Triplet of 'ghostly' galaxies hints at what dark matter is and isn't

I was tempted to comment that there’s no evidence that you can make anything complex out of dark matter, but it turns out that some physicists hypothesize that “self-interacting” dark matter could bind into something like atoms and molecules, so carry on!

> That all three gossamer galaxies existed along a line, as neighbors, didn’t escape the attention of astrophysicists, who began to coalesce around an origin story.

How about this:

What we see is an advanced galaxy-killer alien weapon being fired at one target, and then continuing to travel beyond that target and making 2 more (unintended?) victims.

We noticed that some equations only make sense if mass is greater than what we'd expect from what we can see. So for now we pretend that there's this dark matter that we can't see, and with it all math so far checks out.

But we don't know if that missing mass exists at all, or if some formulas are wrong. So it's definitely a "hack", but it works for now and we don't have anything better. And we do expect to find an explanation at some point.

1. Galactic rotation curves (galaxies spin too fast)

2. Galactic gravitional lensing (galaxies bend too much light)

3. The shape of the cosmic microwave background (it only matches if you have dark matter)

4. The bullet cluster (visible matter collided, but the gravitional lensing kept going)

Non-electromagnetically interacting matter solves all of these, and the same ratio of matter:dark matter solves all of them. That would be one hell of a coincidence.

The standard model has lots of this kind of thing. Leptons don't feel the strong force. Neutrinos are electrically neutral. The weak force only couples to left-handed particles. It seems completely natural to expect some dark matter. The only question is how much.

But we have plenty of observations to lead us to what dark might be and what it definitely isn't.

Any theory of dark matter needs to explain all of the following:

1. The Virial Theorem, galaxy clusters, and the speeds of galaxies in clusters. (both MOND and CDM both do a good job of explaining this)

2. Galactic rotation curves. (MOND and CDM both do a good job of explaining this)

3. Accoustic Baryon Oscillations. (ABO) Immediately after the big bang, everything was ludicrously hot. The most powerful force in the universe was the wash of gamma rays everywhere. These gamma rays carried electrons around. The electric fields of the electrons carried the protons around. (the light dominated phase) As the universe cooled, this flipped; the protons were carried around by momentum, which brought the electrons with them, which mediated the light. (the matter dominated phase, where we are today) During the light dominated phase, sound waves pulsed through the universe, causing dense regions and underdense regions. When the universe flipped from being light dominated to matter dominated, these regions were frozen in time, and we can see them directly in the CMB. (cosmic microwave background) We can analyze the properties of these pressure waves to determine what the properties of the matter and light in the universe were. These waves demand some sort of dark matter. CDM explains these observations, MOND does not.

4. The Bullet Cluster. The Bullet Cluster is a collision of two galaxy clusters. We can measure gravitation lensing and light to determine where the dark matter is and where the regular matter is. The bullet cluster demonstrates that these two properties are not the same. There is matter which has gravity and gravitational lensing, but that we can't see. CDM explains these observations, MOND does not.

5. Ultra diffuse galaxies with very little dark matter. Not all galaxies have galactic rotation curves which require some extra sauce to explain the movement of their stars. Some galaxies are explained perfectly by the matter that we can see in them. So we need dark matter for most galaxies, but some galaxies need to be dark matter free. These galaxies falsify MOND.

6. Leading spiral arms in barred spiral galaxies. The large majority of barred spiral galaxies have their arms trailing from their bars, but a very small number of them have arms which lead in front of the bars. Computer simulations show this happens naturally with CDM, but MOND or dark matter free spiral galaxies don't have a mechanism to cause this to happen.

7. Gravitational waves travel at the speed of light. Most MOND formulations predict gravitational waves travel slower than c; all of those formulations are wrong.

There is a genuinely absurd amount of time, effort, and money that has gone into explaining dark matter. It is not a band aid, we haven't just added 1 to moved on. Everybody is pouring over every step of the algorithm trying to figure out why it's giving us the wrong answer. CDM is the leading hypothesis right now, because it explains all of the observations, and is not falsified by any of the observations. MACHO is/was another theory; it has gone out of fashion because it does not explain all of the observations and makes predictions we do not observe when we check for them. MOND is out of fashion because it is falsified by the data. MOND is still in fashion among a particular brand of people who just want to have their names in the history books; if someone did prove MOND, the history books would all have lists that go "Ptolemy, Copernicus, Kepler, Newton, Einstein, <your name here>."

