A variation on the theory of quantum gravity – the unification of quantum mechanics and Einstein’s 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 several types of measurements of the expansion rate – called the Hubble parameter – produce puzzling inconsistencies.
To resolve this paradox, a new study suggests incorporating quantum effects into a prominent theory used to determine expansion rates.
“We attempted to resolve and explain the mismatch between the Hubble parameter values based on two different prominent types of observations,” study co-author P K Suresha professor of physics at the University of Hyderabad in India told LiveScience via email.
A spreading problem
The expansion of the universe was first identified by Edwin Hubble in 1929. His observations with the largest telescope of the time showed that galaxies further away from us appear to be moving at higher speeds. Although Hubble initially overestimated the expansion rate, subsequent measurements have refined our understanding, establishing the current Hubble parameter as highly reliable.
Later in the 20th century, astrophysicists introduced a new technique to measure the rate of expansion by examining the cosmic microwave background, the ubiquitous “afterglow” of cosmic rays. Big bang.
However, a serious problem arose with these two types of measurements. Specifically, the newer method yielded a Hubble parameter value that was almost 10% lower than that derived from the astronomical observations of distant cosmic objects. Such discrepancies between different measurements, the so-called Hubble strain, point to potential shortcomings 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 align these disparate results. They underlined that physicists derive the Hubble parameter indirectly, using the evolutionary model of our universe, based on Einstein’s general theory of relativity.
The team advocated revising this theory to include quantum effects. These effects, inherent to fundamental interactions, include random field fluctuations and the spontaneous creation of particles from the vacuum of space.
Despite scientists’ ability to integrate quantum effects into theories in 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 many orders of magnitude higher than those that can be achieved in a laboratory.
Suresh and Anupama recognized these challenges and focused on broad quantum gravity effects common to many proposed theories.
“Our equation may not explain everything, but that doesn’t stop us from experimentally testing quantum gravity or its effects,” Suresh said.
Their theoretical exploration showed that taking quantum effects into account when describing the gravitational interactions in the earliest phase of the universe’s expansion, called cosmic inflation, would indeed undermine the theory’s predictions regarding the properties of the microwave background at this time. can change, making the two types of Hubble parameter measurements consistent.
Of course, definitive conclusions can only be drawn once a full-fledged theory of quantum gravity is known, but even the preliminary findings are encouraging. Furthermore, the link between the cosmic microwave background and quantum gravity effects opens the way to study these effects experimentally in the near future, the team said.
“Quantum gravity is thought to play a role in the dynamics of the early universe, so its effect can be observed through measurements of 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 to test quantum gravity. … It offers a promising suggestion to solve and validate the inflationary models of cosmology in combination with quantum gravity. “
Furthermore, the authors propose that quantum gravity phenomena in the early universe may have influenced the properties of the gravitational waves emitted during that period. Detecting these waves with future gravitational wave observatories could further elucidate quantum gravity characteristics.
“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 original gravitational waves with quantum gravity characteristics.”