Hubble catches a cosmic optical illusion predicted by Einstein 86 years ago

Hubble catches a cosmic optical illusion predicted by Einstein 86 years ago

Gravity plays tricks on our telescopes. Consider this image from the Hubble Space Telescope provided by the European Space Agency on August 9, 2021. A ring of light in the center of the image is punctuated by four bright spots, with two more diffuse spots glowing in the middle. This is an Einstein Ring, and its looks are deceiving.

The four brilliant spots are actually the same object, a quasar, whose light is magnified and multiplied by the gravitational influence of the two galaxies in the center. (In reality, Hubble’s data indicates that there is a fifth image of the quasar in the middle of the ring that cannot be seen with the human eye.) The ring is light smeared out by the same illusion as all the stars in the quasar’s home galaxy: Gravitational lensing is a phenomena.

Gravitational lensing is a major prediction of general relativity, and astronomers are now using the rings bearing Einstein’s name to investigate the theory that appears to explain its very existence.


Einstein predicted in a letter to Science in 1936 that the gravity of one star would twist the light of a star directly behind it into a ring.

“Of course, there is no hope of observing this phenomenon directly,” Einstein writes.

He had no idea that one day we might have telescopes powerful enough to image distant galaxies.


The first photograph of an Einstein ring was captured in 1987 at New Mexico’s Very Large Array radio observatory. Hubble discovered the first full one a little more than a decade later. Since then, scientists have discovered many more Einstein Rings, including this one made by Tommaso Treu‘s group at the University of California, Los Angeles’ Department of Physics and Astronomy using the Hubble Space Telescope.

A radio image of the first Einstein ring to be directly observed by astronomers, taken by the VLA.VLA/ NASA

“I think he would love it,” says Treu of Einstein.

”I think he had a sense of the natural sublime. He had a sense of the beauty of the natural world, and so I think he would love just the pure beauty of these images, but also the fact that it’s so comprehensible, the fact that we can make a model of the universe that describes everything that happens to these photons over billions of years.”


Treu’s group is employing Einstein Rings to investigate that model (the standard model of cosmology, which includes general relativity). The rings themselves could be ideal dark matter detectors.

“My take is that since we only know that dark matter and dark energy interact gravitationally, we should use gravity to learn about them,” he explains.

According to Treu, if dark matter exists, it may modify the appearance of the ring of spread out starlight in the same way as scratches on a magnifying glass distort an image. Scientists may learn some properties of dark matter depending on the distortion.

“There are hundreds of theories about what dark matter could be,” says Treu.

“With these lenses, we can rule out some of these ideas, and, if we are lucky, we can pick one that works better than others. If nature helps us, we may be able to find out which one it is.”


Einstein rings, which include quasars and other dynamic objects such as supernovae, provide a different method for investigating the so-called “Hubble tension,” which arises when different methods for measuring the expansion rate of the universe yield different answers when general relativity is used to model them.

“Of course,” says Treu, “there’s always the alternative. It is that general relativity could be wrong.”

Hubble’s fresh picture of an Einstein ring was released in August 2021.ESA

According to Alessandra Silvestri, the Hubble tension is merely one piece of evidence that the mainstream model of cosmology may not fully capture the picture. Silvestri is a theoretical cosmologist at the Netherlands’ Leiden University.

A more puzzling issue is that the universe’s expansion is accelerating, requiring the use of dark energy to counteract the force of gravity.

“The discovery of cosmic acceleration has given an impressive boost to the exploration of alternatives to general relativity,” Silvestri tells Inverse.

“Indeed, among the open questions (dark matter, inflation, and cosmic acceleration), it is the one that really makes us question whether we need to go somehow beyond general relativity in our description of gravity on large, cosmological scales.”

Treu is pessimistic that general relativity will be proven incorrect.

“There’s so much evidence that general relativity works, and there’s so little alternative,” he says.

“I think the easiest solution is still that there are particles that we don’t know about. But in the end, we have to be humble, and nature gets to decide. It’s not our call what the right answer is, so we’ll have to keep trying.”

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