New adjustments to Einstein’s theory of relativity could change our understanding of the Big Bang

The Big Bang is often described as the moment when everything began, a point of infinite density where the laws of physics broke down. But what if the photo is incomplete?

A new study proposes a different explanation for the birth of the universe. Instead of an abrupt start from a singularity, as predicted by Einstein’s theory of general relativity, the early universe may have passed through a more controlled, high-energy phase governed by a modified theory of gravity known as QQG.

“QQG stands for second-order quantum gravity,” study co-author Niaesh Afsholdi, a professor of physics at the University of Waterloo and the Perimeter Institute for Theoretical Physics, told Live Science via email. “Simply put, this is an extension of Einstein’s theory of gravity that includes additional terms that become important at the very high energies that would have existed near the beginning of the universe.”

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The study was published March 18 in Physical Review Letters.

Why Einstein’s theory is not enough

Einstein’s theory of general relativity was extremely successful in describing large-scale gravity. We will explain the movements of planets, black holes, and the expansion of the universe. However, the infinitesimal world of quantum mechanics is difficult to explain and is widely believed to contain fundamental contradictions.

“The main problem is that Einstein’s theory of general relativity predicts failure under extreme conditions, especially the Big Bang singularity,” Afshodi said.

At that point, the density and curvature of spacetime become infinite. This clearly shows that the theory is incomplete. Physicists have long sought a deeper framework that could explain gravity under such conditions.

“What do you make? [quadratic quantum gravity] What’s interesting is that this theory could provide a mathematically consistent way to describe gravity at very short distances and very high energies, where ordinary general relativity would be expected to break down. “In that sense, this provides a possible conservative route to a quantum theory of gravity while remaining close to Einstein’s theory on normal scales,” Afshodi said.

Universe without singularity

In a new study, researchers investigated how QQG, if an exact completion of Einstein’s theory, would reshape the universe’s earliest moments. Their results suggest that the universe did not begin with a singularity.

“Our main result is that within second-order gravity, the very early universe can avoid the usual Big Bang singularity and instead go through a more controlled high-energy phase,” Afshodi said.

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Rather than emerging from a state of infinite density, the universe begins in a smoother, more stable configuration with finite density and finite temperature, the exact properties of which depend on the particles and fields that exist at very high energies and temperatures. This avoids one of the most troubling predictions of standard cosmology.

The theory also provides a new perspective on the inflation of the universe, the short period of extremely rapid expansion that is thought to have occurred immediately after the Big Bang.

“In our analysis, this framework is also able to generate inflation-like periods without manually introducing additional hypothetical fields,” Afshodi said.

In the standard model, inflation is usually caused by a mysterious field known as an inflaton. That field has never been directly observed. In contrast, QQG naturally inflates as a result of gravity itself.

“In other words, some of the important elements that we normally add to cosmology separately may arise directly from the theory of gravity itself,” Afshodi added.

From rare physics to the familiar universe

One of the striking features of QQG is that it behaves very differently depending on the energy scale. At very high energies, new quantum rules follow. But as the universe expands and cools, it returns to the familiar physics described by Einstein.

This theory suggests that gravity becomes simpler at very high energies, a property known as asymptotic freedom, and then evolves into the form we observe today. Eventually, the universe enters a phase filled with hot radiation, which is explained by standard cosmology.

This framework provides a continuing bridge between the exotic early universe and the well-tested physics that followed. However, the key question is whether this idea can be tested.

“Yes, at least in principle,” Afshodi said. “The most promising tests come from cosmology, especially from the signatures of the early universe in primordial gravitational waves and the cosmic microwave background.”

These ancient signals convey information about the universe’s earliest moments. According to the new theory, these signals should contain nuances compared to predictions from standard inflation models.

“One particularly interesting aspect of our scenario is that it could lead to unique predictions of gravitational wave signals generated in the early universe,” Afshodi noted. “As observational sensitivity improves over the coming years and decades, future measurements of primordial gravitational waves may begin to distinguish between this type of model and more traditional inflation scenarios.”

Although this idea is still being investigated, it raises the compelling possibility that the Big Bang may not have been a singular beginning, but rather part of a deeper quantum description of gravity. If confirmed, this framework could reshape the way scientists understand the origins of the universe, replacing deconstructed physics with a new, more complete picture of how the universe began.

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