Friday, August 14, 2015

Superfluid Dark Matter

A new paper proposes that dark matter may be a quantum fluid that has condensed in puddles to seed galaxies.

If Superfluid was a superhero, it would creep through the tiniest door slits and flow up the walls to then freeze the evil villain to death. Few things are cooler than superfluids, an utterly fascinating state that some materials, such as Helium, reach at temperatures close to absolute zero. Superfluid’s superpower is its small viscosity, which measures how the medium sticks to itself and can resist flowing. Honey has a larger viscosity than oil, which has a lager viscosity than water. And at the very end of this line, at almost vanishing viscosity, you find superfluids.

There are few places on Earth cold enough for superfluids to exist, and most of them are beleaguered by physicists. But outer space is cold and plenty. It is also full with dark matter whose microscopic nature has so far remained mysterious. In a recent paper (arXiv:1507.01019), two researchers from the University of Pennsylvania propose that dark matter might be puddles of a superfluid that has condensed in the first moments of the universe, and then caught the matter we readily see by its gravitational pull.

This research reflects how much our understanding of quantum mechanics has changed in the century that has passed since its inception. Contrary to what our experience tells us, quantum mechanics is not a theory of the microscopic realm. We do not witness quantum effects with our own senses, but the reason is not that human anatomy is coarse and clumsy compared to the teensy configurations of electron orbits. The reason is that our planet is a dense and noisy place, warm and thriving with thermal motion. It is a place where particles constantly collide with each other, interact with each other, and disturb each other. We do not witness quantum effects not because they are microscopic, but because they are fragile and get easily destroyed. But at low temperatures, quantum effects can enter the macroscopic range. They could, in fact, span whole galaxies.

The idea that dark matter may be a superfluid has been proposed before, but it had some shortcomings that the new model addresses; it does so by combining the successes of existing theories which cures several problems these theories have when looked at separately. The major question about the nature of dark matter is whether it is a type of matter, or whether it is instead a modification of gravity, or MOG for short. Most of the physics community presently favors the idea that dark matter is matter, probably some kind of as-yet-unknown particle, and they leave gravity untouched. But a stubborn few have insisted pursuing the idea that we can amend general relativity to explain our observations.

MOG, an improved version of the earlier MOdified Newtonian Dynamics (MOND), has some things going for it; it captures some universal relations that are difficult to obtain with dark matter. The velocities of stars orbiting the center of galaxies – the galactic rotation curves – cannot be explained by visible matter alone but can be accounted for by adding dark matter. And yet, many of these curves can also be explained by stunningly simple modifications of the gravitational law. On the other hand, the simple modification of MOND fails for clusters of galaxies, where dark matter still has to be added, and requires some fudging to get the solar system right. It has been claimed that MOG fits the bill on all accounts but on the expense of introducing more additional fields, which makes it look more and more like some type of dark matter.

Another example of an observationally found but unexplained connection is the Tully-Fisher relation between galaxies’ brightness and the velocity of the stars farthest away from the galactic center. This relation can be obtained with modifications of gravity, but it is hard to come by with dark matter. On the other hand, it is difficult to reproduce the separation of visible matter from dark matter, as seen for example in the Bullet Cluster, by modifying gravity. The bottom line is, sometimes it works, sometimes it doesn’t.

It adds to this that modifications of gravity employ dynamical equations that look rather peculiar and hand-made. For most particle physicists, these equations appear unfamiliar and ugly, which is probably one of the main reasons they have stayed away from it. So far.

In their new paper, Berezhiani and Khoury demonstrate that the modifications of gravity and dark matter might actually point to the same origin, which is a type of superfluid. The equations determining the laws of condensates like superfluids at lowest temperatures take forms that are very unusual in particle physics (they often contain fractional powers of the kinetic terms). And yet these are exactly the strange equations that appear in modified gravity. So Berezhiani and Khoury use a superfluid with an equation that reproduces the behavior of modified gravity, and end up with the benefits of both, particle dark matter and modified gravity.

