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As with many controversies, both sides make a good case, and the task of deciding between them falls to observers. Inflation would have had conspicuous gravitational side effects because it’s a highly energetic process. “When you produce density fluctuations at high energy, they also produce fluctuations in space-time itself,” Steinhardt says. So far, searches for gravitational waves from this era have come up empty. If the Simons Observatory doesn’t find any either, inflation is in trouble. Are the null results to date already uncomfortable? “Yup, they are,” Gabadadze says. “Already they kind of are.”recommended you read

Conversely, if the observatory does detect primordial gravitational waves, Steinhardt and Ijjas’ cyclic cosmology is dead. “If we see that, it will disprove many of the competing models,” says Simons Observatory director Brian Keating of the University of California, San Diego.

The distribution of background radiation measurements offers another empirical handle on the problem. Currently, a histogram of temperature readings at different locations on the sky traces out a bell curve — a Gaussian. Any deviation from that generic shape would reveal what physics was in play early on. “Primordial non-Gaussianity has to do with the interactions and the number of fields that were involved in inflation,” says Eva Silverstein of Stanford University.

A bounce would require gravitational effects beyond those of Einstein’s theory, and cosmological observations can look for those. “It’s not something that would typically occur, because gravity is attractive, so if you start contracting, you’re going to collide,” says Claudia de Rham of Imperial College London. She and Gabadadze have explored modifications to gravity that not only might let the universe bounce, but would illuminate the mysteries of dark energy. Modified-gravity theories are a steppingstone to a full quantum theory of gravity and, as such, need to satisfy certain general principles. Those principles, along with observations, narrow the range of allowed modifications. “That really constrains your allowed region of parameter space by combining observations and theory priors,” she says.

Once cosmologists open the door to modified gravity, all sorts of new phenomena come rushing in, and bounces are almost the least of it. Frans Pretorius of Princeton, an expert in computer analysis of Einsteinian gravity, has been simulating post-Einsteinian gravity. In one case, he and his students were tracking the formation of black holes when the modified-gravity equations suddenly ceased to operate in time. They had changed their mathematical character from one that evolves to one that remains in a steady state. “When something like this happens, we have no idea how to interpret it,” he says.