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 a lot of big galaxies made from little particles. But what happened before then? What were the laws of physics like when it all started?
The kind of wild art you expect from a story about the origin of the universe (Image: MIT Computer Science and AI Laboratory)
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 analysed Hawking and Hartle's popular interpretation of the geometry of the big bang, and ran into some trouble. These results shed light on a problem in the universe's beginning; a new hurdle that any future theories will need to hop over.
"We tried to do the calculation more rigorously and obtained this different solution," Job Feldbrugge, a resident PhD student at the Perimeter Institute, told Gizmodo. "This 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."
Researchers usually try to understand the beginning of the universe by taking a look at Einstein's 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 maths 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 Hartle's 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 doesn't make a whole lot of sense. Other ideas, such as the ones posed by Tufts cosmologist Alexander Vilenkin, don't mind this initial singularity.
But this new paper, published recently on the arXiv physics preprint server, introduces a problem. When running Hawking and Hartles' as well as Vilenkin's, maths, the new team didn't get the teeny quantum fluctuations required to create today's universe. Instead, those fluctuations are gigantic, and create a universe complete unlike our own.
"The calculation we do results in violent gravitational waves after the Big Bang," said Feldbrugge — enormous fluctuations in the shape of spacetime itself. "It couldn't result in a universe like today's. The calculations conflict with the stuff we see."
Hartle wasn't so concerned by Feldbrugge's team's results. "In cosmology… we still have a very small amount of data compared to what we might hope to have," he told Gizmodo. "Therefore 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." He felt this new work is another attempt to turn the crank, offering more information and another mathematical pathway that researchers can pursue. "Researchers can decide if they will pursue one of these [ideas] rather than another."
His team has also published another recent paper, reviewing their own mathematics, and demonstrating why things still work.
Still, Feldbrugge and his team's maths seems to show that a smooth beginning to the universe without some sort of singularity is "not an option". Their maths seems to directly dispute Hartle and Hawking's.
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. It's 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 can't escape its pull.
But the main point is that Feldbrugge doesn't think a universe beginning with the laws of quantum mechanics and relativity alone would create small fluctuations that result in a universe like ours — he thinks there has to be something else. "It's not clear which solution will be the ultimate option," 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.
"What's the alternative? It's a non-singular bounce," 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 wasn't entirely convinced by the new results. "The only thing I conclude is that we didn't know how the universe began before the paper was written," she wrote in an email. "And we still don't know how it began after it was written." She said that theorists take maths very seriously, and are performing these calculations for times and spaces far beyond what telescopes can see. The only way to really tell what's 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 theories' implications will turn up as specific signatures in this data.
Can Feldbrugge and the team's work be tested? They're really only at the beginning stages. And then there's the problem of black holes, which also must obey the laws of general relativity and quantum mechanics simultaneously. Figuring them out "is certainly on our to do list," 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, "the details are up in the air."