GRAVITY is supposed to be reliable. It’s the familiar force that keeps our feet on the ground and Earth’s atmosphere from hurtling into space. On grander scales, it has shaped the evolution of the universe. What a shame, then, that it sometimes lets you down. To square the whirligig rotations of galaxies and galaxy clusters with our picture of gravity, we have to invent a whole new form of matter that no one has ever seen: dark matter. To explain why the universe’s expansion is accelerating, we have to conjure up an equally mysterious essence known as dark energy.
But what if we never really knew gravity at all? What if out there, beyond where we can easily keep our eye on it, the universal force doesn’t stick to the rules?
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It’s a heretical idea, if not an entirely novel one. Now though, renewed scrutiny of galaxies and surprises from the realm of quantum information theory are reinvigorating the quest to rethink gravity. Radical ideas are emerging that amount to a fundamental transformation of how we understand space-time – and what gravity really is. In this picture, dark matter ceases to exist. And dark energy, rather than being something that works against gravity, might be part of what creates it.
Pretty much everything we know about gravity comes from Isaac Newton and Albert Einstein. The strength of the pull exerted by a given object declines in proportion to the square of the distance from it, Newton told us, while Einstein explained gravity as the result of massive objects curving space-time.
Newton’s inverse-square law dictates that stars far from the centre of a galaxy should feel less gravitational pull, and therefore orbit more slowly, than stars closer in. But in the 1970s, astronomers including Vera Rubin noticed that farther out from a galaxy’s central bulge, the velocities of stars did not continue to drop as predicted. Instead they levelled off, an observation that could only be explained if there was some invisible form of matter surrounding galaxies to provide an extra gravitational kick. We have been searching for this dark matter ever since.
Bending the rules
Well, not everyone has. In the 1980s, Mordehai Milgrom, then at Princeton University, showed that you can explain the oddball rotation speeds of outlying stars without invoking dark matter. The trick was to ditch the idea that gravity always obeys Newton and Einstein as its strength begins to wane. Milgrom’s theory, known as MOND for “modified Newtonian dynamics”, posited that gravity’s pull tails off more gradually than Newton predicts. As soon an object’s acceleration due to gravity drops below a particular value, 82 billion times weaker than what we experience on Earth, gravity suddenly switches to this new regime.
Milgrom had some success applying his theory to spiral galaxies, but MOND never really caught on. For starters, it failed to account for clusters of galaxies, which couldn’t hold together without dark matter or modifications to gravity beyond what MOND allowed. It also seemed suspiciously ad hoc. Why would gravity’s strength suddenly switch at this seemingly arbitrary point?
And yet MOND never really went away – not least because no one has actually detected dark matter. “There are two possibilities,” says at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Either we find this invisible source of additional gravity, and we reassure ourselves that Newton and Einstein were right all along. Or we don’t. In which case “the alternative is to modify gravity”, says Moffat.
Last year may have finally brought a tipping point. , an astronomer at Case Western Reserve University in Cleveland, Ohio, and his colleagues took a fresh look at more than 150 spiral galaxies similar to our own Milky Way (see “From light into dark, and back again“). When they compared gravity’s inferred strength for each galaxy with its disc’s rotation speed, they found that the stars had anomalously high speeds farther out from the centre.
So what? That’s precisely the sort of behaviour we’ve observed many times before, and you explain it by adding a halo of dark matter around the galaxies. But McGaugh’s statistical survey included a cross-check. Taking a census of all visible matter in every galaxy, he compared its gravitational pull at every point with the rotation speed of nearby stars. The result was between the galaxies’ rotation speeds and the distribution of the visible matter they contain.
Darkness falls
, a theorist at the Perimeter Institute in Canada, was stunned. This relationship is “tantamount to a natural law”, he says – not something you expect to see if something other than visible matter dominates these galaxies.
Even more eyebrow-raising is the fact that this close relationship between visible matter and the movements of stars appears to hold across such a wide range of galaxies, even though those galaxies are not thought to hold identical dark matter distributions. Dark matter is not supposed to slavishly follow the whereabouts of ordinary stuff. So either it interacts with visible matter or itself more than simple models suggest, or there is something up with gravity.
McGaugh’s work is not the only thing that has reawakened this heretical notion. One of the biggest problems for MOND is the behaviour of clusters of galaxies. Like stars at the edges of galaxies, galaxies on the fringes of clusters also seems to orbit too fast – something that has been explained with dark matter. Observations of gravitational lensing, the subtle warping of light by matter, suggest that the source of the additional force catapulting the galaxies around is located somewhere other than the visible matter. You simply can’t explain galaxy clusters without invisible matter, or so the story goes.
The most notorious example is the Bullet Cluster, named for its resemblance to high-speed images of projectiles shooting something to smithereens (see “A massive red-blue herring?“). For many dark matter hunters, this is the best evidence that their quarry must exist. But at the University of Bonn, Germany, argues exactly the opposite – only with MOND can you explain this high-speed intergalactic collision.
“It’s an incredible public relations gag,” he says. Kroupa argues that standard gravity is too feeble to produce galaxy collisions as hot and furious as the Bullet Cluster in a realistic time frame. Dark matter might juice an initial collision up to the high speeds we see, but it would gum up every interaction thereafter. “A dark matter halo is like a spider’s web,” Kroupa says. “It captures any incoming galaxy.” So a pair of post-collision galaxies that are still zipping around at high speeds become really hard to explain. “This is a big, big problem for the standard model of cosmology,” says Kroupa. “But with modified gravity… this problem doesn’t exist.”
