Einstein wins again: Exotic gravity theories take a blow from new observations

Modern astronomy has an unsavory secret: Astronomers don’t understand how the Universe works. Galaxies spin faster than can be explained by the observed matter and well-established laws of physics. Individual galaxies within vast clusters of galaxies move so fast that those clusters shouldn’t exist. And when researchers look at very distant galaxies, they are distorted as space acts like a giant lens. This distortion is more than Einstein’s theory of relativity can explain.

Scientists have proposed a number of different explanations for these observations. The most popular is that the Universe is dominated by an invisible form of matter, called dark matter, that adds to the gravitational pull that governs the cosmos. But dark matter has never been directly observed — and because it has proven elusive, some scientists favor a different explanation. 

If what we observe cannot be explained by the known laws of physics, perhaps the known laws of physics are wrong — or at least incomplete. Maybe the accepted theory of gravity needs to be revised. Or perhaps what describes the behavior of matter in the Solar System doesn’t work when applied to the entire cosmos. Understanding what is going on is one of the most urgent goals of astronomy — and a recent announcement by a consortium of researchers has weighed in on the problem.

The Dark Energy Spectroscopic Instrument (DESI) uses the Mayall telescope at the Kitt Peak National Observatory in Arizona to survey the Universe. DESI can simultaneously image thousands of distant galaxies. Using 5,000 positioning devices, the device can measure the spectrum of light from each galaxy across wavelengths from 380 to 980 nanometers. This range includes ultraviolet light, the entire visible spectrum, and some infrared.

DESI began operations in 2021 and they have announced the results of the analysis of the first year of recorded data. They have studied nearly 5 million galaxies, at distances ranging from our cosmic neighborhood to those located so far away that the light observed by DESI was emitted 11 billion years ago. The DESI instrument has allowed astronomers to create an unprecedented 3D map of the Universe representing 20% of the entire sky. 

Astronomers have long known that the galaxies of the cosmos are not located smoothly throughout space. At length scales of roughly hundreds of millions of light-years, galaxies are clumped together on gigantic sheets surrounding enormous voids, which contain very few galaxies. In many respects, the distribution of galaxies resembles the foam on the top of a quickly drawn pint of beer, with the galaxies located on the surfaces of the bubbles.

The size of the voids — the size of the bubbles in the beer analogy — is related to the speed of sound in the early Universe. Like in air, sound in the early Universe consisted of regions of higher and lower density. The regions of higher density created regions of higher gravity, which pulled the matter in that direction. Meanwhile, lower-density regions lost the gravitational tug of war, resulting in the foamy cosmic structure we see today.

The recent announcement by the DESI collaboration investigated the size of the bubbles at different distances from Earth. Because the speed of light is finite, this is equivalent to looking back in time — when the Universe was only 20% of its current age.

By studying the size of the bubbles as a function of time, scientists were able to test Einstein’s theory of relativity, which is the currently accepted theory of gravity. This provided a perfect testbed to see whether Einstein was right — or if, on cosmic scales, his theory just doesn’t work. If the latter is true, scientists would have to invent an improved theory of gravity.

However, the DESI measurements were in good agreement with relativity. While this measurement doesn’t rule out all possible extended theories of gravity, it certainly rules out some — and it will cause astronomers to revisit their proposed extensions to Einstein’s theory.

This recent result uses only one year of DESI data and reflects a more sophisticated analysis than an announcement last April using the same data. However, there is far more data to be analyzed. DESI is currently in the fourth year of a planned five-year observational program.

The researchers are analyzing the data from the first three years of data, which they hope to announce in the Spring of 2025. The recent announcement agrees with the results of April 2024, which hinted that the energy field that is driving the accelerating expansion of the Universe might be changing over time. If this result is born out by analysis of the larger data set, it will significantly change our understanding of the nature of the cosmos.