Hints of a modification of gravity at cosmological scales

Could Albert Einstein ever be proven wrong? This is the groundbreaking question that Levon Pogosian and co-authors tackle in their recent article published in Nature Astronomy1. And yet, the incredible level of success of general relativity (GR) in describing the effect of gravitation at vastly different scales seems to indicate that proving him wrong will simply never be the case. Perhaps the most impressive test is the discovery of gravitational waves, predicted by Einstein himself in 1918 and observed almost a century later thanks to tremendous improvements in interferometric techniques. Gravitational waves are not the only example, as tests of GR go from table-top experiments to Solar System observations, and include the paramount high-precision test of GR around Sagittarius A* (the central black hole in the Milky Way) that led to the 2020 Nobel Prize in Physics, and its ensuing imaging with the Event Horizon Telescope. Last but not least, the application of GR to describe the Universe and the advent of physical cosmology has proven incredibly successful, as we now have an understanding of about 13.8 Gyr of evolution of our Universe.

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