Earlier this year, researchers found the signal of inflation hidden in the cosmic microwave background—the radiative remnants of the Big Bang took a long time to reveal their secrets. It was a big day. Cosmologists everywhere broke out the Radler , got horrendously drunk, and rioted in front of campus administration buildings. Okay, maybe not—getting drunk while drinking Radler is difficult under the best of circumstances.

No, in reality, they went back to work. Even if those results hold up and inflation is as predicted, that still leaves cosmologists missing two pieces of their puzzle: dark matter and dark energy. While everyone else is searching the skies, a group of physicists has shown that bouncing neutrons are actually very sensitive to variations in gravity. Their research is now putting stringent limits on certain dark energy and dark matter theories.

Three forces walk into a bar

Before we crack open the door to the lab and reveal results, it is important to see how inflation, dark energy, and dark matter all fit together. When we look out into the Universe, its appearance is rather odd. It is surprisingly smooth over very large scales, containing lots of, well, bugger all. And where there is matter, it is clumped up much more than expected. So, the Universe is both smooth and lumpy. It's also still inflating. Our observations show that the expansion of the Universe not only continues, but is getting faster.

The smoothness of the Universe can be explained by very rapid expansion—inflation. If you have the right amount of inflation to explain the size and smoothness of the Universe, however, there is not enough observable matter to get stars or galaxies. The force of gravity is simply too weak to draw matter together. This is one of a number of reasons we need dark matter. And, finally, to explain the acceleration of the expansion of the Universe, we need dark energy; ideally, that would explain both early inflation and today's inflation. Until recently, all of these were placeholder concepts that may or may not take the form implied by their names.

Nevertheless, these names seem to have been good guesses. Observations of galactic collisions tell us that dark matter probably is some form of matter. Observations of the cosmic microwave background radiation tell us that inflation may well have taken place as predicted. Dark energy, though, is still a very open question.

Dreaming up physics

The point about placeholder concepts is that physicists can sit around dreaming up possible ways to explain the placeholder effect. Dark matter might just be a modified theory of gravity; or it could be heavy but cold particles; or light weight particles; or, well, insert your own idea here. The only requirement is that these ideas fit the available data and don't break anything else. After coming up with an explanation, the key step is to figure out how that explanation might affect the Universe in other ways. Once you know that, the real work can begin: the search for evidence.

In amongst the many theories for dark matter, a particle family called the axion is one contender. The axion is not really a family of particles; because no one has found an axion, the particle has a range of possible properties that depend on theoretical considerations. This is much like how Higgs' particle had a range of possible masses and could even have been a descriptor of multiple Higgs-like particles. We couldn't tell the difference until the LHC finally found the actual Higgs particle.

In any case, the axion is a very light particle. And that is important for detecting its presence. The LHC needed protons, smashing together at very high energy, to create and detect the Higgs particle. But, when a particle has very little energy, you can use a very low energy probe to detect it. This brings particle physics back into the realm of nearly ordinary laboratory physics.

I say nearly ordinary because, although you only need a low energy probe, the changes induced by the axion will also be very small.

Hiding neutrons in a beer fridge

And this is where our intrepid group of physicists comes in. Their experiment takes the form of a very cold beam of neutrons. These neutrons float into a chamber containing two neutron mirrors. As the neutrons travel through the chamber, they bounce back and forth between the mirrors, finally exiting at the other end, where they hit a neutron detector.

If you know the angle at which the neutrons hit the first mirror, the distance between the mirrors, and the size of the mirrors, you will know exactly which direction the neutrons will be headed when they exit the chamber. By placing the detector at the right location, you will see lots of neutrons. However, if gravity is not exactly the strength that you expect, or the neutrons are slowed because they stopped to play with a passing dark matter particle, then the detector will see fewer neutrons.

This early version of the experiment saw nothing that would indicate any deviation from ordinary gravity.

This time around, however, the team has increased the sensitivity by vibrating one of the mirrors. The neutrons will keep bouncing for a long time if the mirror is stationary. By moving the mirror, we amplify the motion, speeding the neutrons up. They hit the other mirror sooner, and that changes the final trajectory of the neutrons when they exit the chamber. But the mirror must oscillate at precisely the right frequency in order to amplify the motion. That means that if dark energy is changing local gravity at all, we will see it because the vibrational frequency of the mirror will have to change to come into resonance with the bouncing neutrons.

Again, no changes to the resonance frequency were observed. This closes the window on the coupling between dark energy and ordinary matter by about five orders of magnitude. It does, however, still leave another seven orders of magnitude to be eliminated. It should also be noted that this only applies to a certain family of dark energy theories.

Along with not detecting dark energy, the researchers failed to find axions as well. In this case, the axions are expected to affect neutrons through the neutron spin. The neutron spins were all aligned by performing the experiment in the presence of a magnetic field. Axions are unaffected by the magnetic field because their spin is zero. But, should a neutron bounce off a passing axion, the interaction between the spins will cause the neutron to flip its spin (the spin can either be aligned to the magnetic field, or anti-aligned, but not in between), changing its resonance frequency relative to the mirror yet again. By looking at whether the change in resonance frequency depends on the orientation of the magnetic field, any interactions between an axion and the neutron beam should be made apparent.

But it wasn't. In this case, the coupling strength between the axion and the spin of ordinary matter was shown to be at least 30 times lower than previous measurements. Unfortunately, there is no window to close on axions, because the coupling could be anywhere between the current upper limit and zero. Meaning that higher and higher precision experiments can only get closer to zero, but never exactly eliminate the possibility. Clearly, other experiments will be required for that.

I love these bouncing neutron experiments, though. They are conceptually simple and don't require teams of hundreds of researchers to get great results. I eagerly await more results.

Physical Review Letters, DOI: 10.1103/PhysRevLett.112.151105