Einstein Wins Again! General Relativity Passes Its First Extragalactic Test
“For the first time, we’ve been able to perform a direct test of General Relativity outside of our Solar System and get solid, informative results. The ratio of the Newtonian potential to the curvature potential, which relativity demands be equal to one but where alternatives differ, confirms what General Relativity predicts. Large deviations from Einstein’s gravity, therefore, cannot happen on scales smaller than a few thousand light years, or for masses the scale of an individual galaxy. If you want to explain the accelerated expansion of the Universe, you can’t simply say you don’t like dark energy and throw Einstein’s gravity away. For the first time, if we want to modify Einstein’s gravity on galactic-or-larger scales, we have an important constraint to reckon with.”
For many of the greatest cosmic puzzles today, you can either add two new ingredients, dark matter and dark energy, or you can seek to modify Einstein’s theory of gravity. While Einstein’s General Relativity has been confirmed spectacularly under a wide variety of circumstances, the only robust tests that are independent of dark matter or dark energy assumptions occur on scales of the Solar System or smaller. That’s only for distances that are a tiny fraction of a light year, and for masses no bigger than the Sun, which should trouble you when you’re making inferences about galaxies, clusters, or the entire Universe! But thanks to a very fortunate galactic system
a strong gravitational lens that is only 500 million light years distant
we’ve been able to put Einstein’s theory of gravity to the test for galactic masses and distance scales in the thousands of light years.