A variation of the theory of quantum gravity — unification quantum mechanics and Einstein general relativity – could help solve one of the biggest puzzles in cosmology, new research suggests.
Scientists have known for almost a century that the universe is expanding. But in recent decades, physicists have discovered that different types of measurements of the expansion rate — called the Hubble parameter — produce surprising discrepancies.
To solve this paradox, the new study proposes to incorporate quantum effects into one known theory used to determine the rate of expansion.
“We tried to resolve and explain the discrepancy between Hubble parameter values from two different known types of observations,” study co-author PK Sureshprofessor of physics at the University of Hyderabad in India, told Live Science via email.
A growing problem
The expansion of the universe was first discovered by Edwin Hubble in 1929. His observations with the largest telescope of the time showed that galaxies farther away from us are receding at a faster rate. Although Hubble initially overestimated the expansion rate, subsequent measurements have refined our understanding, establishing Hubble’s current parameter as very reliable.
Later in the 20th century, astrophysicists introduced a new technique to measure the rate of expansion by probing the cosmic microwave background, the ubiquitous “afterglow” Big explosion.
however, a serious problem arose with these two types of measurements. In particular, the new method gave a Hubble parameter value almost 10% lower than that derived from astronomical observations of distant space objects. Such discrepancies between different measurements, called the Hubble stress, signal potential flaws in our understanding of the evolution of the universe.
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In a study published in the journal Classical and quantum gravity, Suresh and his University of Hyderabad colleague B. Anupama proposed a solution to reconcile these different results. They pointed out that physicists derive Hubble’s parameter indirectly using an evolutionary model of our universe based on Einstein’s theory of general relativity.
The team advocated revising this theory to account for quantum effects. These effects, inherent in fundamental interactions, include random field fluctuations and the spontaneous creation of particles from the cosmic vacuum.
Despite scientists’ ability to integrate quantum effects into the theory of other fields, quantum gravity remains elusive, making detailed calculations extremely difficult or even impossible. Worse, experimental studies of these effects require reaching temperatures or energies many orders of magnitude higher than those achievable in the laboratory.
Recognizing these challenges, Suresh and Anupama focused on the broad effects of quantum gravity common to many of the proposed theories.
“Our equation doesn’t have to account for everything, but that doesn’t stop us from testing quantum gravity or its effects experimentally,” Suresh said.
Their theoretical study showed that accounting for quantum effects in the description of the gravitational interaction at the earliest stage of the universe’s expansion, called cosmic inflation, could indeed change the theory’s predictions about the properties of the microwave background today, making the two types of Hubble parameter measurements consistent.
Of course, the final conclusions can only be made when a full-fledged theory of quantum gravity is known, but even the preliminary conclusions are encouraging. Moreover, the connection between the cosmic microwave background and the effects of quantum gravity paves the way for experimental studies of these effects in the near future, the team said.
“Quantum gravity is believed 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 said.
“Some of the future missions dedicated to the study of this electromagnetic background are very likely and promising to test quantum gravity. … This provides a promising proposal for solving and testing inflationary models of cosmology in conjunction with quantum gravity.”
In addition, the authors claim that quantum gravity phenomena in the early universe could have shaped the properties of gravitational waves emitted during that period. Detection of these waves by future gravitational-wave observatories may shed further light on quantum gravitational characteristics.
“Gravitational waves from various astrophysical sources have only been observed so far, but gravitational waves from the early universe have yet to be detected,” Suresh said. “Hopefully, our work will help determine the correct inflationary model and reveal primordial gravitational waves with features of quantum gravity.”