In 1919, British astronomer Sir Arthur Eddington traveled to the West African island of Príncipe to take measurements of starlight passing so close to the sun it could only be seen during a total eclipse. His goal was to test a young German physicist’s prediction that the gravity of the sun warps nearby spacetime, thereby bending any light passing.
Eddington’s results confirmed the theory, propelling Albert Einstein to international fame. But it is only now, 99 years later, that astrophysicists have found a way to test Einstein’s theory of gravity on a larger scale, verifying that what works for stars and planets also works for galaxies. In the process, they have improved the case for a mysterious substance known as dark matter and its even more enigmatic cousin, dark energy.
The new experiment used a galaxy known as ESO 325-G004, 450 million light years away in the constellation Centaurus.
ESO 325-G004 happens to lie almost directly in our line of sight to a much more distant galaxy, billions of light years behind it. This positioning means that light coming from the more distant galaxy is bent by the warped spacetime produced by the gravity of ESO 325-G004.
That produces multiple images of the distant galaxy on all sides of ESO 325-G004, in a feature known as an Einstein ring.
“The radius of the ring depends on how much spacetime is curved by the foreground galaxy,” says Thomas Collett, an astrophysicist from the University of Portsmouth, UK.
“This is exactly the sort of thing Eddington did back in 1919,” he explains.
“The difference is that instead of doing it for the mass of a single star and measuring the curvature a few thousand miles away from the star, we’ve done this with an entire galaxy and are measuring the curvature 6000 light years away.”
Galactic-scale spacetime curvature measurements have been done by other methods, but none this precisely. That’s been a concern to astrophysicists, because there are alternative theories of gravity that, at galactic scales, differ minutely enough from Einstein’s that they have never been ruled out.
“We’re more than twice as precise,” Collett says.
What allowed the increased precision, he says, is that ESO 325-G004 is close enough that new instruments on the European Southern Observatory’s Very Large Telescope, in Chile, allowed his team to map the speeds of stars within it to a spatial resolution of less than 500 light years. From that, he says, it’s possible to measure the mass of the galaxy, and thus calculate the diameter of the Einstein ring if Einstein’s theory is correct.
And just as Eddington found back in 1919 for the gravity of a single star, our sun, their results matched Einstein’s predictions.
That’s important, Collett says, because it plays into the search for dark matter and dark energy.
Dark matter is an invisible substance, so far detected only by its gravity, which appears to comprise more than 80% of the mass of the universe. Dark energy is an even less understood force that appears to be working against gravity to accelerate the rate at which the universe is expanding.
Neither has ever been detected by anything other than these effects — a failure that has led some physicists to suggest that perhaps they don’t actually exist. Perhaps, these physicists say, the problem is with Einstein’s theory of gravity.
That idea, Collett says, has led to “modified gravity theories” that “explain away” things like dark matter and dark energy by predicting that gravity behaves differently at galactic or intergalactic scales than it does in our own solar system.
The findings from ESO 325-G004 say otherwise.
“Our result is showing that if there are deviations from general relativity, they can’t have much effect on the scale of individual galaxies,” Collett says.
The findings also reinforce the case for dark energy.
“It was tempting to have theories of gravity that explain [the accelerating expansion of the Universe] without dark energy,” Collett explains.
“This result says you probably do need dark energy.” Though, he adds, “it doesn’t tell us what it is.”
Brad Tucker, an astrophysicist and cosmologist at Australian National University, Canberra, who was not part of the study team, sees the new study as an important finding.
“They say gravity is the law, so you have to obey it,” he quips. “But it is still a theory that needs to be tested.”
Tests at small scales, within our solar system, have confirmed Einstein’s version of gravity to “astounding accuracy,” he says, but tests like this, on larger scales “have been hard, until now”.
And, he notes, the new study isn’t just an important confirmation of Einstein’s theory: “It is also an independent check on whether our understanding of the existence of dark matter and dark energy is correct.”
The new study is in the journal Science.