Theoreticians claim to love data. Data is the thing that allows them to test their theories and prove that they are right. Unfortunately for them, the data often doesn't support the theory. In those cases, the data has just stabbed your labor of love right in the heart, and you are expected to say "thank you, sir. May I have another?"

The recent observation of the in-spiral and merger of two neutron stars has done just that to a room full of theorists working on gravitational theories that try to get rid of dark matter.

Illuminating dark matter

Dark matter is a particle that is posited to exist in large quantities in the Universe. Physicists did not dream it up because they were bored, but because the internal gravitational structure of galaxies could not be explained by the distribution of visible matter. After the existence of dark matter was first proposed, it got some critical supporting evidence. The cosmic microwave background—the radiation emitted during the Big Bang that permeates the Universe—has features that, at the moment, we can only explain with dark matter.

The difference between dark matter and ordinary matter is how the two respond to light. Ordinary matter has a love/hate relationship with light. It invites light over for dinner, welcomes it, then throws light unceremoniously out the window for dissing its electrons. As a result, that ordinary matter is visible because of the way it absorbs and scatters light. Dark matter, on the other hand, completely ignores light and ordinary matter. Dark matter doesn't even like other dark matter very much. If the Universe were your local neighborhood, dark matter is the family that communicates with everyone, even other family members, only by changing the name of their WiFi network.

More seriously, dark matter particles are massive, but don't seem to have charge or a magnetic moment, so all they feel is the force of gravity. The only way they exert forces on other particles is via gravity.

Not seeing is not believing

Dark matter is not an unreasonable explanation for our observations, but it's not the only possible explanation for galaxies' behavior. You can, for instance, modify the law of gravity. Many of these alternatives, called Modified Newtonian Dynamics (MOND), began as curve-fitting exercises: here is the data, how should gravity be modified to fit?

The stumbling block has been to figure out the underlying physical reason for the new law. This is actually a big ask. In a 3D universe with a flat space-time, conservation of energy and momentum automatically gives forces like gravity and electrostatics a spatial dependence (the force falls as the square of the distance). These laws are a consequence of the nature of the Universe, so changes to the laws also have to be down to the fundamental nature of the Universe. Ad hoc add-ons are not appreciated.

The Universe doesn't have a completely flat space-time, though. Instead space-time warps and flows around massive objects, which provides a sliver of hope for alternative theories of gravity. A new class of theoretical gravities slithered out of this mud, much to the joy of gumboot-wearing theoreticians. For the versions of MOND that we are interested in today, the basic idea is this: there is no dark matter, but there are two different metrics—metrics are space-times coupled to matter.

One metric is coupled to ordinary matter. Light and ordinary matter dance together on this stage and provide us with a spectacular show. Gravtitational waves, on the other hand, have an entirely different metric that, for reasons best known to itself, is warped out of shape. Its warped shape is what we perceive as dark matter.

So there is no dark matter—instead the space-time is naturally warped in the absence of dark matter. It's as if light and ordinary matter were given a nice shiny space-time that is all flat and beautiful, while gravity was given a used space-time that had been bent out of shape by a series of minor parking mishaps. The result of this is that galaxies move like supertankers, but look like dinghies.

These two seemingly independent metrics can explain the structures of galaxies and of collisions between galaxy clusters, but the idea has consequences. For instance, if a something should emit both gravitational waves and light, the two waves will travel by different paths depending on the masses they encounter. So, light and gravitational waves won't arrive at a distant observation point at the same time.

Bring on the dancing neutron stars

And this is the gift gravitational wave observatories have given us. When two neutron stars spiraled into each other and merged, they released a huge amount of energy as both light and gravitational waves. Three gravitational wave detectors picked up the gravitational waves emitted during the in-spiral and merger, while the Fermi telescope caught the gamma ray burst released as the stars merged. Over the course of the next few days, many telescopes observed the light emission from the cooling debris that was hurtling outwards from the merger. It was, in short, a data bonanza the likes of which we may never see again.

The light and the gravitational waves travel along the direct line of sight to us, curling around the gravity wells of intervening galaxies along the way. As a result, the initial burst of light and gravitational waves hid a little gem: the time difference between the arrival of the gravitational waves and the light. All 1.7 seconds of it. Yes, that was the recorded delay between the two signals.

"Hooray," I hear some of you dark matter doubters shout, as the difference would seem to support the idea that they faced two different gravities. That would be premature cheering. Remember, dark matter makes up most of the mass in a galaxy, so the galaxies between us and the neutron stars (including our own Milky Way) should provide two very different paths for the gravitational and light waves. The delay should have been longer if MOND were correct. Much longer: about three years, in fact.

The measured delay was so much shorter than the difference predicted by double metric theories that the researchers didn't even bother performing more detailed calculations. There is, in their view, simply no way to include the intervening galaxies and exclude dark matter. This is a dead MOND theory.

In other news, Einstein is still right

Along the way, the researchers took a look at a related subject, called the weak equivalence principle. The idea is simple: free fall is the same for everyone and everything. It doesn't matter what your internal structure is or what your mass is—if you are in a gravitational field, you'll behave exactly the same as your neighbor. Gravitational waves and electromagnetic waves are both carried by particles: the graviton and the photon. The near simultaneous arrival of the two show that they also experienced the same gravitational field and were influenced in exactly the same way. Or more precisely, the maximum difference is less than four parts in 100 million.

Essentially, this analysis shows that photons and gravitons travel on nearly identical metrics. And, even on the same metric, they experience pretty much the same gravitational fields—that is the coupling between the particle and the gravitational field is the same for both. This also means that any new theory of gravity has to respect the weak equivalence principle.

Zombie MOND will return

Yes, MOND will be back. First of all, this doesn't eliminate all MOND theories. There are those that simply don't posit a underlying physical basis for the change in gravitational laws. Without a reason, they cannot be eliminated. Then there are those that don't use a weird metric to separate gravitation from everything else. These are also not, at first sight, eliminated.

Unusually, I will be a bit cautious here. It is not clear that the remaining MOND theories do not have their success or failure hidden in the neutron star merger data. It may be that no one has tested any predictions they make about gravitational wave behavior (or that the merger does not satisfy the conditions of the predictions). Human nature plays a role too. No theoretician lets a good idea die without a fight. You can be sure that several of them are already working to see if there is some wriggle room that might allow their theory to escape, scarred but alive.

So, as usual, the conclusions aren't quite as straightforward as we would like, but that is science, and it's fun to watch the show.

2017: arXiv.org, ID: 1710.06168v1