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from the Now-you-see-it-now-you-don't-because-it's-dark? dept.
A galaxy has been found containing no dark matter, but that proves dark matter is real?
A distant galaxy that appears completely devoid of dark matter has baffled astronomers and deepened the mystery of the universe's most elusive substance.
[...] In the Milky Way there is about 30 times more dark matter than normal matter. The latest observations focused on an ultra-diffuse galaxy – ghostly galaxies that are large but have hardly any stars – called NGC 1052-DF2.
The team tracked the motions of 10 bright star clusters and found that they were travelling way below the velocities expected. "They basically look like they're standing still," said van Dokkum.
The velocities gave an upper estimate for the galactic mass of 400 times lower than expected. "If there is any dark matter at all, it's very little," van Dokkum explained. "The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter."
Paradoxically, the authors said the discovery of a galaxy without dark matter counts as evidence that it probably does exist. A competing explanation for the fast-orbiting stars is that the way gravity drops off with distance has been misunderstood – but if this were the case, all galaxies should follow the same pattern.
https://www.theguardian.com/science/2018/mar/28/galaxy-without-any-dark-matter-baffles-astronomers. The findings are published in the journal Nature.
(Score: 2) by ese002 on Friday March 30 2018, @10:30PM (6 children)
That would require not just a dense pocket of dark energy but that the dark energy nearly balances the dark matter. It's not impossible but those kinds of balancing acts are usually either wildly improbable or mandatory. It is clearly not mandatory because we don't see it elsewhere.
(Score: 0) by Anonymous Coward on Friday March 30 2018, @10:47PM (5 children)
I'd like to see distribution plot of the ratio of estimated dark matter to "regular" matter among a random sample of galaxies. If it's a roughly continuous curve then this particular galaxy could simply be one with a coincidental balance. If there are spikes or sudden drop-offs in the plot, it could imply a variety of things.
If we are dealing with 3 factors (regular matter, dark matter, dark energy) and we can only directly measure 1, interpreting such a curve could be tricky and may await other clues.
(Score: 5, Insightful) by HiThere on Saturday March 31 2018, @06:06AM (4 children)
Actually, we measure two related things:
1) The stars which we see have a mass that we deduce from the spectrum and a few other things and so we think we know their mass, more or less. (This is a lot more definite in multiple star systems.)
2) The orbital speeds of the stars around the galaxy.
We add up the estimated mass of the stars to come up with an estimated mass of visible matter. (A bit of finagling goes on here, because there are planets, comets, dust clouds, etc., but most of the mass is expected to be in stars so it's probably about right.)
We look at the orbital velocities, and calculate the mass of galaxy needed for those speeds to be orbital velocities. Usually there's way too little observable mass. "Dark matter" is "whatever causes stars at a faster velocity to be in orbit". This time they don't need that bugger factor. The easiest answer is that "dark matter" really is matter that we can't detect, but for various cosmological reasons it can't be baryonic matter. (If it's baryonic matter, the amount of Lithium that got produced in the early days is just totally wrong.)
This isn't informal just to be funny. I don't really understand the details, and this is the closest I can come to a real answer to your question.
O, Yes, dark energy. We can't detect that at all on scales as small as a galactic cluster. Sorry. The only way we can detect that is by measuring the apparent speed of the expansion of space-time over areas that are much more loosely bound than a galactic cluster.
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(Score: 2) by PiMuNu on Saturday March 31 2018, @07:27AM
I think that's a good summary. But it's worth adding that there are other ways to observe the dark matter, like looking at the way light is bent around galaxies by their gravitational field; everything points to gravity being stronger than expected. The best model that fits the data is that there is some extra matter that doesn't emit light at any wavelengths i.e. dark matter.
(Score: 0) by Anonymous Coward on Saturday March 31 2018, @01:42PM (2 children)
It is more correct to say that dark matter is matter we cannot see. We can obviously detect it, otherwise how would we even know it exists? However we can only detect it indirectly through its gravitational effects.
(Score: 3, Interesting) by HiThere on Saturday March 31 2018, @05:08PM
The problem here is "What do you mean 'matter'?". All we know is that *something* is acting on gravity the way matter would. That's something that we usually attribute to matter, but we don't see it, so we're calling it dark matter until we detect something. Or come up with a better explanatory theory.
The thing is, just because is has a name doesn't say much about it. I could take a bagel and name it "hopscotch", but that wouldn't make it jump. All that we really "know" is a perturbation in gravity.
Even that's overstating the case, but here we're starting to get closer to psychology than physics. Just accept that actual knowledge is impossible, and the best we can do is make estimates worth acting on. Normal English usage doesn't map well onto either physics or psychology, but the words we use shape the thoughts we communicate. Within your own mind you've got lots of things that modify the words you use, attached images, feelings, perhaps even sub-text dialogs. Those get stripped off when you talk to someone else. So the Whorfian Hypothesis is true for spoken, and even more written, thoughts, even though the strong form isn't true for personal thought.
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(Score: 2) by Fluffeh on Wednesday April 04 2018, @11:15PM
Not really. We can see the effects of *something* but it is not anything that we can in fact detect directly - just its symptoms. We can only see the effects that we would associate with more matter. It might be matter we for some reason don't see, it might be that gravity works differently to what we understand - or there might be some completely new physics we haven't scratched at yet.
The things/effects that we can see don't match the formulas we have to explain them.