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stormwyrm writes:

Dark matter, long theorised but remaining controversial, may have found yet another piece of evidence in its favour. The theory of dark matter has it that billions of years ago, not so much dark matter should have fallen into the galaxies yet, so instead of the flat rotation curves that are observed in the galaxies of today, younger galaxies should exhibit falling rotation curves that slow further from the centre. The measurement of the rotation curves of such younger, more distant galaxies has so far been elusive, but astronomers have now succeeded in doing so. In a paper just submitted to the Astrophysical Journal, they show how they have measured the rotation curves of 101 distant galaxies with redshifts between 0.6 to 2.6 (or 7.2 billion to 19 billion light years away comoving distance, 8 billion to 2.5 billion years after the Big Bang). These galaxies all show a precipitous drop-off in rotational velocity as one goes further away from the centre. From an article by Ethan Siegel:

When they use a technique called "stacking" — where they calibrate each galaxy to one another to examine their overall, average properties — they find that there is, in fact, a precipitous drop-off in rotational velocity as you move away from the center of these galaxies.

This is, remarkably, a strong piece of evidence that points to dark matter and not to modified gravity! As Philipp Lang and his coauthors write in a paper just submitted to the Astrophysical Journal:

Our stacked rotation curve exhibits a decrease in rotation velocity beyond the turn-over radius down to ∼ 62% of the maximum normalized velocity Vmax, confirming the drop [...] as a representative feature for our sample of high-z disk galaxies. The drop seen in our stacked rotation curve strikingly deviates from the average rotation curves of local spirals at the same mass at > 3σ significance level.

This is just a 3-sigma effect so far, but it should be improved upon by future telescopes like the Giant Magellan Telescope, E-ELT, and WFIRST that are coming in the 2020s.

Related: Dark Matter Is Missing From Young Galaxies

Original Submission