But for Pete's sake it's not as if people have their fingers in their ears.

explanation of acronyms/terms:

1. Dark Matter. Dark Matter is the observation that the universe acts as if there's a lot more matter in it other than the stuff we can see. Basically no one says that Dark Matter isn't real; that would imply that the observations are wrong.

2. MOND. Modified Newtonian Gravity. MOND is a family of models which aim to explain the dark matter observation. Instead of using F=Gm1m2/r^2 to determine the force of gravity, we have better math, that gives a different equation, that accurately explains Dark Matter. When you say "physicists discover a much more logical explanation" that's what you're talking about.

3. CDM. Cold Dark Matter. CDM is a family of models which aim to explain the dark matter observation. CDM implies that Dark Matter is explained by particles. CDM implies there is some sort of particle of family of particles which interact with normal matter via gravity as normal, via the electromagnetic or strong nuclear force not at all, or via the weak nuclear force super weakly or not at all. CDM interacts with other CDM particles via gravity as normal, but only weakly via either the weak nuclear force or some other as-yet undiscovered force. People often use the terms Dark Matter (referring to the observation) and Cold Dark Matter interchangeably.

4. CMB. Cosmic Microwave Background radiation. After the Big Bang, the universe was opaque. Light would be absorbed by the stuff in the universe and then re-emitted. During recombination, the universe became transparent. The last moment of the various bits and parts of the universe switching from being opaque to transparent has the entire universe glowing in microwaves. This moment is captured in a snapshot, known as the cosmic microwave background. As we understand the CMB better, we learn a lot--a lot a lot--about the early universe.

Everything aligns perfectly with matter that only interacts with gravity. This means it is invisible and nearly intangible. "That sounds crazy, nothing like that should ever exist" you might say. Except it already does - neutrinos. We just don't think neutrinos quite fit the bill.

Imagine if scientists were trying to identify some sort of sea monster, where cameras have gotten minor glimpses here and there. They aren't sure whether it is a giant squid, or octopus, or whale, or whatever and there's a lot of debate around it. And some non-scientist comes along and says "that's obviously a band-aid, we all know that animals can't exist in the ocean, so maybe you are wrong about what the ocean even is."

The problem here is two-fold. First, the non-scientist is misunderstanding the debate. The scientists actually have a pretty good idea of what's going on, they just lack the evidence to nail down the exact thing they are seeing. Meanwhile the non-scientist thinks that they are completely stumped and need an Einstein level change in direction in their thinking. Second, the non-scientist thinks that nothing acts like what the scientists are looking for. However, that's not true, the "weird" attributes aren't all that weird, they just aren't familiar to the non-scientist.

The irony is that while everyone thinks that dark matter is a band-aid solution and there's not much room for it to be, there is currently an idea that is likely to be a bandaid solution - dark energy. The main idea behind the acceleration of the expansion of the universe is that empty space contains energy, which we call dark energy. This makes logical sense and quantum physics agrees it does. However, any kind of actual measurement of the amount of energy per unit of space that we try just doesn't make sense. The theory doesn't align with our measurements to a vast degree (to where it's known as the worst prediction in the history of physics), and our measurements don't even align with each other. There is room here for a truly novel solution, such as varying amounts of energy per unit of space, or the cosmological principle being incorrect.

This should set off crank alarms when you read it.

Ok then

https://arxiv.org/abs/2104.13961

my first impression was this could actually be a counterexample, because there we can see the heavy, dark matter-full clusters continuing on their trajectories at full speed whereas the gas slowed down and is being left behind. compared to these 3 lightweight galaxies which seem to be alone, rather than trailing behind their dark matter-hosting parents

but perhaps these 3 _are_ going slower than their full-scale origin galaxies. I would stake my view as a layman on whether anyone can find the dark matter galaxies that spawned these farther along their expected trajectories

Friction between regular matter causes it to lose energy and thus velocity. Dark matter doesn't have self-interaction outside of gravity, so it just keeps going.

van Neumann's quote comes to mind - "With four parameters I can fit an elephant, and with five I can make him wiggle his trunk."