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The universe began. But what did it begin from? What did it begin into? We know it began by expanding rather quickly, and ended up with lots of big galaxies made from little particles. But what happened before then? What were the laws of physics like when it all started?

Famous physicists James Hartle and Stephen Hawking proposed some answers to some of these problems several decades ago. A new paper from another team of physicists analyzed Hawking and Hartleâs popular interpretation of the geometry of the big bang, and ran into some trouble. These results shed light on a problem in the universeâs beginning; a new hurdle that any future theories will need to hop over.

âWe tried to do the calculation more rigorously and obtained this different solution,â Job Feldbrugge, a resident PhD student at the Perimeter Institute, told Gizmodo. âThis theory we use sheds some new light on this existing idea and shows that it might not work the way [we] expected it to work.â

Researchers usually try to understand the beginning of the universe by taking a look at Einsteinâs laws of gravity, called general relativity, and playing them in reverse. Eventually, you should get to a point long ago in which the universe was really tiny. But questions arise as to what the baby universe looked like, including whether it was small enough to obey the laws of quantum mechanics, which seems to govern the way that the smallest particles like atoms and photons behave.

There are a few ways our present universe could have started. Maybe, Hawking and Hartle thought, this condensed universe was just one point in space with a special quantum state, a so-called wave function that described the whole thing with quantum mechanics. Then, time started. Philosophy and religion require a lot more conversation for that statement, but math sort of just requires a pen and paper. This point-sized universe evolved based on the mathematics of general relativity with the initial probabilities of quantum mechanics built into its structure. The teeny, random energy fluctuations in space would then, through a quick-expanding process called inflation, turn into the large-scale density differences we see in our universe today, with galaxies and voids. Hawking and Hartleâs theory is one of several ways to account for the start of the universe without a singularity, a point of zero volume and infinite mass that doesnât make a whole lot of sense. Other ideas, like the ones posed by Tufts cosmologist Alexander Vilenkin, donât mind this initial singularity.

But this new paper, published recently on the arXiv physics preprint server, introduces a problem. When running Hawking and Hartlesâ as well as Vilenkinâs, math, the new team didnât get the teeny quantum fluctuations required to create todayâs universe. Instead, those fluctuations are gigantic, and create a universe complete unlike our own.

âThe calculation we do results in violent gravitational waves after the Big Bang,â said Feldbruggeâenormous fluctuations in the shape of spacetime itself. âIt couldnât result in a universe like todayâs. The calculations conflict with the stuff we see.â

Hartle wasnât so concerned by Feldbruggeâs teamâs results. âIn cosmologyâŚ we still have a very small amount of data compared to what we might hope to have,â he told Gizmodo. âTherefore we do the best job we can, supplying the piece of the theory which is motivated by all of our observations and see how it does.â He felt this new work is another attempt to turn the crank, offering more information and another mathematical pathway that researchers can pursue. âResearchers can decide if they will pursue one of these [ideas] rather than another.â

His team has also published another recent paper, reviewing their own mathematics, and demonstrating why things still work.

Still, Feldbrugge and his teamâs math seems to show that a smooth beginning to the universe without some sort of singularity is ânot an option.â Their math seems to directly dispute Hartle and Hawkingâs.

Linking quantum mechanics and general relativity to explain the start of the universe is neither a new story nor a problem close to being solved. Itâs one of the core things that theoretical physicists are trying to do, given its importance both for understanding the origin of the universe when both sets of laws applied at the same scale, and its importance in black holes, places where gravity is so strong that light canât escape its pull.

But the main point is that Feldbrugge doesnât think a universe beginning with the laws of quantum mechanics and relativity alone would create small fluctuations that result in a universe like oursâhe thinks there has to be something else. âItâs not clear which solution will be the ultimate option,â he said.

Other physicists had varying opinions on the matter. Paul Steinhardt, physics professor at Princeton University, told Gizmodo that there are already alternative routes to go down to avoid the problems in the new paper, as well as some of the other complaints with the Hawking-Hartle model. This so-called no boundary model requires some mathematical workarounds to create a universe like ours.

âWhatâs the alternative? Itâs a non-singular bounce,â he said, a model that he and Princeton theoretical cosmologist Anna Ijjas are working on, where a past universe collapses and then turns around into our own universe, all before having to worry about the effects of quantum mechanics.

I also reached out to Sabine Hossenfelder, Research Fellow at the Frankfurt Institute for Advanced Studies, who wasnât entirely convinced by the new results. âThe only thing I conclude is that we didnât know how the universe began before the paper was written,â she wrote in an email. âAnd we still donât know how it began after it was written.â She said that theorists take math very seriously, and are performing these calculations for times and spaces far beyond what telescopes can see. The only way to really tell whatâs going on is with experiments, she said.

Today, most of these theories can be proven or disproven by observing the oldest light to reach us, the cosmic microwave background. Researchers all hope that their theoriesâ implications will turn up as specific signatures in this data.

Can Feldbrugge and the teamâs work be tested? Theyâre really only at the beginning stages. And then thereâs the problem of black holes, which also must obey the laws of general relativity and quantum mechanics simultaneously. Figuring them out âis certainly on our to do list,â he said.

Obviously, all of this work is going to take a very, very long time. Scientists need to end up with a universe that acts like ours, but, said Hartle, âthe details are up in the air.â

[arXiv]