Checking in on troubles with dark matter | Hacker News Reader

Checking in on troubles with dark matter

tritonstation.com

58 pointsxqcgrek237 comments

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If I understand correctly, that is, to simplify almost to the point of absurdity.

Observations about the universe don't match our understanding of physics.

DarkMatter handles the problem by adding stuff we can't see to make it match the physics.

MOND handles the problem by changing the physics to match what we can see.

The problem with DM is why can't we see the stuff.

The problem with MOND is why are local physics different than remote physics.

MOND does a better job at predicting observed phenomenon, because it is based on observed behavior. but fails to provide a good underlying reason for the behavior.

Both have edgecases they fail to handle.

^ this. IMO, We simply don't have enough data, nor the instruments capable of collecting the needed data yet, to explain the observations.

I don't think that's the problem. Data and observations are the same thing. You are saying we need more observations to explain the current observations.

No, we need new theories to explain the current observations.

MOND does a good job at only some observations, in particular galactic scale ones, but it's original form falls over completely when talking about observations of cosmological scales.

So DM does better at matching observations than MOND does at the largest scales, where ironically the modifications of MOND are most important.

This article isn't totally fair, in that the theory of cold dark matter is based mostly on observations of the cosmic microwave background rather than on galaxy behavior. Also, it's not like the 𝛬CDM crowd says they are certain of the nature of DM. And there are plenty of holes in alternative explanations as well.

If I were going to critique the anti-CDM arguments, I'd say that they aren't advancing their own alternative explanations. MOND is a very interesting observation for sure, but advocates freely admit 𝝰 is sort of a magical number and they don't really offer a physical/geometric justification for why it should exist or why it should take the value that it apparently has.

All the same, I love reading these lists of things that don't fit. I'm super glad observers are poking holes in (tearing apart?) the dominant paradigm. Their evidence is coming from the other side of the theory, it's totally valid and very interesting.

The best treatment I've see is Ethan Siegel's write up [0].

[0] https://bigthink.com/starts-with-a-bang/modifying-gravity/

> Also, it's not like the 𝛬CDM crowd says they are certain of the nature of DM.

They seem pretty certain given how easily the epicycles keep getting adopted to keep DM alive, and how readily they dismiss MOND.

> and they don't really offer a physical/geometric justification for why it should exist or why it should take the value that it apparently has.

There are papers that do this actually, such as superfluid DM. Regardless, the lack of justification for an empirical law does not refute the existence of the effect it describes. Unfortunately, DM proponents seem adamant in not trying to explain this effect in terms of DM.

I wasn't aware of MOND's successes beyond rotation curves; the author lists three more. It does seem like LDCM and MOND are both feeling different parts of the same elephant and that someone will eventually see the whole critter. This TritonStation fellow is worth reading.

So many sophisticated experiments have failed to detect dark matter particles. So many theories have predicted still unobserved phenomena. On the other hand, many of us side-eyed LIGO for decades but when it worked, it worked. Black holes were unphysical and abominations until they weren't.

That blog is worth a deep dive, it goes into deep and fascinating detail about how this stuff is measured, as well as the sociology of the field, which has continued to add epicycles to dark matter theory to an extent that is starting to make even true believers uncomfortable.

>which has continued to add epicycles to dark matter theory to an extent that is starting to make even true believers uncomfortable.

You certainly get this impression from pop-sci, but are there prominent people in the field actually saying this? Obviously sometimes the scientific consensus can be wrong (see the planetary nebula 'debate' for example) but normally contrarians turn out to be incorrect.

In peer review academia, ideas progress when a generation dies. Let’s see what the next twenty years show us.

From my layman perspective, either cosmologists enjoy lying down in front of buses or the field advances way faster than they could possibly be dying off.

In areas where prevailing theory lines up with reality.

Where it doesn’t, you get situations like dark matter: theoretically consistent extensions with the framework where it intersects with reality or the additions of free variables to make it just so.

Well. It is hard to tell from the perspective of a person who is a consistent contrarian what exactly the consensus / mainstream viewpoint is, or how much it is perceived to have cracks in it. You would have to survey people who aren't writing from a point of extreme skepticism to get a better sense of it.

> I wasn't aware of MOND's successes beyond rotation curves; the author lists three more

Here's another that he calls the bullet cluster for MOND, something that follows easily from MOND and was predicted in advance, but DM can't really explain even after the fact:

https://tritonstation.com/2016/12/23/crater-2-the-bullet-clu...

Why were you side-eyeing LIGO? Gravitational waves were a prediction of an extremely well tested theory, and all that was needed was to engineer an instrument sensitive enough to see them.

The Hulse and Taylor results seemed pretty conclusive for gravitational waves.

LIGO: I have some experience in optics. The thought of an interferometer detecting a displacement of a small fraction of a proton in the presence of all that noise was way, way out there. They'd also been working at it for decades with null result after null result. It was like fusion, always around the corner.

And then it worked handing all of us doubters a nice cup of STFU.

Was just watching an "intro" lecture[1] on DM over at PIRSA, as part of TRISEP 2023[2], a summer school for grad students. There he goes over some of the arguments for DM, but with more math and details than the usual popsci stuff, but still at a fairly accessible level.

