Does dark energy's sound speed reveal gravity's true nature?

Physicists tested whether different dark energy models can be distinguished by how sound waves propagate through cosmic structures. They compared General Relativity against modified gravity theories using data from the Planck satellite, supernovae, galaxy surveys, and weak lensing. Models with changing dark energy sound speeds show significant deviations from standard gravity only when combined with evolving dark energy—but adding more data from lensing pulls constraints back toward Einstein's theory.