Biggest puzzle in cosmology could be explained by a new theory of quantum gravity

A new concept of quantum gravity, which tries to unite quantum physics with Einstein’s relativity, could help unravel the puzzle of the universe’s expansion, suggests a theoretical paper.

For almost a century, scientists have known that the cosmos is expanding. But in recent decades, physicists have found that different kinds of measurements of the expansion rate — termed the Hubble parameter — produce perplexing inconsistencies. To resolve this conundrum, a new study suggests incorporating quantum effects into one theory used to determine the expansion rate. “We tried to resolve and explain the mismatch between the values of the Hubble parameter from two different prominent types of observations,” said study co-author P.K. Suresh (a professor of physics at the University of Hyderabad in India).

An expanding problem

The universe’s expansion was first acknowledged by Edwin Hubble in 1929. His observations with the largest telescope available at that time showed that galaxies farther from us seem to move away at faster speeds. Even though Hubble initially overestimated the expansion rate, ensuing measurements have refined our understanding, establishing the current Hubble parameter as very reliable. Later on in the 20th century, astrophysicists presented a novel technique to gauge the expansion rate by scrutinising the cosmic microwave background, the ubiquitous “afterglow” of the Big Bang.

But, a grave problem arose with these two types of measurements. Specifically, the newer technique produced a Hubble parameter value nearly 10% lower than the one deduced from the astronomical observations of distant cosmic objects. Such inconsistencies between different measurements, termed the Hubble tension, signal possible flaws in our understanding of the cosmos’s evolution.

In a paper published in the journal Classical and Quantum Gravity, Suresh and his colleague B. Anupama proposed a solution to align these incongruent results. They underlined that physicists infer the Hubble parameter indirectly, employing our cosmos’s evolutionary model based on Einstein’s theory of general relativity. The team recommended revising this theory to incorporate quantum effects. These effects, inherent to fundamental interactions, incorporate random field fluctuations and the spontaneous formation of particles from the vacuum of space.

Despite researchers’ ability to integrate quantum effects into theories of other fields, quantum gravity remains vague, making detailed calculations tremendously difficult or even impossible. To make matters even worse, experimental studies of these effects require attainment of temperatures or energies many orders of magnitude higher than those achievable in a lab. Recognising these challenges, Suresh and Anupama concentrated on broad quantum-gravity effects common to several proposed theories. “Our equation doesn’t need to account for everything, but that does not prevent us from testing quantum gravity or its effects experimentally,” Suresh stated.