One of the greatest mysteries in cosmology may be resolved by a variant on the theory of quantum gravity, which unites general relativity with quantum mechanics, according to recent study.

The universe is expanding, as scientists have known for almost a century. However, scientists have discovered in recent decades that various methods of measuring the expansion rate, also known as the Hubble parameter, result in perplexing discrepancies.

A recent study proposes to incorporate quantum effects into a well-known theory that is used to calculate the expansion rate in order to address this conundrum.

“We tried to resolve and explain the mismatch between the values of the Hubble parameter from two different prominent types of observations,” emailed P.K. Suresh, co-author of the paper and physics professor at the University of Hyderabad in India.

An Increasing Issue

Edwin Hubble discovered the expansion of the universe for the first time in 1929. Galaxies that are farther away from us seem to be moving away at quicker speeds, according to his observations made with the greatest telescope available at the time. Hubble overestimated the rate of expansion at first, but later measurements improved our knowledge and established the current Hubble value as quite dependable.

Astrophysicists later in the 20th century developed a fresh method to measure the rate of expansion by looking at the cosmic microwave background, the ubiquitous “afterglow” of the Big Bang.

But there was a significant issue with these two kinds of measurements. In particular, the more recent approach yielded a value for the Hubble parameter that was over 10% less than the value inferred from the astronomical observations of far-off cosmic objects. These disparities between various measurements, known as the Hubble tension, indicate possible weaknesses in our comprehension of the evolution of the universe.

A solution to align these inconsistent results was offered by Suresh and his University of Hyderabad colleague B. Anupama in a work published in the journal Classical and Quantum Gravity. They emphasized that physicists use the evolutionary model of our cosmos, which is based on Einstein’s theory of general relativity, to indirectly infer the Hubble parameter.

An Illustration of Galaxies Bent Due to Gravity

The group made the case for changing this theory to take quantum effects into account. These phenomena, which are inherent to fundamental interactions, include oscillations in random fields and the impulsive production of particles from space vacuum.

Although quantum effects can be incorporated into other sciences’ ideas, quantum gravity is still a mystery that makes precise calculations nearly hard. Even worse, in order to conduct experimental research on these phenomena, temperatures or energy that are many orders of magnitude higher than those that can be achieved in a lab must be reached.

Recognizing these difficulties, Suresh and Anupama concentrated on general quantum-gravity phenomena found in a wide range of 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.

Their theoretical investigation showed that the inclusion of quantum effects in the description of the gravitational interactions during the cosmic inflationary period could, in fact, modify the current predictions of the theory about the properties of the microwave background, reconciling the two kinds of Hubble parameter measurements.

Of course, only until a complete theory of quantum gravity is understood can definitive conclusions be made, but even these preliminary results are promising. The scientists added that the connection between quantum gravitational effects and the cosmic microwave background makes it possible to investigate these effects experimentally in the near future.

“Quantum gravity is supposed to play a role in the dynamics of the early universe; thus its effect can be observed through measurements of the properties of the cosmic microwave background,” Suresh stated.

“Some of the future missions devoted to studying this electromagnetic background are highly probable and promising to test quantum gravity. … It provides a promising suggestion to resolve and validate the inflationary models of cosmology in conjunction with quantum gravity.”

The authors also suggest that the characteristics of gravitational waves released in the early cosmos could have been influenced by quantum gravitational phenomena. Future gravitational-wave observatories may be able to detect these waves, which could shed more light on quantum gravitational properties.

“Gravitational waves from various astrophysical sources have only been observed so far, but gravitational waves from the early universe have not yet been detected,” Suresh stated. “Hopefully, our work will help in identifying the correct inflationary model and detecting the primordial gravitational waves with quantum gravity features.”