Einstein Predicted How Gravity Should Work at the Largest Scales. And He Was Right

When Albert Einstein introduced his theory of general relativity in 1915, it changed the way we viewed the Universe. His gravitational model showed how Newtonian gravity, which had dominated astronomy and physics for more than three centuries, was merely an approximation of a more subtle and elegant model. Einstein showed us that gravity is not a mere force but is rather the foundation of cosmic structure. Gravity, Einstein said, defined the structure of space and time itself.

But in the past century, we have learned far more about the cosmos than even Einstein could have imagined. Some of our observations, such as gravitational lensing clearly confirm general relativity, but others seem to poke holes in the model. The rotational motion of galaxies doesn’t match the predictions of gravity alone, leading astronomers to introduce dark matter. The expansion of the Universe is not steady but is accelerating, pointing to the presence of dark energy. For some astronomers, this points to the need for a new model. Something that can account for the motions of stars and galaxies without the need for those dark materials that remain undetected in the lab. The most popular alternatives focus on theories of modified gravity.

The standard model of cosmology is known as the LCDM model. The L, for lambda, is the symbol used in general relativity to represent the rate of cosmic expansion and represents dark energy, while CDM stands for cold dark matter. This model describes an expanding Universe that began as a hot, dense state about 13.78 billion years ago. It is a Universe made up of about 5% regular matter, 25% dark matter, and 70% dark energy. It is currently the model best supported by observational evidence. Modified gravity models have a big hill to climb. To topple LCDM they have to account for everything it predicts as well as eliminate the need for dark matter and energy.

This year, that hill has become much steeper. In a series of publications released by the Dark Energy Spectroscopic Instrument (DESI) collaboration, the standard cosmological model has been confirmed to be in complete agreement with Einstein’s model. The DESI survey mapped nearly six million galaxies across 11 billion years of cosmic time, allowing astronomers to see not just how galaxies cluster but how that clustering changes over time. It is the largest 3D map of the Universe made thus far.

The LCDM model makes very stringent predictions of cosmic structure. If dark energy were a kind of repulsive force rather than an inherent property of spacetime, clustering would evolve differently than observed. If dark matter was an illusion of modified gravitational forces, the scale of galactic clustering would be different. This latest survey shows in explicit detail that modified gravity models don’t hold up. The results strongly constrain which modified gravity models are possible and rule out many of the models currently proposed. Based on these new results, the standard cosmological model of Einsteinian gravity, dark matter, and dark energy is the one that best fits the observed Universe.

There are still mysteries that still need to be solved, most significantly the issue of the Hubble tension problem. Perhaps a novel modified gravity model will solve this mystery and finally topple Einstein, but for now, the wild-haired genius remains king of the hill.

Reference: Adame, A. G., et al. “DESI 2024 II: Sample Definitions, Characteristics, and Two-point Clustering Statistics.” arXiv preprint arXiv:2411.12020 (2024).

Reference: Adame, A. G., et al. “DESI 2024 V: Full-Shape Galaxy Clustering from Galaxies and Quasars.” arXiv preprint arXiv:2411.12021 (2024).

Reference: Adame, A. G., et al. “DESI 2024 VII: Cosmological Constraints from the Full-Shape Modeling of Clustering Measurements.” arXiv preprint arXiv:2411.12022 (2024).