s some kind of freak occurrence we can’t really sort out from here.) Generally speaking, Weinberg’s Cosmology (not to be confused with his GR textbook Gravitation and cosmology from the 70s, the cosmology parts of which are much too old to be useful) is a bit peculiar in a way that might be helpful here: Weinberg is undeniably one of the giants of modern physics, but not in cosmology, astronomy, gravitation, or similar—he’s a high-energy person first and foremost, with connections to other field-theoretic pursuits. So AFAIU writing a cosmology textbook was just his way of learning how the subject developed over the three decades when he wasn’t looking (spoiler alert: a lot). Thus he tends to pay attention to which pieces of reasoning and evidence hold up the subject to an extent that people immersed in it don’t—including things that a normal astronomer considers elementary knowledge not really part of cosmology proper, like Hertzsprung—Russell (and, to be fair to the astronomer, it really isn’t). That doesn’t mean that he’ll hold your hand: if you don’t know special relativity, the book’s probably going to be mostly unreadable (Taylor—Wheeler and a bit of Lightman—Press—Price—Teukolsky be with you); if you don’t know enough Riemann geometry to write down a geodesic equation, ditto (try Misner—Thorne—Wheeler maybe? it really is a cursed subject); same for not being able to derive said equation from an action, or not knowing how blackbody radiation works. All of those are proven as well as anything can be, though, and hold up much more than just cosmology. |