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The availability to detect visible light from stars or detect that light being blocked by baryonic matter. With dark matter it’s two steps removed where we’re
by Retric 2y ago
The availability to detect visible light from stars or detect that light being blocked by baryonic matter.
With dark matter it’s two steps removed where we’re inferring the behavior of baryonic matter and then inferring the amount of baryonic matter we aren’t observing and then calculating the existence of dark matter to get that behavior after accounting for undetected baryonic matter.
- andrewflnr 2y agoYeah, that's a pain, but calculating mass from photons is still pretty indirect. More importantly, and independently of "directness", no one pretends that galaxies having different masses introduces two billion parameters into our models of cosmology. Because that's not what a model of cosmology is.
- Retric 2y agoCalculating the percentage of the universe’s observable mass is dark matter adds 200+ billion parameters because the mass fraction of each galaxy varies. So there’s no simple way to calculate it from say looking at the Milky Way alone and extrapolating from the baryonic mass of the rest of the universe. Trying to approximate things from a representative sample is its own problem.
- andrewflnr 2y agoYou're still confusing a physics model with a map of the universe. That said, it's sure a heck of a coincidence that the number they get from adding up estimated dark matter in galaxies lines up with the number they get from other cosmological measurements, isn't it? Almost like galaxy rotation curves aren't the only evidence for dark matter and haven't been for a long time. https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
- Retric 2y agoGravitational lensing, velocity dispersions, etc circles back to the total mass of galaxies. So it shows up on many of the ways we calculate the total mass fraction not just rotation anomalies.
- andrewflnr 2y agoMany of the ways? I guess that's it then, there's no reason to look at the whole picture, and especially no point in reading all the way to https://en.wikipedia.org/wiki/Dark_matter#Cosmic_microwave_background https://en.wikipedia.org/wiki/Dark_matter#Cosmic_microwave_b...
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- Retric 2y agoYour being redundant, Many in this instance obviously implied not every.
- andrewflnr 2y agoThen, in what way is it relevant to my claims here? Namely, 1. Dark matter does not meaningfully introduce billions more parameters into cosmological models than they already have, and 2. Individual galaxies' dark matter fractions are not essential to (not proving, but) strongly suggesting dark matter exists.
- Retric 2y agoSomething being consistent with a model is different than something being sufficient evidence on its own to support a model. If the observed dark matter fractions of all known galaxies were 0% but the CMB was unchanged we wouldn’t assume dark matter exists. Thus your #2 is false. There’s infinite models consistent with any observation so finding something after a model was created for other reasons is useful as validation, but the chain of logic is still dependent on the prior observations not the model. In a meaningfully different cosmos different observations would have happened and different models would exist. Trying to pick out specific experiments as sufficient on their own glosses over that particular limitation.