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How Sabine Hossenfelder’s opinion of dark matter has changed over 20 years

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At first it was a simple idea that could account for a lot of observations. But then “observations have piled up that dark matter cannot explain”:

Citing a number reasons via the video and transcript. she says,

So, if it’s not dark matter then what else could it be? The alternative explanation to particle dark matter is modified gravity. The idea of modified gravity is that we are not missing a source for gravity, but that we have the law of gravity wrong.

Modified gravity solves all the riddles that I just told you about. There’s no friction, so high relative velocities are not a problem. It predicted the Tully-Fisher relation, it explains Renzo’s rule and satellite alignments, it removes the issue with density peaks in galactic cores, and solves the missing satellites problem.

But modified gravity does not do well with the cosmic microwave background and the early universe, and it has some issues with galaxy clusters.

So that looks like a battle between competing hypotheses, and that’s certainly how it’s been portrayed and how most physicists think about it.

Sabine Hossenfelder, “Dark Matter: The Situation Has Changed” at BackRe(Action)

She thinks the solution now is to combine dark matter with modified gravity. Now that will be a challenge. We kind of wish she would go back to denying free will and fine-tuning of the universe.

Dr. Hossenfelder posted an amazing graph comparing the expected relationship between orbital velocity versus distance. This graph encapsulates the frustrating problem involving galactic rotation. Unlike the velocities of solar orbits, which are slower the farther you get from the sun, galactic orbital velocities aren't simply deviations (as I was once led to believe) but they are actually mirror opposites--they travel faster the father they are from the galactic core! This is stunning! But gravity is still in operation, so what could flip the graph? Some new force or perhaps a radically different distribution of (presumably dark matter) mass. For example imagine a in a toroidal shape exterior to the galaxy spinning at a relativistic angular velocity. -Q Querius
Rather than try to force dark matter to fit, let it go. It has not been witnessed by anyone under any conditions. If something is not witnessed, it cannot be a valid theory. As far as gravity goes, gravitational balance should be considered. Einstein and Newton do not need to be at odds when it comes to pushing or pulling. BobRyan
OCKHAM! No unnecessary entities. If Dark Matter can't be fitted into reality it's unnecessary. Toss it. If a theory can't explain reality, toss it. polistra

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