Einstein’s theory of gravity is a cornerstone of modern cosmology. It has been tested and proven correct over and over and is supported by the discovery of countless cosmic phenomena: from gravitational lensing discovered by Arthur Eddington in 1919 and anomalies observed in the orbit of Mercury, to galactic redshifts and gravitational waves. The theory of 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 may work differently on larger scales. Now, some scientists propose that Einstein’s theory of gravity can be improved by adding a simple “footnote” to his equations, which constitutes a “cosmic error” in the scientific understanding of gravity.
Cosmologist Niayesh Afshordi is a senior author on a new research paper, published in Journal of Cosmology and Astroparticle Physics, which describes this “cosmic error” model as an extension of Einstein’s gravitational theory. He and his colleagues suggest that their footnote would not only account for observed large-scale discrepancies, but could also help alleviate other “tensions” in astronomy, where the predictions of the best theories disagree. astronomical observations – including the expansion scale of the universe and the abundance of superclusters of galaxies.
The cosmic flaw model derives from Einstein’s theoretical challenges to gravity.
“From an observational standpoint, there have been these anomalies in the data for over a decade now,” says Afshordi, a professor of astrophysics at Canada’s University of Waterloo and a researcher at the Perimeter Institute.
Scientists have made dozens of attempts over the past few decades to modify Einstein’s gravity to better fit observations. One of these is the theory of “mass 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 some early theories of dark energy, which suggest that the dark energy thought to 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 inconsistencies in the data, the cosmic flaw model stems from fundamental theoretical challenges specific to Einstein’s gravity that have developed in recent decades, Afshordi says. These challenges include the Hořava-Lifshitz proposal—the idea that quantum gravity works differently at high energies—and the Einstein-aether framework, which reintroduces a dynamical form of the “aether” that Einstein intended to eliminate.
“It’s a top-down approach,” Afshordi says of their cosmic flaw theory. Only after they developed their theory to reconcile these theoretical issues did they decide to see if the theory fit observational data from the Planck Space Telescope, which studied the cosmic microwave background between 2009 and 2013.
Afshordi says the results were remarkable.
The usual value for the gravitational constant in Einstein’s field equations—the essential mathematical equations of general relativity—can accurately explain almost everything observed in the cosmos, he says. But the field equations associated with observations made 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 that gravity becomes about 1 percent weaker at distances that span billions of light years.
The researchers found that applying their cosmic error model also reduces two important tensions in astronomy. Most notable is the famous Hubble tension: a discrepancy in the values for the Hubble constant, a number that represents the expansion rate of the universe. Observations of the cosmic microwave background radiation produce one value for the Hubble constant, while observations based on “standard candle” supernovae in distant galaxies produce another value. The cosmic fault model also reduces a key component of the “cluster tension,” which measures the sudden abundance of galaxy superclusters in the universe.
At the same time, however, the cosmic flaw model degrades the accuracy of predictions of baryonic acoustic oscillations, or BAOs—effectively “bunching” at the average distances between galaxies, which appear to be caused by pressure waves generated during the formation of the early universe. But the authors hope that the BAO discrepancies can be improved with better modeling and observations.
Afshordi says that over the next few years, the CMB Stage 4 observatory and the Euclid Space Telescope are planned to collect new observations of the cosmic wave background and billions of galaxies 10 billion light years away, but with four times that precision. that Afshordi and his colleagues used in their calculations.
If the cosmic flaw is there, that will be enough to detect it, he says.
This article originally appeared in Nautilus, a science and culture magazine for curious readers. Sign up for the Nautilus newsletter.