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Milky Way Rotation Data Challenges Dark Matter and Modified Gravity

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Milky Way Rotation Data Challenges Dark Matter and Modified Gravity

Clip title: Headache: New Milky Way Data Just Defies Explanation Author / channel: Sabine Hossenfelder URL: https://www.youtube.com/watch?v=FExs0euUFHI

Summary

The video discusses a recent paper challenging established theories about dark matter and modified gravity (MOND) based on new data concerning the Milky Way’s rotation curve. Traditionally, astronomers believed our galaxy contains about ten times more dark matter than normal matter, with its distribution primarily inferred from the “flat rotation curves” observed in galaxies – meaning stars maintain a roughly constant orbital speed even far from the galactic center, contrary to Newtonian predictions. This discrepancy has been largely attributed to the gravitational pull of an unseen, spherical dark matter halo surrounding galaxies.

However, new data from the Gaia DR3 mission, as analyzed in the paper by Melchiorri and Ruchika, suggests a different picture for the Milky Way. Instead of a flat rotation curve, the data indicates a “Keplerian decline” in the orbital speeds of stars beyond approximately 15 kiloparsecs from the galactic center. This implies that the total dynamical mass of the Milky Way is significantly lower (three to five times below previous estimates) and that the expected large, spherical dark matter halo in the outer regions of our galaxy does not appear to be present.

This finding poses a substantial challenge to both the prevailing Lambda-CDM model (which posits dark matter halos) and the alternative Modified Newtonian Dynamics (MOND). For dark matter, the absence of a significant outer halo in the Milky Way contradicts models of galaxy formation, which typically rely on dark matter clumping to initiate structure. While dark matter may still exist near the galaxy’s center, its overall fraction in the Milky Way would be less than the cosmological average, requiring complex explanations to reconcile. For MOND, which predicts a consistently flat rotation curve in the outer regions without dark matter, the observed Keplerian decline is even more problematic, as MOND is a less flexible theory in its predictions.

The video highlights that while the data itself has considerable error bars, making definitive conclusions challenging, the implications are significant. The difficulty in accurately measuring the Milky Way’s rotation curve from within the galaxy adds to the complexity compared to observing distant galaxies. Ultimately, the new data suggests that both the dark matter hypothesis and the MOND theory, as currently understood, struggle to fully explain the observed dynamics of our home galaxy, potentially opening new avenues for understanding gravity and matter distribution in the universe.

Video Description & Links

Description

For decades, astrophysicists have told us that the Milky Way sits inside a huge halo of dark matter. But new measurements of how the outer stars of our galaxy move don’t seem to fit that picture, nor the main alternative, modified gravity. So what’s going on? Let’s take a look.

Paper: https://doi.org/10.48550/arXiv.2608.10189

darkmatter astrophysics space sciencenews physics

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Tags

hossenfelder, science with sabine, science humour, science news, physics news, astrophysics, astronomy, cosmology, space news, milky way dark matter, dark matter halo, dark matter halo missing, galaxy rotation curve, galaxy rotation problem, keplerian decline, kepler in, lambda cdm, lcdm, standard model of cosmology, dark matter vs mond, mond gravity, is dark matter real, dark matter evidence, milgrom, milky way mass, sabrina

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