New Hubble data suggests an ingredient missing from current dark matter theories
phys.org
phys.org
As I understand, they see a discrepancy between simulation and observation of some lensing observables. Could be something wrong with the calculation of the observables, something wrong with the implementation of the simulations, or (the exciting bit) with the physics put into the simulations. As a complete non-expert on lensing observables and cosmological simulations, I'd guess it's one of the first two. As mentioned in the text, there are many other discrepancies between observations and simulations that are still unresolved. This adds to that list.
People need to understand that physicists do NOT consider dark matter to be settled, in any sense! The experiments are not done to polish up any theory... they're done because all of the theories are awful, and guidance from observations is necessary for further progress.
I also still fully stand by my previous comment on this subject: https://news.ycombinator.com/item?id=23666620
Contrary to your linked comment, to this non-physicist, nearly all physicists in public seem to say that, while the details are unknown, dark matter is pretty much definitely a thing. AFAICT they're not wrong about there being multiple lines of evidence for it. But if you mean "no one worth listening to" says even that much, then I don't know any physicists worth listening to.
It does not seem like it to me (non physicist with some physicist friends from other fields who try to dumb down stuff to me).
In my world (SE) it seems like a proprietary binary that must be included for the rest of an open source product describing the observable universe to work very well. Everyone seems to hate it, but nobody wrote something better, yet. Every competing implementation fails to work in other areas that we already are using in production, extensively.
And when some professional, who read and understood the source code, starts talking about it, a lot of people voice stupid ideas that they would not have if they passed some lectures on the subject.
Edit to add: I envy physicists a little - the best thing for “them” to happen seems to be if something does not work in a very peculiar way.
Almost every honest physicist will say that. But not all physicists are honest, and I don't trust that there is a positive correlation between honest scientists and loudness in the scientific community, especially where it interfaces with the general public. And if you want an explanation of why dark matter brings out the crazies, I think that's as good a reason as any.
For example, throughout my career, whatever the commercial domain, there is almost always a noisy competitor whose product is an amateur hour shit-show, usually a superficial clone of the market leader, that still picks up market share from naive customers by sheer volume of message.
When you’re trying to focus on making something that actually creates value for others this can be an irritating distraction. “Pay attention to competitors, but obsess over customers” isn’t always a comfortable mantra to fulfil.
Most of my peers from high school went into high energy physics (one works at JLab, another is a professor at an east coast college; the others quit or went into... other fields, like finance). Although I am no longer a practicing biochemist, when we got together we complained about the exact same things. So yeah, I don't know about the intricacies of what happens in physics. But that doesn't mean I don't have real insight about what's going on.
It was nearly a millenia between the sacking of Rome and the Renaissance. Intellectuals seem to have the idea that we can’t have that type of reset again. As populations options narrow with climate change and vaunted scientists fail to provide solutions, those populations will turn elsewhere.
Because everybody thinks they can "intuit" their way to knowledge.
No. No you can't. We KNOW you can't. We have experiments and statistics and double-blind trials for exactly this reason.
"Intuition" only works when coupled with huge amounts of knowledge and effort.
Part of the problem is that too many people never suffer the consequences of "truth" in modern society. Consequently, far too many people get the idea that "truth" is negotiable.
But, this is not new:
“There is a cult of ignorance in the United States, and there has always been. The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.”
― Issac Asimov
The existential confrontation of research processes and discourse is such anathema to a prevailing mindset of certainty and explanation that a psychosis may develop.
So bring your kids up to be comfortable with doubt, to inoculate them against becoming conspiracy nuts.
It's intuitive, it's not suggested by evidence, it's based in imagination rather than reason, but I imagine the universe as a matter / energy creation machine in which the forces that tear everything apart seem to be generative. I think perhaps over time we will observe a correlation between the entropic forces at the edge of the universe and the gravitational forces of dark matter in the center of galaxies. I think dark matter is dark because its light hasn't reached us as its trapped by gravity, yet its gravity is there. It's new matter, and we don't know how it got there. I think we'll find the bridge by looking into quantum mechanics, and how particles behave in relation to one another when they're in conditions similar to those described at the point of heat death of the universe, everything broken down, even time. When that happens, what's the difference between any one thing and another? I'm not entirely convinced: "well, nothing at all" is the answer. Do we know nothing at all is stable? The big bang seems to suggest that nothing at all is extremely unstable.