The whole point of MOND is that over galactic and extragalactic distances, where we can never directly test its strength, gravity is stronger than we assumed. That, rather than some invisible form of matter, would be the simplest explanation as to why things at these scales appear to move faster and collide more furiously than Newton or Einstein predict.
This is not to say MOND doesn’t have some problems when it comes to gravitationally interacting galaxy clusters. In the Bullet Cluster, our telescopes point to two distinct regions of stronger gravitational lensing, and thus higher concentrations of mass, that are separate from what you would expect given the mass of the ordinary matter we observe.
Milgrom insists that his model is not nearly as imperilled by that as many claim. “You need only a little amount of unobservable matter, which could just be some normal matter like dead stars or cold gas clouds that has not been detected yet,” he says.
In the absence of such observations, however, others are seeking new theoretical solutions. One is a hybrid model featuring a shape-shifting version of dark matter that flows unimpeded in galaxies, creating a MOND-like extra pull, then behaves like orthodox dark matter in galaxy clusters.
The other option, suddenly in vogue again, is to modify MOND. That is precisely what Moffat has been doing. In his version, the strength of gravity varies thanks to the addition of a repulsive force that itself varies with distance, making gravity follow a Newton-like inverse square law closer in, only to peter out at distant reaches of a galaxy. In that realm gravity is stronger than Newton would allow, behaving as MOND predicts.
Moffat claims his theory can account for the rotations of galaxies and the anomalous motions of the Bullet Cluster. But what really sets it apart is that it produces stronger gravitational effects than even MOND would predict near black holes, which might give us a chance to put it to the test.
If we could see a black hole, we would see a dark disk surrounded by a shadow caused by extreme gravitational lensing. In 2015, Moffat worked out that, according to his revisions of gravity, the shadow around the supermassive black hole at the centre of the Milky Way will appear . Enter the Event Horizon Telescope, a global network of radio dishes capable of capturing detailed images of black holes for the first time, set to come online this April. In principle at least, it should be able to see this bloated shadow – if it’s there.
Even so, whether we go for traditional MOND or Moffat’s modified gravity, there is still a massive elephant in the room: the glaring absence of an underlying theory. Why would gravity suddenly deviate from what Newton and Einstein laid down, and at a seemingly arbitrary scale? The answers might lie in a radical rethink of what it actually is.
Last year, at the University of Amsterdam in the Netherlands came up with a fresh vision. Gravity, he suggests, is really an emergent phenomenon – a consequence of interactions between entangled bits of quantum information.
Entanglement is a deep but deeply counter-intuitive quantum mechanical connection between pairs or groups of particles in which actions performed on one affect the others, even if they are separated by large distances. Physicists have been able to make Einsteinian and Newtonian gravity emerge from networks of entangled quantum bits since the late 1990s. The problem is that it only works in a model universe known as anti-de-Sitter space, which doesn’t behave like the one we inhabit.
The key difference is that our universe’s vacuum is not conveniently quiescent. Instead, it is roiling with what we call dark energy, a mysterious substance or force thought to be responsible for the accelerating expansion of space-time.
Rather than try to work around the problem, Verlinde explored how emergent gravity might behave in a universe infused with dark energy. The result is in which this background energy imbues the entanglement of quantum bits with something akin to an extra degree of elasticity.
“It’s like dark energy is acting like an elastic medium,” Verlinde says. “And the matter we put in starts deforming that medium.” The extra elasticity provided by dark energy, he adds, boosts gravity’s strength at long range, resulting in an additional far-field effect that resembles Milgrom’s MOND.
A stretch too far?
Verlinde’s ideas made a big splash, but it’s still not clear how coherent they are. “He starts with dark energy, and he says this leads to something that looks like dark matter,” says at the Frankfurt Institute for Advanced Studies in Germany. “He makes a lot of effort to embed this into the large idea of space-time emerging from entanglement, which has become very popular in recent years. But I’m not sure that is necessary.”
One recent study has shown that Verlinde’s recasting of gravity can explain gravitational lensing anomalies in the vicinity of some 30,000 foreground galaxies. But his theory has come under fire for predictions it makes that, in fact, diverge from MOND. A study co-authored by McGaugh, for instance, suggests that – explaining the anomalous rotations of galaxies. Another found they predict planetary motions that bear no relation to what we see in our solar system.
For his part, Smolin has come up with a more modest attempt to derive MOND-like physics from first principles of quantum gravity – and unlike Verlinde’s theory, it doesn’t produce anything that diverges from MOND. Neither of them is claiming to have a complete theory of quantum gravity. It is clear, though, that when it comes to the question of why gravity acts strangely far from home, theorists are starting to come up with answers.
“We don’t know where the final theory takes us, because we don’t have it yet,” says McGaugh. “So there needs to be a period of uncertainty and scattershot, in order to find our way forward.”
A massive red-blue herring?
The Bullet Cluster (above), actually a collision between two clusters of galaxies, is often invoked as the smoking gun for dark matter. Although the individual galaxies glided past each other, the hot gas (pink) around them collided and slowed, leaving a trail behind each cluster. Oddly, most of the mass (blue), inferred by the way it bends light, has stuck with the galaxies. Hot gas is thought to form the bulk of visible matter, so the mismatch between where the mass should be and where it is shouts dark matter.
But in recent years dissenting voices have said the ferocity of the collision is impossible in a universe dominated by dark matter. In fact, tweaking the laws of gravity might better explain this smash-up (see main story).
This article appeared in print under the headline “Strangely attractive”
