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by privong·7y ago·view on hn ↗
> if requiring dark matter to explain galactic physics can be dispensed with by a distance correction, doesn't "we've been underestimating distances all this time" serve as a simpler fact of galactic physics than needing "there is mysterious dark matter everywhere" to explain galactic physics?

It would be a simpler explanation, but that would require that the distances to _all_ galaxies are overestimated. Presently there's no good evidence that our distance estimates for galaxies are systematically overestimated.

Something like this did happen in the 1940s, when it was discovered that a key type of star in the cosmic distance ladder (Cepheid variables) actually came in two types, with different period-brightness relations. Recognizing these two types and correcting distances doubled the distances inferred to nearby galaxies: https://en.wikipedia.org/wiki/Cepheid_variable#History

2 comments
> that would require that the distances to _all_ galaxies are overestimated.

No, underestimated. The estimate of the distance to this particular galaxy was reduced--that's what made its observed parameters consistent with dark matter. So if it were really true that no galaxies have dark matter, our current distance estimates would all have to be too low--in the case of this particular galaxy, 13 Mpc (consistent with dark matter) vs. 20 Mpc (if there were no dark matter).

> No, underestimated.

Oops, yes. Wrote that backwards. Thanks for the correction.

To be fair, the cepheid correction was presaged by good data from elsewhere in science. Until the 40's, it was possible to date the earth (via isotope ratios in decay products trapped in rocks) as older than the universe.