There are few things that get us more excited than the mysteries of dark matter and the warping of spacetime, but when you have both wrapped into a stunning image of an Einstein ring, you know you're onto something special.
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In 2014, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile observed a striking cosmic quirk during its Long Baseline Campaign. It saw a distant galaxy, warped beyond recognition, by the gravitational field of a massive galaxy in the foreground. This "Einstein ring" is so-called after Einstein's theory of general relativity, which predicts spacetime can become bent by the presence of a powerful gravitational field.
In this case, the foreground galaxy had drifted in front of the more distant galaxy located some 12 billion light-years away, causing the distant galaxy's light to be redirected around the warped spacetime. The result was a near-perfect circle of galactic light received by ALMA as one of the more extreme examples of gravitational lensing. Gravitational lensing is common in observations of the deep cosmos, where massive galaxies and galactic clusters bend spacetime like a malleable rubber sheet, often creating a "funhouse" mirror-like effect, distorting the observed shapes of distant galaxies whose light has taken a helter-skelter path through the confused spacetime landscape.
But sometimes, as this example proved, the alignment can be so perfect that the distant galaxy's light can be warped around the symmetrical foreground galaxy, creating a ring that resembles a candle flame passing behind a magnifying glass. Gravitational lenses are the universe's natural magnifying lenses and they are being used by the Hubble Space Telescope, for example, to superboost its observational power as part of the Frontier Fields project.
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Though it looks like a pristine ring, this particular observation of the "SDP.81″ gravitational lens holds some tiny distortions in the shape of its ring and astronomers have used these distortions to reveal the presence of an invisible dwarf galaxy situated right next to the more massive lensing galaxy. And this tiny cluster of stars is packed with dark matter.
"We can find these invisible objects in the same way that you can see rain droplets on a window," said Yashar Hezaveh at Stanford University, Calif., in a statement. "You know they are there because they distort the image of the background objects." Raindrops will subtly refract light, distorting the light passing through a window; in much the same way, the invisible dwarf galaxy's gravitational field is creating a minute distortion in the Einstein ring, revealing its presence in a tiny spacetime warp.
Finding this distortion and realizing it was due to the presence of an unseen galaxy was no easy task and required a huge computational effort, requiring, in part, time on one of the world's most powerful supercomputers, the National Science Foundation's Blue Waters.
Because of its close proximity to the larger galaxy, its estimated mass and lack of optical data, Hezaveh's team thinks they've found a very dim dwarf galaxy that is dominated by dark matter.
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It is predicted that large galaxies should have a large population of satellite dwarf galaxies, but astronomical surveys can only seem to detect a few examples. Our galaxy is known to have around 40 such satellites, but models predict that there should be thousands of them.
"This discrepancy between observed satellites and predicted abundances has been a major problem in cosmology for nearly two decades, even called a 'crisis' by some researchers," said team member Neal Dalal, of the University of Illinois. "If these dwarf objects are dominated by dark matter, this could explain the discrepancy while offering new insights into the true nature of dark matter."
Now it is hoped that many more gravitational lenses can be studied to look for the distortions caused by other dark matter-dominated dwarfs to hopefully explain why there's such a strange discrepancy in observations when compared to theory. If we can do this, then perhaps we can better refine dark matter models and move a step closer to understanding why dark matter constitutes 85 percent of all the mass in the universe.