Galaxy Without Any Dark Matter Baffles Astronomers (arstechnica.com)

A distant galaxy that appears completely devoid of dark matter has baffled astronomers and deepened the mystery of the universe's most elusive substance. The Guardian reports: The absence of dark matter from a small patch of sky might appear to be a non-problem, given that astronomers have never directly observed dark matter anywhere. However, most current theories of the universe suggest that everywhere that ordinary matter is found, dark matter ought to be lurking too, making the newly observed galaxy an odd exception. Dark matter's existence is inferred from its gravitational influence on visible objects, which suggests it dominates over ordinary matter by a ratio of 5:1. Some of the clearest evidence comes from tracking stars in the outer regions of galaxies, which consistently appear to be orbiting faster than their escape velocity, the threshold speed at which they ought to break free of the gravitational binds holding them in place and slingshot into space. This suggests there is unseen, but substantial, mass holding stars in orbit. 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 traveling way below the velocities expected. The velocities gave an upper estimate for the galactic mass of 400 times lower than expected. The researchers described their discovery in the journal Nature.

Comments

Except rotation speeds have already been explained

[Anonymous Coward]

... in other ways. I remember reading a paper that explained exactly that away very nicely. I can't find it anymore, but I know it was mentioned in the Scientific American, many years ago.

Our math still does not fit reality, especially energy-wise, but not because of the rotation of galaxies.
It's just that in pop-sci, "dark matter/energy" is commonly presented as if our theories were right and it was just our observations of the universe that are wrong, when in reality, "dark matter/energy" is merely a convenient identifier for the discrepancy and is merely saying "we don't know yet". Implying that, obviously, it's our theories that are still wrong.

So saying "without any dark matter" is already highly questionable. Rather, this galaxy might help us fix our silly theories, to match the cold hard reality that we simply observe. Not the other way around.

Re:Except rotation speeds have already been explai

[Megol]

You are wrong. Dark matter and dark energy are used as they are the only things that help explain our observations of nature.

Yes our theories are still wrong. They will be until we can describe everything - something not likely to ever happen. That's science. What you are doing is hand waving without understanding the basics.

Re:Except rotation speeds have already been explai

[BlueCoder]

I'm more surprised this hasn't happened before. To me I believe it's simple. And YES a BELIEF. Gravity spans dimensions. I am more surprised it hasn't been observed before. My question has always been what does that neutrino look like from a different angle.... It's taken us this long simply to detect them. It is really so hard to postulate particles in other dimensions which we can detect through gravity. It it really so hard to believe that Plato couldn't have been partially right?

Re:Except rotation speeds have already been explai

[OneAhead]

If machinery would refuse to work for people who don't correctly understand their working principles, we'd be living in some kind of stone age.

Science vs Religion

[Anonymous Coward]

I am not GP, but:

"Dark matter and dark energy are used as they are the only things"
That statement is completely false. "Only" isn't a science word (it requires you disprove all other theories, even theories you haven't had yet), and neither of those claims is proven or even likely, given they don't address even currently understood data.

"hand waving without understanding the basics"
i.e. to paraphrase you make the claim that "dark matter and dark energy" are the basics of science, and that they are set in stone, and deeper understand requires knownledge/acceptance of these basics.
Again this is not science. You will likely have to throw away a lot of theories built on false logic (e.g. QM, standard model, mass/gravity) if the current understanding of these hit a dead end. You cannot say "this theory is broken, yet must form the basis of better theories", because that's nonsense logic. A broken theory is broken, it must be wrong.

"You are wrong"
And acceptance of mistakes is necessary for science, i.e. you might be wrong.

Really the danger to science is people like you. You learn things as though they're true, you build your careers based on this, and when experiments point to faults, you gloss over the failure and defend the broken model. It's more religion than science.

Re:Science vs Religion

[Daetrin]

That statement is completely false. "Only" isn't a science word (it requires you disprove all other theories, even theories you haven't had yet), and neither of those claims is proven or even likely, given they don't address even currently understood data.

You're confused. Dark matter and dark energy aren't concrete things. They're placeholders for the remainders that don't fit the currently understood data. They are the X and the Y in an equation we haven't yet solved. X and Y are in fact the "only" solutions to the equation, because that's a tautology. The solution to the equation is the solution to the equation. But because we don't know what they are they could turn out to be almost anything, or a combination of things. There are a lot of theories as to what they might be, but none of them is definite, and none will be until we gather more data.

You cannot say "this theory is broken, yet must form the basis of better theories", because that's nonsense logic. A broken theory is broken, it must be wrong.

All of science is based on "broken" theories. Copernicus' ideas were wrong, but they led to Galileo. Galieo's ideas were wrong but they led to Newton. Newton's ideas were wrong, but they led to Einstein. Einstein is probably also wrong, and we're not quote sure what the next wrong theory will be. And yet those and the many other wrong theories have led to better things. Improved theories and actual engineering improvements.

Satellites can be launched into orbit using only Newton's "broken" theories, but it's Einstein's "broken" theories that allow accurate GPS using those satellites. The computer you are using to post to this site was built based on "broken" theories that nevertheless provided a stepping stone to new and better things.

Dark matter is a kludge

[bradley13]

Dark matter has always struck me as a kludge. It amounts to "we don't know WTF is going on, so here's our fudge factor". There is no evidence that dark matter exists, other than the fact that gravity on large scales doesn't behave the way cosmologists expect. Two other possibilities receive too little attention:

- Our current theory of gravity does not apply on the scales we are observing, i.e., the theory is incomplete.