Superfluids aren’t usually purely super, instead they generally are a mixture between a normally flowing component, and a superfluid component. The ratio between these components depends on the temperature – the higher the temperature the more dominant the normal component. In the new theory of superfluid dark matter the temperatures can be determined from the observed spread of velocities in the dark matter puddles, putting galaxies at lower temperatures than clusters of galaxies. And so, while the dark matter in galaxies like our Milky Way is dominantly in the superfluid phase, the dark matter in galactic clusters is mostly in the normal phase. This model thus naturally explains why modified gravity works only on a galactic scales, and should not be applied to clusters.

Moreover, on scales like that of our solar system, gravity is strong compared to the galactic average, which causes the superfluid to lose its quantum properties. This explains why we do not measure any deviations from general relativity in our own vicinity, another fact that is difficult to explain with the existing models of modified gravity. And since the superfluid is matter after all, it can be separated from the visible matter, and so it is not in conflict with the observables from colliding clusters of galaxies. In fact, it might fit the data better than single-particle dark matter because the strength of the fluid dark matter’s self-interaction depends on the fraction of normal matter and so depends on the size of the clusters.

Superfluids have another stunning property which is that they don’t like to rotate. If you try to make a superfluid rotate by spinning a bucket full of it, it just won’t. Instead it will start to form vortices that carry the angular momentum. The dark matter superfluid in our galaxy should contain some of these vortices, and finding them might be the way to test this new theory. But to do this, the researchers first have to calculate how the vortices would affect normal matter.

I find this a very interesting idea that has a lot of potential. Of course it leaves many open questions, for example how the matter formed in the early universe, so as the scientists always say: more work is needed. But if dark matter was a superfluid that would be amazingly cool – a few milli Kelvin to be precise.

When it comes to superpowers, I’ll chose science over fiction anytime.


Uncle Al said...

"Superfluid...freeze the evil villain" Pulsar core neutrons are hot superfluid, remaining core protons hot superconductor. "full with dark matter" or angular momentum Noetherian leakage from a testable vacuum symmetry breaking selective to hadronic matter. Look. "whose microscopic nature has so far remained mysterious" consistent with no particles.

"superfluid that has condensed in the first moments of the universe" If bound by gravitation and thermally inflated, distribution as threads is metastable unless tuned (laminar flow nozzles). From the paper: "giving rise to a superfluid core within galaxies" swallowed by the central black hole over time, falsified by Fisher-Tully relationship vs. redshift.

"human anatomy is course" "coarse." English homonyms are now "homophones." sigh

Sabine Hossenfelder said...

Hi Uncle,

Thanks for pointing out the typo, I've fixed that. Best,


Arun said...

What is the effect on cosmological nucleosynthesis of the superfluid dark matter coupling to baryons?

vttoth said...

Dear Sabine: as we discussed in e-mail, and since it is not the first time that you mentioned MOG in your blog (for which I thank you!) I wanted to point out that MOG is not really an "improved version of MOND": MOND is a phenomenological formula (perhaps retroactively justified by TeVeS) whereas the former is a proper classical relativistic field theory. I also wanted to mention that whereas MOND indeed needs some form of DM to deal with clusters, this is not the case for MOG; indeed, MOG can also account for the Bullet Cluster, as demonstrated by Brownstein and Moffat (MNRAS 382:1, 29-47; 2007).

Thierry Periat said...

Dear SH I recently discovered the work you develop on your blog. I am trying to do a similar job on mine -but at a lowest level- just as French amateur loving sciences and trying to diffuse that passion all around me) and I know what energy such entreprise needs. That's the reason why I wanted first to thank you for that.

Second, concerning the article, I find it fascinating how people are actually "brainsturming" on the real nature of these empty regions that we call "vacuum", certainly by exageration. Where is the limit (if any) between these regions and the subatomic elements? A territory for sciences and fictions certainly but why should the fiction not contain the key ingredients for the future scientific developments?