I think the physicists are touchy because imagination has replaced reasoning in key areas. Whereas you and I would probably love to be told exactly why we might be wrong about our crackpot musings in a field we upfront claim to know little about.
Why haven't MOND or other theories become more popular?
Edit: MOND isn't great either - it just has a parameter that they tune until it matches the observation.
Edit 2: I am not a physicist, I am simply a concerned citizen.
There's a fundamental difference though. With dark matter, the evidence of the stuff is much more direct and precise. We can determine the shape and concentration of it. Aether on the other hand was a hand-wavy metaphysical concept from the beginning. There was never any experimental evidence that aether was a real thing; it was just a concept invented to patch up the inherent inconsistencies in electromagnetic theory near the speed of light.
But I think you might be after something:
> Given that, "dark matter" is as inoffensive a name as possible.
Precisely. Dark matter is boring. People want Klingons.
> We know there's something out of ordinary, it behaves like matter, and it doesn't interact with light
That's wrong, and presupposes the existence of dark matter, rather than treating it as a possibility. What we know is that our formulas don't match up with our observations. We don't know why that is. One suggestion is that there is some sort of invisible stuff out there that we cannot detect, but would make our formulas add up, so it could exist. Its perfectly legitimate to speculate about the existence of dark matter, and to set up different experiments to try to detect it, but its not fine to pretend that its "settled science" that dark matter is out there and its only a matter of finding it. Unless and until someone detects "dark matter" or figures out another reason why our calculations don't add up its existence will remain an open question.
DM: we see complexity, let's treat it as fundamental.
MOND: we see complexity, let's ignore the inconvenient parts.
I think that is a mischaracterization; IIRC, MOND fits the overwhelming balance of galaxies with a single parameter, and there are only a handful of exceptions (probably around in the hundreds or so?) out of all of the galaxies.
also: MOND doesn't explain intergalactic movement, or the clumpiness of the universe. But there's ALSO no good reason to believe that that "LCDM dark matter" does either, because by the "curve-fitting nature" of LCDM it could literally explain anything. If all of the oxygen in my room went to the northeast corner of my room and suffocated me, you could come up with a dark matter field that explained that phenomenon. As a scientist, that worries me. Also doesn't mean that LCDM is wrong.
There are however other issues with MOND including that gravity is still instantaneous (in most variations) which we know with gravitational waves it isn't, as well as that most of them don't lend to the formation of stars and galaxies. If we take vanilla MOND then the universe as we know it shouldn't have been formed matter wouldn't clump up to form the formations we can see around us and the one we live on.
MOND isn't a career ender as much as say cold fusion is, but it's pretty close.
And in all honesty if we look at Modified Gravity theories as a whole they shouldn't be anymore more offensive to one's career than say string theory and despite string theory being essentially a dead end for decades now it doesn't get anywhere near that level of stigma that MOND does and that's more for a cultural / dogmatic reason than purely scientific ones.
I don't think you can even be a 'concerned citizen' on this topic without some introductory knowledge of dark matter.
There are things to be frustrated about with the dark matter hypothesis (and I am too), but lack of evidence isn't one. Dark energy seems to be another beast though...
https://en.m.wikipedia.org/wiki/Dark_matter#Observational_ev...
It seems quite difficult to modify GR or make a new theory of gravity which explains the new observations while matching GR in the old ones, where GR has been verified. But if someone manages, then you'll see scientists flock to it.
I'm not shunning the idea, I've worked on alternative explanations myself (with no concrete result), but for the moment we remain convinced that dark matter is the better theory.
I _really_ like what you did here :-)
Key to note that this doesn't mean it's right. But one wonders why there isn't more skepticism about conventional dark matter theories.
Like what if I said "there isn't global warming, there's just a mysterious unobservable dark thermal input adjustment that we have to apply to every month's reading to make our models work out". You'd say that I was crazy.
The appropriate MOND/DM analogy here would be to propose general relativity as an explanation for Mercury but then find another planet(oid) in the system where GR suggests an effect, but to notice that the effect is measurably not present. You can’t just say “got most of them right” when you propose such a theory. You need to revise your theory until it explains the observations.
There is a difference between observations and _experimental_ evidence, which I think the GP was referring to.
The only thing we haven't done is figured out what it is in terms of other concepts we know, such as elementary particles.
