Stacy McGaugh On Astronomy v. Astrophysics

There is more good stuff in the latest post at Triton Station, but this quote clears up important semantic issues and makes an important observation about the scientific effort to understand dark matter phenomena. 

When I say dark matter, I mean the vast diversity of observational evidence for a discrepancy between measurable probes of gravity (orbital speeds, gravitational lensing, equilibrium hydrostatic temperatures, etc.) and what is predicted by the gravity of the observed baryonic material – the stars and gas we can see. When a physicist says “dark matter,” he seems usually to mean the vast array of theoretical hypotheses for what new particle the dark matter might be. . . . 

To date, the evidence for dark matter to date is 100% astronomical in nature. That’s all of it. Despite enormous effort and progress, laboratory experiments provide 0%. Zero point zero zero zero. And before some fool points to the cosmic microwave background, that is not a laboratory experiment. It is astronomy as defined above: information gleaned from observation of the sky. That it is done with photons from the mm and microwave part of the spectrum instead of the optical part of the spectrum doesn’t make it fundamentally different: it is still an observation of the sky.

One could arguably slightly amend one sentence of the post to say instead: "To date, the positive empirical evidence for dark matter to date is 100% astronomical in nature." 

This is because while there is no positive empirical evidence for dark matter from any source other than observational evidence from astronomy, there are two other important means by which we better understand of dark matter phenomena.

One is computational work (both analytical and N-body) that looks at existing theories and select modifications of them to see what those theories predict and whether those theories are internally consistent and consistent with other laws of physics that are believed to be true.

The second is negative empirical evidence from laboratory-type experiments, such as particle collider experiments. Empirical evidence that rules out a possible explanation of something is still important empirical evidence, even though it can't provide us with an answer all by itself. Efforts to understand dark matter phenomena benefit greatly from negative empirical evidence that rules out a wide swath of dark matter particle theories including most of the parameter space for what was initially the most popular dark matter particle candidate: the supersymmetric WIMP.

Now, in fairness to the original language, negative empirical evidence is strictly speaking evidence "against dark matter", rather than "for it" even though it is still important evidence in conducting the overall scientific inquiry. And, arguably, the computational work is something you do with evidence, rather than evidence itself. But, the output of an analytic analysis or an N-body simulation are used in a manner very similar to that of observational evidence from astronomy and laboratory work, so maybe it is a distinction without a difference.

Meanwhile:

* "Brace for the oncoming deluge of dark matter detectors that won’t detect anything" with Twitter commentary.

* Dark matter interpretations of gamma ray excesses at the galactic center seen by the Fermi Gamma-Ray Space Telescope also doesn't look promising.

* This February 2019 conference on dark matter and modified gravity would have been great to attend.