The universe had a beginning. But where did it start? What did you become in the beginning? We know that it all started with a fairly rapid expansion and ended with a large number of galaxies made of small particles. But what came before that? What were the laws of physics when it all started? Famous physicists James Hartl and Stephen Hawking offered several answers to these questions several decades ago. A new work by another group of physicists analyzed Hawking and Hartle's popular interpretation of Big Bang geometry and ran into some trouble. These results shed light on the problem of the beginning of the universe. A new obstacle that all theories of the future will have to overcome.
“We tried a more rigorous calculation and came up with a different solution,” says Job Feldbrügge, a resident graduate student at the Perimeter Institute. "The theory we are using sheds new light on existing theory and shows that it may not work as we expected."
Scientists usually try to understand the beginning of the universe by looking at Einstein's laws of gravity, called general relativity, and playing them backwards. After all, they want to get to the point where the universe was very small. But the most interesting questions arise about what the young universe looked like, whether it was small enough to obey the laws of quantum mechanics, which governs the smallest particles, atoms and photons.
There are several ways to start a universe like ours. Maybe, Hawking and Hartle thought, this condensed universe was just one point in space with a special quantum state, the so-called wave function, which describes it all in the language of quantum mechanics. Then time came. Philosophy and religion need to talk a lot on this topic, but mathematicians just need pen and paper. This point-like universe evolved from the mathematics of general relativity with the original properties of quantum mechanics built into its structure. Thus, these tiny random fluctuations of energy in space should have, in the course of rapid expansion - inflation - turned into large-scale density differences that we observe in the modern Universe, with galaxies and voids. Hawking and Hartle's theory was one of several ways to mark the beginning of a universe without a singularity, a point of zero volume, and infinite mass that didn't make much sense. Other ideas, like those suggested by Alexander Vilenkin, did not imply this initial singularity.
A new article that recently appeared on the arXiv preprint server introduces a problem. In calculations in the mathematics of Hawking, Hartles, and Vilenkin, the new team did not get the tiny quantum fluctuations needed to create today's universe. Instead, these fluctuations are gigantic and create a universe completely different from ours.
“The calculations we have made lead to strong gravitational waves after the Big Bang,” says Feldbrugge - huge fluctuations in the shape of spacetime itself. “It couldn't lead to a universe like it is today. The calculations contradict what we see."
Hartl is not particularly worried about the results of the Feldbrugge team. “In cosmology, we still have too little data compared to what it could have been,” he says. "So we do our best to support the piece of theory that best suits our observations." He sees the new work as another attempt to turn the game around by offering more information and a different mathematical path that scientists can follow. "Researchers have the right to choose whether to pursue this idea or another."
His team also recently published another paper revising his own mathematics and demonstrating why his theory still works.
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Yet the mathematics of Feldbrugge and his team seem to show that a smooth appearance of a universe without any singularity is "not an option." Their mathematics directly disputes Hartle and Hawking.
Linking quantum mechanics and general relativity to explain the beginning of the universe is neither new nor nearly resolved. In fact, this is one of the main problems that theoretical physicists are trying to solve, given its importance for understanding the origin of the universe, when both sets of laws are applied on the same scale, and the importance for black holes in which gravity is so strong that light cannot leave her.
But most importantly, Feldbrugge does not believe that a universe that begins with the laws of quantum mechanics and relativity could create small fluctuations that would lead to a universe like ours - he thinks there must be something else. "It is unclear which solution will be the final option," he says.
The opinions of physicists on this issue are very different. Paul Steinhardt, professor of physics at Princeton University, says there are already alternative ways to avoid problems in a new job, as well as other complaints about the Hawking-Hartle model. This so-called infinite model requires some mathematical workarounds to create a universe like ours.
“What is the alternative? A bounce without a singularity,”he says, referring to a model he is developing with Princeton-based cosmologist Anna Idjas. According to this model, the universe collapses and then unfolds into our own universe, long before one can begin to think about the effects of quantum mechanics.
Sabine Hossenfelder, a research fellow at the Frankfurt Institute for Advanced Study, is unsure of the new results. “The only thing I can conclude is that we did not know how the universe began before this work was written. And we didn’t know it after this work was published”. Theorists take mathematics seriously and have been doing these calculations with time and space long before telescopes would confirm them. The only way to know for sure what's going on is through experimentation.
Today, most of these theories can be confirmed or disproved by observations of the oldest light that has come down to us, the cosmic microwave background. Scientists hope the insights from their theories will help isolate important signatures from this data.
Is it possible to check the work of Feldbrugge and his team? They're just getting started. Obviously, it will take a long time to check. Scientists need to ultimately create a universe that resembles ours. But the details of this process have not yet been determined.
ILYA KHEL