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from the if-only-you-knew-the-power-of-the-dark-side dept.
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
(Score: 5, Informative) by Immerman on Wednesday March 22 2017, @11:00PM (2 children)
Yes, dark matter was initially created to explain the lack of falloff in the rotation curves of nearby galaxies (and has since found other supporting evidence.)
But it could also be that gravity just behaves differently at galactic scales, and there's *lots* of alternative hypothesis along those lines.
Now, we find that when we look back in time at much younger galaxies, those whose light is reaching us from 6-12 billion years ago, we *don't* see the same rotational anomalies that we see in modern galaxies.
Note that the rotational anomalies are still there in nearby galaxies - it's not like they were just mis-measured, it's that modern galaxies do show the falloff, while early galaxies don't. Something has changed.
And, if something has changed, then many/most alternative hypothesis to explain the anomalies can be thrown out - for example if gravity behaves strangely at the galactic scale, then it would be expected to also have behaved strangely in the distant past, and these ancient galaxies would be showing similar anomalies to new ones.
As for Newton and Einstein having it nailed between them - it doesn't work that way. Einstein flat out said Newton was wrong, threw out his theory of gravity, and replaced it with general relativity. And all subsequent experiments have shown that he was right to do so. Newton made a rough approximation, Einstein a much more accurate one. You can't just say "what the heck, we'll average them out and call it good" - if we could do that, then experiments would have shown that GR was also wrong, rather than accurately predicting everything we've thrown at it.
(Score: 2) by Immerman on Wednesday March 22 2017, @11:03PM
Correction: it's that modern galaxies don't show the falloff, while early galaxies do
Or perhaps more accurately, they show a *different* falloff - it sounds like they probably still don't match the Newtonian prediction either.
(Score: 3, Funny) by bob_super on Thursday March 23 2017, @12:14AM
My bad, sorry.
After a few Billions years of random trials, in order to make the Earth form at just the right place, I had to tweak a couple fundamental constants on the universe control console.
I'm glad you guys didn't notice the side effects of the cat jumping on there a few thousand years ago. Both the Stonehenge crew and the Egyptians were pissed for a while...