I'm just a layman but to me the Bullet Cluster[3] seems very hard to reconcile with just modified theories of gravity. On the other hand it seems unreasonable to me that General Relativity doesn't need corrections due to quantum mechanics, and as this article mentions LCDM has a bunch of other issues.

So how about both? Some non-interacting "dark" particle(s) and some quantum corrections to General Relativity. Anyone know about work trying to combine the two approaches, and see how things stack up?

[1]: https://pirsa.org/23060067

[2]: https://pirsa.org/C23020

[3]: https://en.wikipedia.org/wiki/Bullet_Cluster

It's a bit of a false dichotomy. Even the MOND people recognize that you need dark matter (even if they don't use the term) to explain the bullet cluster, so it isn't really dark matter vs MOND.

It's "dark matter" vs "MOND + dark matter"

I admit I haven't followed the field closely, but when reading papers about MOND I've never gotten that impression.

Do you have some examples?

edit: I hadn't read (or forgotten) the notes by Milgrom as shared in the other reply.

Milgrom's view on the bullet cluster:

http://astroweb.case.edu/ssm/mond/moti_bullet.html

LCDM also apparently fails to explain the velocity of the bullet cluster, so it's not like it's slam dunk for DM (collision velocity better matches MOND).

Some MOND proponents posit "sterile neutrinos" to complete the theory in some cases, like the bullet cluster. And then there are bullet cluster-like failures for LCDM that get papered over:

https://tritonstation.com/2016/12/23/crater-2-the-bullet-clu...

There have also been recent measures that suggest that much existing data is actually more consistent with warm regular matter that has so far gone undetected rather than cold dark matter.

The dark matter "consensus" is a flimsy facade covering a lot of gaping holes. Something's gotta give.

Thanks, I had missed that note by Milgrom.

With JWST it seems we might be able to get closer to an answer[1]. Sterile neutrinos[2] seemed like an obvious candidate, but we'll see how that pans out. Current searches aren't positive[3][4], but I suppose it's far to early to call it quits on that given the theoretical motivation.

[1]: https://arxiv.org/abs/2302.00030

[2]: https://arxiv.org/abs/2106.05913

[3]: https://phys.org/news/2022-11-analysis-closer-sterile-neutri...

[4]: https://phys.org/news/2023-01-results-stereo-sterile-neutrin...

Aren't sterile neutrinos dark matter candidates?

Can the Modified Newtonian Dynamics (MOND) theory adequately explain the Cosmic Microwave Background (CMB) radiation and the persistent mass discrepancies observed in galaxy clusters?

Furthermore, as a laiman, I'm puzzled by how certain versions of MOND requires introduction of new fields. Doesn't this effectively the same as saying there is some new particle we neeed to discover?

Are you still a layman if you’re able to ask these questions though?

If you got at least halfway in to Sean Carroll’s recent 4 hour podcast on the subject — yes ;)

I thank you for this reference. Having a blast listening to him.

MOND is really a rule without a theory; it simply posits that there is an acceleration cutoff below which gravity does not obey an inverse square law. This conjecture has been shown to have impressive predictive power, and it naturally explains a bunch of observations that are not obviously related and require fine-tuning in a dark matter paradigm. The blog linked here goes into a lot of detail on those observations, as well as observed phenomena that don't seem to fit well with MOND.

What the theory underlying MOND actually is is anyone's guess, and an area of active conjecture. But that's just as true for dark matter.

Another equally plausible MOND model is one in which the gravitational constant G increases with distance. This is what is observed in the orbits of stars around the center of a galaxy.

Of course, this is a pseudo-increase caused by, you guessed it, dark matter.

There is a relativistic MOND theory that has predicted the CMB spectrum[0]. And perhaps. MOND is first of all observational. AeST is just the latest (and AFAIK best) attempt to create a theory that reproduces the MOND acceleration artifact.

0: https://tritonstation.com/2023/01/13/what-we-have-here-is-a-...

I would like to hear about some alternatives to LCDM that are not MOND for once. Surely, they're out there... and some may even be compatible with general relativity.

Quantized inertia might be worth looking into. Proposed by Mike McCulloch https://twitter.com/memcculloch/status/1179077969345732608

I poked around that dude's twitter and found this recent tweet: https://twitter.com/memcculloch/status/1688260680929931264 I'll grant the possibility that he's doing some kind of loaded question to talk up his theory, but going by the rest of his tweets (which seem to be mostly basic promotion and culture war stuff), that's a long shot.

The observation that we seem to live in the emptiest region of known space should cast doubt on our understanding of universe. Are we watching through fish-eyed lens? Something kills life after critical concentration? Funny third option?

Wow this is one quality article. Thanks a ton.

Sooner or later, the physics community will come to their senses and realize that every massive particle is surrounded by its own dark matter halo. This is the reason that measurements of the gravitational constant G suffer from a high uncertainty problem not seen with other physics measurements.

Then it will become obvious that the halo is a neutral electric field created by real photons, not virtual photons.

Rebel Science

hello crankgpt!

Haha, Hi there.

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