How Superfluid Dark Matter Mimics an Old Idea About Gravity

Two physicists have been developing a theory of superfluid dark matter. Perhaps most intriguing is that the new theory reproduces in many ways the predictions of a model called Modified Newtonian Dynamics, or MOND, which was first proposed in 1983. MOND abandons dark matter particles entirely. Instead, it posits that there should be a way to tweak the laws of gravity to explain key astronomical observations.

MOND is something of a mirror image to the dominant view of dark matter — that it’s made of weakly interacting massive particles, or WIMPs. MOND accurately describes the rotation curves of galaxies — precisely the regime where WIMP models fall short — but doesn’t work well for galaxy cluster

Few physicists believe that the laws of nature change at different scales; for this reason and others MOND has been largely abandoned by the mainstream research community. But physicists must still grapple with reconciling their preferred dark matter models with the predictions that MOND gets right.

In the view of Justin Khoury, a physicist who co-developed the superfluid dark matter model, MOND is best viewed as an empirical statement about galaxies. Something is going on that makes it appear as though gravity is changing. Dark matter exists, but must act in such a way as to mimic the predictions of MOND inside galaxies.

The new model offers just such a way. Khoury originally hit on the idea for superfluid dark matter when he noticed a striking similarity between the equations for sound waves in a superfluid and those for MOND. “This strange-looking theory, interpreted as a theory of sound waves, looks quite natural,” said Khoury.

Here on Earth, superfluids aren’t exactly commonplace. But physicists have been cooking them up in their labs since 1938. Cool down particles to sufficiently low temperatures and their quantum nature will start to emerge. Their matter waves will spread out and overlap with one other, eventually coordinating themselves to behave as if they were one big “super atom.” They will become coherent, much like the light particles in a laser all have the same energy and vibrate as one. These days even undergraduates create so-called Bose-Einstein condensates (BECs) in the lab, many of which can be classified as superfluids.

Superfluids don’t exist in the everyday world — it’s too warm for the necessary quantum effects to hold sway. Because of that, “probably ten years ago, people would have balked at this idea and just said ‘this is impossible,’” said Tait. But recently, more physicists have warmed to the possibility of superfluid phases forming naturally in the extreme conditions of space. Superfluids may exist inside neutron stars, and some researchers have speculated that space-time itself may be a superfluid. So why shouldn’t dark matter have a superfluid phase, too?

To make a superfluid out of a collection of particles, you need to do two things: Pack the particles together at very high densities and cool them down to extremely low temperatures. In the lab, physicists (or undergraduates) confine the particles in an electromagnetic trap, then zap them with lasers to remove the kinetic energy and lower the temperature to just above absolute zero.

Inside galaxies, the role of the electromagnetic trap would be played by the galaxy’s gravitational pull, which could squeeze dark matter together enough to satisfy the density requirement. The temperature requirement is easier: Space, after all, is naturally cold.

Outside of the “halos” found in the immediate vicinity of galaxies, the pull of gravity is weaker, and dark matter wouldn’t be packed together tightly enough to go into its superfluid state. It would act as dark matter ordinarily does, explaining what astronomers see at larger scales.

But what’s so special about having dark matter be a superfluid? How can this special state change the way that dark matter appears to behave? A number of researchers over the years have played with similar ideas. But Khoury’s approach is unique because it shows how the superfluid could give rise to an extra force.

In physics, if you disturb a field, you’ll often create a wave. Shake some electrons — for instance, in an antenna — and you’ll disturb an electric field and get radio waves. Wiggle the gravitational field with two colliding black holes and you’ll create gravitational waves. Likewise, if you poke a superfluid, you’ll produce phonons — sound waves in the superfluid itself. These phonons give rise to an extra force in addition to gravity, one that’s analogous to the electrostatic force between charged particles. “It’s nice because you have an additional force on top of gravity, but it really is intrinsically linked to dark matter,” said Khoury. “It’s a property of the dark matter medium that gives rise to this force.” The extra force would be enough to explain the puzzling behaviour of dark matter inside galactic halos.