One of the greatest unsolved problems in physics concerns unifying quantum mechanics and general relativity—two equally successful theories that infamously don’t get along. And the latest attempt to bridge the pair brings into the equation the explosion that started everything: the Big Bang.
In a recent Physical Review Letters paper, physicists at the University of Waterloo and the Perimeter Institute in Canada propose a new theory that the rapid expansion of the universe in its earliest moments aligns nicely with another model for quantum gravity. According to the team’s mathematical predictions, the Big Bang emerges rather naturally from the theory, called quadratic gravity, a revised account of Einstein’s standard theory of gravity.
“Think of Einstein raised to the second power,” Jerome Quintin, the study’s co-author and a theoretical cosmologist, told Gizmodo. The new work “takes formal calculations from quantum field theory and creates a bridge between them and actual cosmological scenarios and observations, which allows us to test these formal, theoretical ideas and calculations.”
The universe’s early days
The Big Bang refers to that gigantic explosion responsible for setting up the universe to grow into the world we observe today. In the “standard” picture, the universe starts small, hot, dense, and generally homogenous and isotropic, explained Ruolin Liu, the study’s lead author and a postdoctoral student, to Gizmodo.
A popular explanation for this phenomenon is called the inflationary scenario, which postulates that the hypothetical inflaton particle drove a “burst of rapid, accelerated expansion” in the earliest days of the universe, Liu said. But this theory breaks down the earlier we go in astronomical time, which corresponds to higher energies in the universe, he added.
Finding a quantum solution
The team wondered if there was a way to explain the Big Bang without tacking on too many new variables. And if quantum effects could somehow enter the equation, that would be even better. They decided on quadratic gravity, which remains mathematically consistent at very high energy levels like that of the Big Bang, according to the paper.
Fascinatingly, the team’s calculations revealed that the quadratic terms of the model organically triggered cosmic expansion, after which the spacetime structure fell into the known effects of general relativity. What’s more, Liu added, the study’s mathematical predictions fit nicely with observations of the universe made by the latest technologies, which have been “in conflict with more mainstream models of inflation.”
A testable hypothesis
Most importantly, the team believes its theory is completely testable—which isn’t usually the case for models on quantum gravity. The model predicts a minimum level of gravitational waves generated during inflation, which next-generation detectors will be able to catch.
Niayesh Afshordi, the study’s senior author and a physicist, told Gizmodo that quantum gravity is often conveyed as something “purely theoretical,” but the new work “shows that quantum gravity can absolutely be studied and bridged to concrete cosmological scenarios, which come with specific predictions that we can test now and in the future as well.”
Awaiting the revolution, sort of
Certainly, if the team’s results can be validated—both theoretically and empirically—the implications will be nothing short of revolutionary. But as is the case with any prediction, there’s not much to say conclusively until the new proposal weathers independent review and experimentation.
That said, it’s also true that we’re living in a particularly great time for cosmology. For example, LISA, the next-generation detector that could do what the team is looking for, is slated to launch as early as 2035. NASA’s Nancy Grace Roman Telescope, after weathering a budgetary storm, will launch as planned. And the young Vera C. Rubin Observatory is bombarding astronomers with hundreds of thousands of observations.
So even if the latest proposal doesn’t end up being “the one,” we might be entering an era that allows us to drag out the hypothetical into the realm of testable ideas. And that’s exciting!
Related article: Radical New Theory Rewrites the Story of the Earliest Universe
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