- Physical laws are not constant. e are looking at very distant objects, and seeing them in the distant past. Perhaps universal constants are not, in fact, constant across large spans of space and/or time.

So now they've discovered a galaxy where the kludge factor of dark matter is not needed. Maybe this will prompt more cosmologist to consider the alternatives...

Re:Dark matter is a kludge

[Michael Woodhams]

From Wikipedia:

Dark matter's properties are inferred from observations in gravitational lensing, from the cosmic microwave background, which shows the structure of the universe early in its history, from astronomical observations of the observable universe's current structure, and from evidence about the formation and evolution of galaxies, from mass location during galactic collisions, and from the motion of stars within galaxies, and of galaxies within galactic clusters.

That is way more than a kludge. That is many observations explainable by a simple hypothesis - that is, a scientific theory.

Re:Dark matter is a kludge

[Anonymous Coward]

Dark matter has always struck me as a kludge.

Astrophysicist (but not a cosmologist) here: this is true! In its favour, though, this makes "dark matter" an umbrella term that covers any phenomenon that fits the data. It might be stray, undetected, planet-sized objects; it might be some exotic neutrino variant; it might be little clumps of antiquarks; it might be one of any number of things. All these possibilities are referred to under the term "dark matter".

On the particular possibilities you mention:

Our current theory of gravity does not apply on the scales we are observing, i.e., the theory is incomplete.

This is certainly possible, and some theorists work on it. My understanding, though, is that these approaches postulate energy (with an equivalent mass) resulting from large-scale gravitational fields; that is, you can think of this as a form of dark matter that arises from the gravitational field itself.

This sort of approach has trouble explaining the formation of small-scale dark-matter halos, which depend on some kind of dark-matter self-interaction. It's also incompatible with the example in this article: if "dark matter" results directly from gravitational fields, how can you have a galaxy without it?

Physical laws are not constant. e are looking at very distant objects, and seeing them in the distant past. Perhaps universal constants are not, in fact, constant across large spans of space and/or time.

If this were the case, we'd expect the rotation curves of nearby galaxies to be well-behaved, while more distant galaxies would show gradually increasing evidence for the influence of "dark matter". We don't see that. There are theories that fundamental constants do change over time - I've seen some interesting tests for the speed of light changing based on gamma-ray absorption spectra - but they don't work as an alternative explanation to dark matter.

Re:Dark matter is a kludge

[pots]

You should know that "dark matter" is just matter that doesn't emit light. In other words, it's anything which isn't a star or a hot gas. You are dark matter. It doesn't seem so mysterious when you look at it like that, does it?

There's reason to believe that known types of matter can't account for all, or even most, of dark matter. All this means is that there's another type of matter which doesn't emit light, which we don't know about. Given that neutrinos were only discovered in 1970, and yet they're hugely abundant in the universe, this doesn't seem so implausible.

Re:President without any sense baffles historians

[Anonymous Coward]

And prison guards alike

Yeah.

One wonders what Obama was thinking - "If I give the crazy mullahs in Iran a few hundred billion dollars, they'll STOP trying to build nukes!"

It's so much better to have an Obama-fueled nuclear arms race in the Middle East, right? Because once Iran gets nukes, Saudi Arabia and Egypt won't be far behind...

That'll work out well.

Maybe Obama should have just told the Iranians that there's a "red line" they can't cross....

Re:Except rotation speeds have already been explai

[_Sharp'r_]

Dark matter and dark energy are used as they are the only things that help explain our observations of nature.

Not "the only things that help explain our observations of nature", but rather, "the best things physicists have currently considered that help explain our observations of nature". The OP is right, it's not proven by direct evidence, so it's basically a placeholder for "we don't know, here's a guess of a possibility". The more we do know, the less likely it looks as an explanation, it's just that no better explanation has caught on yet. But in terms of evidence, it's certainly at the gods granted fire to mortals level of explanation. At best, you can say it might be possible. Just because we don't have a better explanation currently, doesn't make dark matter a good explanation. It's not competing with a whole lot. :)

Aliens of the gaps.

[fazig]

Unfortunately "baffles scientists" is a phrase I've come associate with people who use it as a strawman, because there's actually plenty of hypotheses out there. And if they present any of them it's a usually one of the more if not most ridiculous sounding ones. Then they proceed with a false dichotomy by presenting 'maybe it's aliens' with more reasonable sounding arguments than the alleged, opposing position has to offer.

Model error cannot be dismissed (yet)

[sjbe]

The thing is, our theory of gravity (general relativity) makes a lot of other predictions on scales of the same order, and they seem to work fine.

So do Newtonian mechanics but that was proven to be a useful but incomplete model. Likewise it's hardly inconceivable that there are aspects of gravity not adequately described by general relativity. That doesn't mean general relativity is wrong or useless just like Newtonian mechanics are still useful.

Now obviously it very well could be some sort of matter and there is evidence to suggest that is a reasonable proposition. But until we get more evidence the possibility of it being an error in our mathematical models remains non-zero. I think this fact tends to get dismissed because it's a lot less glamorous than to imagine some sort of exotic matter or new particles. But we've seen it happen before where we invoked fanciful solutions (epicyles anyone?) to explain something that was better explained with an improved model.

The Standard Model is incomplete (there are observations it can't account for), and when extending the standard model in ways to account for those observations, many models wind up including particles that would behave consistently with dark matter.

Exactly. The Standard Model is amazing and highly predictive but we still haven't reconciled it with gravity and we know for a fact that it is incomplete. Therefore it's not at all a stretch to imagine that dark matter is evidence of what the Standard Model is still missing. And that is an exciting prospect. I hope we figure out this mystery during my lifetime.