It might not be obvious to those of us only grappling with more mundane concerns, but for cosmologists bent on unlocking the universe’s deepest secrets, there’s no shortage of problems keeping them up at night. “Dark matter” is the shorthand explanation for stars and galaxies moving much more quickly than the gravity of their luminous matter should allow. Let’s not forget “dark energy,” too—the preferred solution to the mystery of the universe expanding faster than anyone expected and doing so at an accelerated rate. Meanwhile a hypothesized “evolving” form of dark energy might resolve something called the Hubble tension—the term used for a major disagreement among researchers about the present-day cosmic expansion rate.
Cosmologists have been losing sleep over such quandaries for generations, wondering what missing ingredients they need to add to their models to fix what seem to be glaring gaps in their understanding. But what if the answer to some—maybe even all—of these problems isn’t a radical new theory but rather an old one, devised almost a century ago by none other than Albert Einstein himself? It’s called teleparallel gravity, and according to a loose collection of theorists who study it, this theory deserves a closer look by the wider scientific community.
Dark matter, dark energy, the Hubble tension: underpinning these theories is Einstein’s general theory of relativity, which treats space and time as a unified “spacetime” and considers gravity as spacetime’s curvature. Perhaps, then, the answer is to modify, change or update relativity itself to gain a new understanding of gravity rather than hypothesizing mysterious dark substances and forces. But across the decades, theorists pursuing this general approach have delivered mixed results at best.
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The best example may be Modified Newtonian Dynamics (MOND), an effort to banish dark matter that, according to some research, still has to allow for the existence of some dark matter. A more recent addition, dubbed “timescape” cosmology, seeks to account for dark energy by asserting that gigantic, matter-sparse “voids” in the cosmos are much larger than most other measurements say they can be. None of these possible solutions come without their own problems.
So if these new ideas aren’t working out, why not return to the old master? In 1928, about a decade after completing his greatest scientific achievement, general relativity, Einstein began work on an alternative form of this cherished idea. His dream was to find a single set of equations that could describe both gravity and electromagnetism. His idol James Clerk Maxwell achieved such a feat in the early 1860s, using a single set of equations to describe electricity, magnetism and radiation, and Einstein hoped to follow in Maxwell’s footsteps.
Curvature is the core concept of general relativity. Matter and energy tell spacetime how to bend, curve and warp. In turn, that bending of spacetime tells matter how to move. Einstein was able to use this mathematical language to describe gravity, which was a huge success in its own right, but it wasn’t enough for him. The theory couldn’t also describe the electromagnetic force. So he turned to another approach that offered more flexibility: torsion. In this “teleparallel” version of gravity, matter and energy tell spacetime how to twist, and that twisting ripples outward to affect everything else.
Even though Einstein hoped that this new concept could bring both gravity and electromagnetism into the same theory, he never found a path to his longed-for unification, and teleparallel gravity seemed to die with him as physicists focused their efforts on exploring the powerful and paradoxical quantum world.
But across the years, theorists here and there revisited Einstein’s model, rummaging through its remnants for anything interesting buried in the calculations. They found that if they abandoned Einstein’s attempt to incorporate electromagnetism, they could formulate versions of teleparallelism that were equivalent to the typical mathematical language of curvature in general relativity. In other words, if you’re trying to solve some problem in gravity, you can take your pick of basic model, curvature or torsion, and get the same results.
What’s more, in 2017 a single observation rocked the world of modified gravity theories. That year observations of a merging neutron star revealed that gravitational waves and electromagnetic waves arrived at Earth within three seconds of each other—after traversing a distance of 130 million light-years. This strongly implies that gravity and light travel at essentially the same speed. Because many theories of modified gravity predicted small-but-significant differences between those speeds, they were ruled out almost instantly.
But teleparallelism survived because it allows for gravity to move at light speed.
Compared with Einstein’s magnum opus, teleparallel gravity has a much richer, and more complicated, mathematical structure. And that’s really saying something, considering that general relativity consists of 10 fiendishly complex, interconnected equations. The numerical complexity of teleparallelism is both a blessing and a curse. On one hand, it offers many opportunities to create tweaks and adjustments to gravity—tweaks that can survive all current experimental tests yet still manifest in ways that explain dark matter and dark energy.
On the other hand, the complexity sets a brutal learning curve for aspiring new theorists while also making it harder for the theory to generate viable, testable predictions. And for the wider community, all this makes discerning what’s a good teleparallel-based idea and what’s rubbish exceedingly difficult. And the extra complexity on top of relativity gives the theory some troubling ambiguity. It’s not always clear that the rich mathematical structures can reliably connect to physical reality—in other words, that the math won’t just blow up in your face when you try to apply it to realistic scenarios. That is probably why much of today’s research on teleparallel gravity exists largely outside the mainstream.
But still, progress has been made. Inquiries into teleparallel gravity follow two basic branches. One branch explores the theory itself and its connection to general relativity. Einstein’s theory has survived a host of experimental verification, from the orbits of planets in the solar system to the behavior of black holes. Can teleparallelism really be considered equally viable? Under its auspices, do black holes still look like black holes? Does the big bang still proceed? Do stars and galaxies still act like stars and galaxies?
So far, the answer seems to be yes, which drives the other branch of teleparallel research: using teleparallelism to explain aspects of the universe that remain mysterious under vanilla general relativity. For example, it might be possible to formulate a version of teleparallel gravity that passes every single experimental test yet does away with the need for dark matter—or one that explains the accelerated expansion of the universe. or resolves the Hubble tension.
These attempts are only in their initial stages. There is a tremendous body of evidence for the existence of dark matter and the accelerating expansion of the universe and the reality of the Hubble tension. Finding explanations for everything while making sure all observations remain consistent and obeyed is hard—especially when dealing with a complex, poorly understood theory.
And then there’s an even greater challenge: convincing the ultimate skeptics—scientists themselves—that this is a valid approach. That would take an enormous amount of effort, with nature itself as the ultimate arbiter. But there’s a powerful payoff: if a promising theory emerges that seamlessly fits with our prevailing big picture of the cosmos while eliminating at least one of the corresponding cosmological conundrums—and, most importantly, while also making some truly testable new prediction—that would be a breakthrough at least as momentous as Einstein’s when he first proposed a warped spacetime. That is a tall order, to put it mildly.
But who knows? Certainly not you or me—not yet anyway. Maybe Einstein was right all along—even if he didn’t realize it at the time. And all it took was a little twist.