Healthy skepticism is a good thing for science. Until DM is proven, it is worth exploring other avenues--may the best experiment win. If you don't understand this, you are a zealot rather than a scientist.
> Honestly, I'm a little tired of the whole dark matter thing. There's no evidence for it, and physicists keep coming up with more and more complex models to fit the data when they don't actually understand what's going on there.
Reads as:
"I know nothing, but I assume that there's no evidence for this because I haven't done any research, and I assume that physicists are foolishly sticking to the theory because they're desperate".
This couldn't be further from the truth, and it could be dispelled with 5 minutes of googling.
You're being downvoted for the same dismissive cluelessness:
> Last I checked, it had not been verified via experimental evidence. Is that still the case?
In reality, there has been mountains of experimental evidence ever since the theory was first proposed, which is why it has been taken seriously for decades. That's why the "Observational evidence" section on Wikipedia has 11 sections in it, reflecting thousands of papers. We still don't know what it is at an _atomic_ level but we know where it is, what it weighs, how it moves, and what it interacts with.
Perhaps that is the complaint here - and asking for academic background is an argument from authority...
- it cannot be detected the way you detect your phone in your hand or see light so your definition of detection seems to escape the basic premise of dark matter: its ability to avoid electromagnetic force, and also,
- we have detected it through gravitational anomalies
With the DM effect, we ONLY see gravitational anomalies as you said. Though DM may very well be the reason, it does not automatically mean it MUST be some matter we cannot see
No it isn't. It isn't even an argument of any sort. I think most reasonable people would interpret that as an attempt to gather evidence to support applying a heuristic filter to the question of "is it worth my time to continue engaging with this person or not?"
Except for all the ways we have to detect actual matter (i.e. gravitation) that fits its properties.
Another problem with MOND is that no matter how you tune it, it can't explain the Bullet cluster for example.
If the bullet cluster is a great counter example for MOND, then conventional dark matter is highly opportunistic curve fitting (which doesn't mean it's not correct).
However, that leads to your follow-up question why MOND and other modified models of gravity are not more popular. It's not for lack of trying:
https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
Also IIRC, Rubin's seminal observations on dark matter were in the 60s too, and dark matter was postulated in 1930.
is that really a huge difference? plenty of eventually-proven math conjectures were initially posed with no knowledge of how a proof will be found.
perhaps with the higgs it was easier because it was conceivable that we could produce the necessary collision energies here on earth, while other problems remain at unattainable scales for a clear path to exist today.
As a theoretical math major and a practicing scientist, I'm just going to have to say that these are not comparable, neither in process, nor ontologically.
> conceivable
that's a very weak way of saying the scaling factor of collision energies was known, and the technology to build the required machine right around the corner (we could have found the higgs with technology proposed for the hole in texas, only 20-or-so years after the prediction of the higgs; at the time higgs was postulated superconductors were already known for 10 years and the first superconducting NMR was 10 years away).
Anyways, my point is that there is a categorical difference. At the time of its postulation, building an experiment that could say yay or nay about the existence of the higgs was largely an engineering problem.
With DMwe're shooting in the dark, so to speak, in hope something will turn up
There are essentially 3 (or 2.5) explanations, that we have (a lot of) missing mass, that we don't understand gravity or that we don't understand spacetime.
Most theories revolve around the missing mass part this includes all possible theoretical candidates for Dark Matter from both within and beyond the standard model, overall a single candidate or a combination of a few likely ones seems to fit our observations the most.
Modified Gravity; these theories actually have gotten plenty of attention in the 80's and 90's but the problem is that most of them don't fit observations especially modern ones like the bullet cluster and gravitational waves, most of them aren't relativistic (or don't have a relativistic formulation) and they overall suck at predicting everything else including the formation of galaxies and stars in the first place.
"Modified Spacetime"; mostly relegated to the dark corners of the web and to pseudo scientific forums technically possible but I haven't seen anything that comes close to being an actual theory as in having an actual mathematical basis. They basically revolve around various possible ways of affecting spacetime curvature, basically if you could curve spacetime without out mass to the same extent as say the earth's mass does you'll technically will end up with the same gravity well. The more "serious" of these tend to revolve around various attempts at gauge gravity.
I usually consider the "Modified Spacetime" as being an offshoot of Modified Gravity theories.
[citation needed]