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The reasons given for Dark Matter always begin with galaxy rotation curves (the galaxy rotation problem and the galaxy winding problem). But the problem with this "evidence" is that the observations were made of galaxies in isolation, but even galaxies are not gravitationally isolated.[1] I can not speak to the veracity of the rest of the evidence for Dark Matter, but galaxy rotation curves can no longer be counted as evidence of Dark Matter because they are more simply explained by a gravitational phenomenon caused by the out of frame mass of other nearby galaxies.

[1] https://www.youtube.com/watch?v=PL0ewiwqoTw&t=4m5s

There are many theories of dark matter given at conferences like this. Around that time (2013-2014) I remember hearing about axion theories too. I don't know anything about this person's presentation so I can't say how it displaces the problem of galaxy rotation curves. If the math checks out, I think it would have, but you never know. I would like to see a reaction from cosmologists.

I would hesitate leaning too much on the always part. Fritz Zwicky was the first to notice the anomalous galaxy rotation curves and propose dark matter (is the story that I remember, anyway.) This is probably why it's presented first. It does not mean that it's the most important piece of evidence.

I'm intentionally writing with a light touch here because I don't consider myself an expert in the field.

> a gravitational phenomenon caused by the out of frame mass of other nearby galaxies

There have been papers published on this, but it is still speculative, so it's premature to say that this kind of alternative theory rules out dark matter.

> it's premature to say that this kind of alternative theory rules out dark matter

No, it only eliminates the need for Dark Matter to explain galaxy rotation curves.

Explanations always begin with Galaxy rotation curves because that's how it was first notified that something about our understanding of the universe was off.

It's been a long time since then and there's been a ton more data collected about wide ranging phenomenon since.

A naive question from a hobbyist with over-average interest in particle physics: quantum theory is based on complex numbers. Has anyone considered that mass/gravity may also have an imaginary component that manifests itself as "dark matter"? (I.e., it's not "matter" at all, just an incomplete understanding of gravity.)

That's exactly the kind of question a curious physics student would ask. This might be what you're looking for: https://en.wikipedia.org/wiki/Tachyonic_field

> In physics, a tachyonic field, or simply tachyon, is a quantum field with an imaginary mass.

I hear the word Tachyon so much in science fiction that I was under the impression it was a sci-fi word.

It almost is. https://en.wikipedia.org/wiki/Tachyon#History:

The term tachyon was coined by Gerald Feinberg in a 1967 paper titled "Possibility of faster-than-light particles". He had been inspired by the science-fiction story "Beep" by James Blish.“

I don’t think “Beep” introduced the word, though (https://archive.org/details/galaxymagazine-1954-02)

Good question. Imaginary mass -> negative 4-momentum -> momentum exceeds energy -> object is moving faster than light (backwards in time in some reference frames).

There are some reasons to suspect/hope such things might not exist, such as protecting causality.

In general, asking "what if <simple field type> was <more complex field type>" can actually produce super fruitful lines of inquiry. You can use "complex numbers", "quaternions", "finite fields", whatever you like. There's plenty of unexplored ground here.

Given the quantum eraser experiment and general lack of time directionality in particle physics it seems causality as an "axiomatic" physical truth is already broken. At least at a quantum level. My suspicion is that event-based causality falls into the 'its only resolved for >= hbar/2' realm. Then even if you had a bunch "tachyons" it'd be impossible to send a signal back in time more than a few planck constants of time much like its hard to entangle more than a few qbits.

Superluminality isn't required by all interpretations of QM. MW doesn't need it, for example.

I'm not aware of a mechanism by which causality would arise from known QM, so I wouldn't expect a priori planck scale to play into it. There would also need to be an energy term for h to make sense as the relevant constant.

The most compelling source of temporal direction I'm familiar with is that it's a regional cosmological entropy gradient rather than a fundamental aspect of physics, although it strikes me as suspicious that the time direction lies inside the 4-cone generated by the odd-dimension-out in our spacetime metric. That certainly lends some credence to theories which would predict the direction of time as fundamental to physics.