Best regards

JimV said...

Very small typo: "one of the main reason" should be "one of the main reasons".

Thanks for another excellent article. (What a best-selling book they would make!) (Of course then you would be too busy giving interviews to do any science.)

Sabine Hossenfelder said...


Thanks, I've fixed that typo :o) And I've started to seriously think about the book idea... Best,


nicolas poupart said...

A superfluid is interesting but also extremely annoying to experimentally obtain some macroscopic gravitational effects. How to generate micro-vortexes that may combine to generate macroscopic vortices... To motivate the troops: One million Euro for an antigravity phenomenon experimentally reproducible offered by in / institute-for-gravity-research / and of course one million US dollards to solving the Navier-Stokes equation

CapitalistImperialistPig said...

I like the idea of you putting your stuff in a book.

Arun said...

The mention of "book" and "Bullet Cluster" above reminded me of this, from the Bulwer-Lytton contest ( ) which is a competition to write the worst-possible opening line for a book.


Galileo Galilei gazed expectantly through his newly invented telescope and then recoiled in sudden horror – his prized thoroughbred’s severed neck, threateningly discarded in a murky mass of interstellar dust (known to future generations as the Horsehead Nebula), left little doubt about where the Godfather and his Vatican musclemen stood on the recent geocentric/heliocentric debate. — Don Mowbray, San Antonio, TX

Plato Hagel said...

Mostly definitely interesting to see a possible physical manifestation of some kind being injected into the cosmos around us. Perhaps such a correlation may be found in the lighthouse effect we see where such idea of a space to provide for information to move freely and uninhibited down that projection to seed an affect in the Cosmo. The collapse of the supernova has ended in such a transmission perhaps?

Arie Nouwen said...

Sabine, thanx for your article about this interesting idea of dark matter as a superfluid. I have one question regarding the vortices, that will develop when the superfluid is rotating. In the article you say that it's unknown what effect these will have on normal matter. You would suspect that these vortices will appear where the dark matter is present, that is in the halo of our galaxy. Is it possible that there is a connection between these vortices and the globular clusters, that are present as a swarm of big star clusters, also in the halo of the galaxy. Could it be that these clusters are the beginning or entrance of the superfluidal vortices of dark matter - just a wild guess of mine.

Uncle Al said...

I wrote, "[dark matter] distribution as threads is metastable unless tuned" Here is a threaded mass distribution being shepherded. I don't see this mechanism stabilizing universal dark matter thread-like foam interstices.

Phillip Helbig said...

It would be interesting to know what Bekenstein (who tried to bring a physical foundation to MOND via other means) would think of this, but, alas, he died yesterday.

Sabine Hossenfelder said...

yes, I already heard. quite shocking :/

Jeff said...

I wonder when Sidney Harris drew that cartoon? For the record, the quote "Sometimes the magic works, sometimes it doesn't" appears in the move Little Big Man (1970). A short clip:

Approximation Prophet said...

Am I confused in thinking that this refers to spacetime itself as a superfluid and not some sort of dark matter "particles" in a superfluid state? If spacetime IS the superfluid then there are no DM particles to talk about to couple with anything or annihilate with another DM particle. I thought this was the point of these superfluid space theories. To show that there really are no dark matter particles, just a property of spacetime itself. This supposedly explains both dark energy and dark matter. Near baryonic matter spacetime clumps up and causes rotation curves, clusters, etc and in the voids it spreads put and flows and is responsible for cosmological expansion. This is why no gravitationally bound objects will experience expansion in the vicinity of galaxies, clusters, or superclusters.

So now I'm confused...does superfluid space do away with actual DM particles? WIMPs, etc?

Sabine Hossenfelder said...

Yes, you are confused, it's stuff in spacetime. Yes, it does away with other dark matter particles, WIMPS, axions, whathaveyou.