## Monday, January 15, 2018 ... /////

### Superfluid dark matter, an example of excessive hype

Presenting such papers as revolutions in physics is a full-blown scam

The most recent text on Backreaction is titled Superfluid dark matter gets seriously into business. At this moment, this popular text celebrates a November 2017 preprint by Justin Khoury and two co-authors which added some technicalities to Khoury's program that's been around for some three years.

Justin Khoury is a cosmologist who is well-known for his work on colliding branes cosmologies, chameleon fields, and a few other topics. You should also search Google Scholar for Justin Khoury superfluid. You will find several papers – the most famous of which has 62 citations at this moment. That's fine but much fewer than Khoury's most famous papers that are safely above 1,000 citations. The "revolutionary" November 2017 paper on the "superfluid dark matter" only has one self-citation so far.

Hossenfelder's popular text ends up with this short paragraph:

I consider this one of the most interesting developments in the foundations of physics I have seen in my lifetime. Superfluid dark matter is without doubt a pretty cool idea.
These are big words. Is there some substance for such big words? Well, I could imagine there could be and 1% of the time, I could get slightly excited about the idea. But 99% of the time, I feel certain that there is no conceivable justification for such big words, and not even a justification for words that would be 90% smaller.

Superfluid dark matter is supposed to be a hybrid of the "cold dark matter" paradigm which is the standard way to explain the anomalies in the rotation curves of galaxies and "corresponding" aspects of the expansion of the Universe; and the "modified gravity" which tries to modify the equations of gravity, fails to provide us with a satisfactory picture of physics and cosmology, but could be a "simpler" theory that intriguingly explains some universal phenomenological laws that seem to be obeyed even though "cold dark matter" has no explanation for them.

OK, according to superfluid dark matter, the Universe is filled with some low-viscosity fluid, a superfluid, and it acts like dark matter. But a standardized description of the dynamics within this fluid may also be interpreted as "modified gravity".

It seems like a plausible combination of approaches but the devil is in the details. However, what I find extremely weird is the idea that this rough paradigm is enough for a revolution in cosmology or physics. You know, the "anomalous" galactic rotation curves are either explained with the help of some new matter – which may carry some variable entropy density and which is assumed not to be visible in the telescopes – or without it. This is a Yes/No question. So if there's some extra matter which is a superfluid, it's still some extra matter – in other words, it must be considered an example of dark matter. After all, even superfluid dark matter has to have some microscopic behavior which may be studied by local experiments – it must be composed of some (probably new) particle species.

The Universe must still allow the idealized "empty space" phenomena that have been measured extremely accurately and incorporated into the state-of-the-art theories of particle physics. For this reason, whether or not someone (e.g. Erik Verlinde) gets completely lost in vague, almost religious musings saying that the "spacetime might be a fluid", any "dark matter superfluid" or anything of that sort simply has to be some extra matter added on top of the things we know to exist. Any such dark matter may also be captured by some macroscopic, "hydrodynamic or aerodynamic" equations, and if the dark matter is a superfluid, they may have some special features.

(The empty space might in principle be a "fluid" but if the entropy density were nonzero and variable, the conflict with the tests of relativity would be almost unavoidable because such a fluid would be nothing else than a variation of the aether even though, in this case, it wouldn't be the luminiferous aether but rather the lumo-prohibiting aether. Lumo is light, not only in Esperanto, just to be sure. The entropy density, along with an entropy flux, is a 4-vector and its nonzero value breaks the Lorentz invariance. So any matter with some entropy density does so which is bad. A Lorentz-covariant spacetime fluid could in principle exist but it would have to be a new dual description of string/M-theory and it's clearly hopeless to dream about any Lorentz-covariant "fluid" without a glimpse of evidence of such a connection to string/M-theory.)

But because every dark matter model has such emergent, "hydrodynamic" field equations, I think it's just wrong to sell the "dark matter superfluid" as a totally new paradigm. These authors still add dark matter; and they must still decide whether Einstein's equations hold at the fundamental classical level. One may spread lots of hype about a "revolution" but at the end, it's just another technical model of dark matter, like e.g. the ultralight axion model by Witten et al.

Note that Witten et al. have employed an extremely modest, technical language – which is appropriate despite the fact that their proposal is clever and attractive. This approach is so different from the approach of Ms Hossenfelder.

I don't think that the "superfluid dark matter" papers contain something that would make their reading irresistible. But I find the "framing" of these superfluid dark matter papers in the media and the blogosphere – and the "framing" of many other papers – more important and highly problematic. It seems utterly inconceivable to me that an honest yet competent physicist could consider these papers "one of the most interesting developments in her lifetime".

When you look at the response (followups) by the other physicists and cosmologists, these papers don't even make it to top 100 in the year when they were published. Especially because I know quite something about Ms Hossenfelder, it seems vastly more likely that she has a completely different agenda when she overhypes such papers. What is it?

She has written at least one paper about these MOND and Verlinde issues – the 300th most important derivative paper commenting on the 101st most influential paper in a year ;-) – and she simply has personal incentives to make the world think that this kind of work is very interesting even though it is not. She is working on similar things because she doesn't have the skills (and vitality) needed to work on more interesting and deeper things. She says "it is most interesting and cool" but she really means "its fame is beneficial for her".

The financial news servers (e.g. SeekingAlpha) usually require the authors to disclose their positions in assets that they discuss in their articles. That has good reasons. Someone's being long or short may reduce his or her integrity and encourage him or her to write positive or negative things about the asset. The readers have the right not to be scammed in an easy way – which is why fair publishers insist on informing the readers whether there could be a clash of interests. One should expect the scientific integrity to be much deeper than the integrity of the journalists in the financial media. Sadly, it isn't so these days. Self-serving scammers such as Ms Hossenfelder face no restrictions – they are free to fool and delude everybody and lots of the people in the media want to be fooled and be parts of this scam because they're as unethical as Ms Hossenfelder herself.

Readers should learn how to use Google Scholar to acquire some rough clue about the importance of a paper or idea as evaluated by the body of the genuine scientists. If the folks learned how to use this "simple branch of Google", they could instantly figure out that 99% of the hype is probably rubbish (well, of course, this method isn't waterproof so there would be false positives as well as false negatives). It's too bad that almost no laymen – and, in fact, almost no journalists – are doing these things. So they're constantly drowning in hype and in a superfluid of fairy-tales that overhype papers that are either average or totally wrong.

Self-serving, fake scientists such as Sabine Hossenfelder are obviously the main drivers that propagate this fog and misinformation.

P.S.: In an older popular article about the topic, one at Aeon.CO, Hossenfelder emphasized the point that superfluidity represents a quantum behavior across the Universe. This assertion – which is just another way to add the hype – is really a deep distortion of the issues. A superfluid is nicely described by a classical field theory. Some of the fields seem to behave like the wave function but because this is a macroscopic limit of many particles in the same state, it is really a classical limit, with no minimal uncertainty etc., so the function of the spacetime coordinates isn't a wave function and shouldn't be called a wave function. It is a classical field. The classical limit isn't really any different in the case of a superfluid and in the case of electromagnetism or any other pair of a quantum field theory and its corresponding classical field theory!