Einstein’s theory of gravity is a cornerstone of modern cosmology. It has been tested and proven correct time and time again and is supported by the discovery of countless cosmic phenomena: from the gravitational lens discovered by Arthur Eddington in 1919 and the anomalies observed in Mercury’s orbit, to galactic redshifts and gravitational waves. General relativity – to give Einstein’s theory of gravity its proper name – predicted them all exactly.
But astronomical observations near the ‘cosmological horizon’ – where the most distant galaxies are moving away from us at nearly the speed of light – suggest that gravity might work differently on the very largest scales. Some scientists now propose that Einstein’s theory of gravity could be improved by adding a simple “footnote” to his equations, amounting to a “cosmic error” in the scientific understanding of gravity.
Cosmologist Niayesh Afshordi is senior author of a new research paper published in the Journal of Cosmology and Astroparticle Physics, which describes this “cosmic glitch” model as an extension of Einstein’s theory of gravity. He and his colleagues suggest that their footnote would not only explain the observed large-scale discrepancies, but could also help alleviate other “tensions” in astronomy, where the predictions of the best theories do not match astronomical observations – including the expansion rate of the universe and the abundance of superclusters of galaxies.
The cosmic glitch model is derived from Einstein’s theoretical challenges to gravity.
“From an observational point of view, there have been anomalies in the data for well over a decade,” said Afshordi, professor of astrophysics at Canada’s University of Waterloo and researcher at the Perimeter Institute.
Scientists have made dozens of attempts over the past decades to adjust Einstein’s gravity to better fit observations. One of these is the theory of ‘massive gravity’, proposed by Claudia de Rham, a theoretical physicist at Imperial College London. Another is MOND, which applies modified Newtonian dynamics and was developed as an alternative to dark matter theories; In addition, there are several early theories about dark energy, which state that the dark energy that would drive the expansion of the universe was much stronger in the first 100,000 years after the Big Bang.
Unlike these other theories, which are driven by discrepancies in the data, the cosmic glitch model is derived from specific fundamental theoretical challenges to Einsteinian gravity that have been developed over the past decades, Afshordi says. These challenges include the Hořava-Lifshitz proposal – the idea that quantum gravity works differently at high energies – and the Einstein-ether framework, which reintroduces a dynamic form of the ‘ether’ that Einstein wanted to eliminate.
“It’s a top-down approach,” Afshordi says of their cosmic glitch theory. Only after developing their theory to reconcile these theoretical issues did they decide to see if the theory fit the observational data from the Planck Space Telescope, which studied the cosmic microwave background between 2009 and 2013.
Afshordi says the results were remarkable.
The common value for the gravitational constant in Einstein’s field equations – the key mathematical equations of general relativity – can accurately explain almost everything observed in the cosmos, he says. But field equations related to observations at the cosmological horizon seem to require a different value for the gravitational constant.
According to Afshordi’s colleague and co-author Robin Wen, a recent graduate of the University of Waterloo and now a doctoral student at the California Institute of Technology, the effect is to make gravity weaker by about 1 percent over distances of billions of light years.
The researchers found that applying their cosmic glitch model also alleviates two major tensions in astronomy. The most notable is the famous Hubble tension: a discrepancy in values for the Hubble constant, a number that represents the expansion rate of the universe. Observations of cosmic microwave background radiation yield one value for Hubble’s constant, while observations based on the “standard candle” supernovae in distant galaxies yield a different value. The cosmic glitch model also reduces an important component of the “cluster tension,” which measures the unexpected abundance of galaxy superclusters in the universe.
At the same time, however, the cosmic glitch model worsens the accuracy of predictions of baryonic acoustic oscillations, or BAOs – essentially “ripples” in the average distances between galaxies, which appear to be caused by pressure waves generated during the formation of the galaxies. early universe. But the authors hope that the BAO differences can be improved with better models and observations.
Afshordi says that in the coming years the CMB Stage 4 Observatory and the Euclid Space Telescope will collect new observations of the cosmic wave background and of billions of galaxies spread across 10 billion light-years, but with a precision four times greater than the one that Afshordi and his colleagues used in their calculations.
If the cosmic disturbance is present, that will be enough to reveal it, he says.
This article originally appeared in Nautilus, a science and culture magazine for curious readers. Sign up for the Nautilus newsletter.