As someone who is similarly and over-interested hobbyist, I wonder what happens when you throw in even more, for lack of the proper term, deviant math systems. The cousins of imaginary numbers using either epsilon (where epsilon^2 = 0, epsilon not a member of R) or j (where j^2 = 1, j not a member of R). Or quaternions. Or the family of quaternions similar to the family of complex numbers. And so on.

My guess is that it falls back to Occam's Razor and there has yet to be any physics to use it thus it isn't a serious contender for current problems until much more work has been done on all the threads already being followed.

Is it possible that Dark Matter is simply not possible to observe from our frame of reference?

But we are only talking about it because we can observe it. Or rather observe how it interacts with other things, but that's just semantics: all observation happens via interactions.

Dark matter could be a neighboring universe interacting with ours or something like that where the actual particle causing it is fundamentally out of reach (if it even is a particle), but just the fact that it interacts with our universe means we should in principle be able to interact with it and characterize it with experiments.

Thats more along the lines of what I was thinking - we can observe this specific interaction but without also observing other interactions it seems very difficult to define exactly what it is. Or maybe it is just that our current frame of reference (both physically and in terms of time) is just a bad vantage point to do so.

An important property of dark matter need to fit what we are seeing is that it also doesn't interact with itself. It would seem weird if it can't interact with itself because of some innate property, but we can't interact with it because we are somehow out of phase with it.

Might dispersed matter have a higher mass than more condensed matter as an alternative hypothesis? Where the more distant and small a unit of matter is, the more curved the spacetime around it. Lonely particles longing to coalesce? These particles would reflect and scatter too little to be visible and hence be "dark matter". If this was the case, stronger attraction at either extreme of distance might be an effecient accelerator of formation of celestial bodies. This kind of wonky gravitation could also explain the accelerating expansion of the universe.

No.

"Dark" matter refers to something that does not interact at all with photons - not that it scatters too little light.

What evidence do we have that the dark matter isn't interacting _at all_ with light? I would have thought there would be limits in our instruments.

>I left feeling disenchanted

In scientific study you want and need failure. Knowing what doesn't work is as important as knowing what does. This way we can make lists of dangerous or useless things to be avoided. As long as things are documented and reproducible to the best of your abilities, you've won.

I do not consider the search for dark matter a waste of time. But when you are working with a team of WIMP-detector-building specialists, it is (unsurprisingly) hard to have a conversation about whether we should continue building WIMP detectors. People’s egos and emotions are all wrapped up in the science, and that is the part I came away disenchanted about. Definitely don’t misinterpret my feelings to be about science in general, or about getting a negative result (the Michelson-Morley experiment comes to mind.)

I know how to build a dream catcher.

Dark matter is possibly the best evidence in support of Dr. Mills' Hydrino theory https://brilliantlightpower.com/theory/ which deserves a fair reading. There are about 20 experimental results supporting this theory that models electron orbits as spherical shells instead of probability clouds. He has machines that produce 100s of kW from small quantities of hydrogen - 200 times the energy produced by burning hydrogen. https://brilliantlightpower.com/tpv-suncell-rectanguloid-cav...

    In 1999, the Nobel prize winning physicist Philip Warren Anderson said he is "sure that it's a fraud",[12] and in the same year another Nobel prize winning physicist, Steven Chu, called it "extremely unlikely".[23] The following year, a 2000 patent based on its hydrino-related technology[24][25] was later withdrawn by the United States Patent and Trademark Office (USPTO) due to contradictions with known physics laws and other concerns about the viability of the described processes, citing Park and others.[26]

Source: https://en.wikipedia.org/wiki/Brilliant_Light_Power

Every submission you've made seems to be about this company (https://news.ycombinator.com/submitted?id=dave333).

So how do these Nobel prize winners explain dark matter? Do "known physics laws" not evolve as new discoveries are made? Wikipedia is policed by skeptics that only support the status quo. IMO that article is not a fair description of Brilliant Light Power. As for my submissions, if as I believe hydrinos turn out to be real, it will be the greatest scientific discovery since the genome, so worth harping on about.