Late in the 20th century, scientists discovered something amazing: gravity doesn't just suck, it also blows. This knowledge comes to us by looking at distant supernovae and determining how fast they are receding from us. It turns out that the rate at which objects are receding from us is accelerating. The Universe isn't just expanding; it is expanding faster each day.

General relativity can cope with that—sort of—by adding a cosmological constant. This constant turns up naturally from the math, but neither the math nor the physics tells us what its value should be. Explaining what this constant might mean physically also turns out to be a bit tricky. There are many models, but the big obstacle to most of them is that they don't just add a constant term to general relativity. Instead, most of them have additional physical consequences.

Now, many of these models that seemed to fit with all the data may not fit after all. That's because these models predict unreasonable gravitational wave distortions within galaxies.

The accelerating expansion of the Universe has consequences, which might be best highlighted by looking at the alternatives. If the rate of expansion of the Universe was slowing, that would mean gravity sucks, and there is enough mass in the Universe to draw it all back together (let us prepare for our fiery gravitational death). If the rate of the expansion of the Universe was constant, we could say: gravity sucks, but there simply isn't enough mass in the Universe to draw it all back together. Our destiny is boredom, and nothing special is going on.

To be accelerating, something has to be pushing. At some length scale, gravity has to push, not pull. The tricky part is that general relativity makes some pretty accurate predictions about the Universe at scales ranging from the orbit and wobble of Mercury right up to the formation of galaxies and galaxy clusters and beyond. Any new version of gravity that includes a push has to match all of that data.

This is best summed up by saying that reality has a nasty habit of slapping people who try to modify general relativity. After a couple of black eyes, even the most hard-headed scientist gets the message. Yet, a modification has to be made. So a popular approach is to build in a modification that pushes at very large scales and dies away at shorter scales through some kind of screening mechanism. This should preserve general relativity as we observe it and still give the Universe the acceleration it needs.

This is also not an unnatural approach for physics. Consider an electron orbiting a nucleus. For hydrogen and helium, the electrons feel the full force of attraction from the protons at the center. But, for larger elements, like silicon or oxygen, the outermost electrons don't feel the full force of the nucleus because there are lots of electrons in between that partially shield them from the central charges. The trick is to apply similar reasoning to gravity.

But does screening work?

The problem is that it is difficult to solve the equations and actually test if the effects of the gravitational push term die away at small scales. To check this, researchers generate solutions by assuming that the Universe changes slowly with time. Effectively, this means that the rapid changes brought by gravitational waves have to be excluded from the calculation. Under these conditions, many models seem to fit. Screening works, apparently.

On the face of it, ignoring gravitational waves seems reasonable: we only detected them last year, after all. And we needed a measurement device that could detect physical movement on the order of one part in 1021. Surely something on that scale couldn't interfere with things like solar systems and galaxies, right?

According to a trio of physicists, maybe they can. Using some common extensions to general relativity, the physicists calculated how waves impinging on a galaxy, like the Milky Way, would show up. Now, to be clear, they didn't simulate a galaxy in huge detail. Instead, they used a model of a kind of spherical blob of matter with a mass distribution that follows that of a typical galaxy. In their model, the galaxy was subject to gravitational waves from outside. They allowed the galaxy to evolve in the presence of these waves and then measured the curvature of space-time in various locations within the galaxy.

Now, galaxies have a lot of mass, but that mass is relatively spread out. So the curvature should be very close to zero. And, indeed this has been confirmed by satellite measurements. However, the researchers' simulations show that popular modifications to general relativity and their screening mechanisms fail to predict this. Instead, the curvature is greater by several orders of magnitude (though still close to zero) in the presence of gravitational waves.

The calculated result is relatively general*, but the authors argue that the work applies to many more models than the one they presented in the paper. With that, you might think that this spells the end of these theories.

But it won't. The point of argument will be over relevance: what is the intensity of gravitational waves with wavelengths of the right scale? We know that neutron-star collisions, rotating pulsars, and black-hole mergers all generate gravitational waves. But do the very long period waves—those that have the right length scale to curve space-time on the scale of a galaxy—have much amplitude?

Back where we started?

And that brings us around in a circle, because the amplitude and frequency of gravitational waves is determined by matter, space-time, and the theory of gravity that describes their interaction. That sounds hopeless, but the circle is more like a spiral, since we know more now. Any theory of gravity, when coupled to the mass distribution of the Universe, should meet two additional criteria: it should either predict that the amplitude of long-period gravitational waves is small enough that they can be neglected, or it should have a screening mechanism that is not disrupted by gravitational waves.

Physical Review Letters, 2017, DOI: 10.1103/PhysRevLett.118.101301 (About DOIs).

*Pun intended. I would be expelled from the science-writing club if I were to avoid a pun.