Einstein’s theory of gravity has received its most precise validation yet in a setting larger than the Solar System.According to the general theory of relativity, a massive object’s gravity should bend light passing nearby. Researchers have exploited this effect to test general theory on light deflected by the Sun. But to do so on galactic scales is difficult.
To put Einstein’s theory to an extragalactic test, Thomas Collett at the University of Portsmouth, UK, and his colleagues studied the galaxy ESO 325-G004 (E325), which is massive enough to warp light into what seems to be a ring encircling E325. Einstein’s theory holds that the ring’s radius would depend on the strength of E325’s gravitational pull, which in turn depends on its mass. The researchers used that relationship to determine the galaxy’s mass from the ring’s size.
To confirm their measurements, the team obtained a second estimate of E325’s mass by observing how quickly the galaxy’s stars orbit its centre. The ratio of the two mass measurements was close to 1 — the value predicted by general relativity.