A variation of the theory of quantum gravity – the unification of Quantum mechanics and Einstein’s general relativity – could help solve one of cosmology’s biggest mysteries, new research shows.
For almost a century, scientists have known that the universe is expanding. But in recent decades, physicists have discovered that different ways of measuring the expansion rate – the so-called Hubble parameter – lead to puzzling inconsistencies.
To resolve this paradox, a new study proposes incorporating quantum effects into an important theory for determining the rate of expansion.
“We sought to resolve and explain the discrepancy between the values of the Hubble parameter from two different prominent types of observations,” said study co-author PK Suresha professor of physics at the University of Hyderabad in India, told Live Science via email.
A growing problem
The expansion of the universe was first identified by Edwin Hubble in 1929. His observations with the largest telescope at the time showed that galaxies that are further away from us appear to be moving away faster. Although Hubble initially overestimated the expansion rate, subsequent measurements have refined our understanding and proven the current Hubble parameter to be highly reliable.
Later in the 20th century, astrophysicists introduced a novel technique to measure the expansion rate by studying the cosmic microwave background, the omnipresent “afterglow” of the universe Big Bang.
However, A serious problem has occurred with these two types of measurements. In particular, the newer method gave a Hubble parameter value that was almost 10% lower than that derived from astronomical observations of distant cosmic objects. Such discrepancies between different measurements, the so-called Hubble voltage, point to possible flaws in our understanding of the evolution of the universe.
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In a study published in the journal Classical and quantum gravitySuresh and his colleague from the University of Hyderabad, B. Anupama, proposed a solution to reconcile these different results. They emphasized that physicists derive the Hubble parameter indirectly, using the evolutionary model of our universe based on Einstein’s general theory of relativity.
The team argued for a revision of this theory to include quantum effects. These effects, inherent in fundamental interactions, include random field fluctuations and the spontaneous emergence of particles from the vacuum of space.
Although scientists are able to integrate quantum effects into theories from other fields, quantum gravity remains elusive, making detailed calculations extremely difficult or even impossible. To make matters worse, experimental studies of these effects require reaching temperatures or energies that are many orders of magnitude higher than those that can be achieved in the laboratory.
Suresh and Anupama recognized these challenges and focused on long-range quantum gravity effects that are common to many proposed theories.
“Our equation doesn’t have to take everything into account, but that doesn’t stop us from experimentally testing quantum gravity or its effects,” Suresh said.
Their theoretical investigation found that taking quantum effects into account when describing gravitational interactions at the earliest stage of the universe’s expansion, called cosmic inflation, could actually change the theory’s predictions regarding the current properties of the microwave background radiation, thereby creating the two types of Hubble parameter measurement would be brought into harmony with each other.
Of course, final conclusions cannot be drawn until a comprehensive theory of quantum gravity is known, but even the preliminary results are encouraging. Moreover, the connection between the cosmic microwave background and quantum gravity effects opens the way for experimental investigation of these effects in the near future, the team said.
“Quantum gravity is thought to play a role in the dynamics of the early universe; therefore, their effect can be observed by measuring the properties of the cosmic microwave background,” Suresh said.
“Some of the future missions dedicated to studying this electromagnetic background are very likely and promising for testing quantum gravity. … It provides a promising proposal for solving and validating the inflationary models of cosmology in connection with quantum gravity.”
In addition, the authors hypothesize that quantum gravity phenomena in the early universe may have influenced the properties of gravitational waves emitted during that time. Detecting these waves with future gravitational wave observatories could further shed light on the properties of quantum gravity.
“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 said. “Hopefully our work will help identify the correct inflation model and detect the primordial gravitational waves with quantum gravity features.”