There are several things that excite us more than the secrets of dark matter and the curvature of space-time, but when you are wrapped in a stunning image of Einstein's ring, you know that you are in something special.
In 2014, the Atakam Big millimeter / submillimeter Array Grid (ALMA) in Chile observed a striking space freak during its Long Baseline campaign. She saw a distant galaxy distorted beyond recognition by the gravitational field of a massive foreground galaxy. This is the so-called "Einstein ring", named after Einstein's general theory of relativity, which predicted that space-time can be bent by the presence of a powerful gravitational field.
In this case, in the foreground is a galaxy in front of a more distant galaxy, located about 12 billion light years from Earth, with the result that the light of a distant galaxy bends to pass around a curved space-time. The result was an almost perfect circle of galactic light, taken by ALMA as one of the most prominent examples of gravitational lensing. Gravitational lensing is common in deep space observations, when massive galaxies and clusters of galaxies bend space-time as a ductile rubber sheet, often creating a “curved” mirror effect that distorts the observed forms of distant galaxies whose light has taken a messy path through an entangled space-time landscape. But sometimes, as it turned out in this example, the alignment can be so perfect that the light of a distant galaxy can be deformed around the symmetrical foreground of the galaxy, creating a ring resembling a candle flame behind a magnifying glass. Gravitational lenses are the natural magnifying lenses of the universe and they are used with the Hubble Space Telescope, for example, to increase its observational power in the framework of the Frontier Fields project.
Although it looks like a perfect ring, this particular observation of the “SDP.81” gravitational lens has some tiny ring distortions and astronomers used these distortions to detect the presence of an invisible dwarf galaxy located in the immediate vicinity of the more massive lens 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 raindrops on the window," said Yashar Hezaveh of Stanford University, California, in a statement. "You know that they are there because they distort the image of background objects." Raindrops subtly refract light, distorting the light passing through the window; in much the same way, the gravitational field of an invisible dwarf galaxy creates minor distortions in Einstein's ring, revealing its presence in the tiny deformation of spacetime.
Finding this distortion and understanding that it was due to the presence of an invisible galaxy was not an easy task, and it required tremendous computational costs, requiring, in particular, time on one of the world's most powerful supercomputers, the National Science Foundation Blue Waters. Because of its proximity to a larger galaxy, its estimated mass and the lack of optical data, the Hezaveh team thinks that they have found a very dim dwarf galaxy dominated by dark matter.
According to predictions, large galaxies should have a large population of satellite dwarf galaxies, but astronomical studies can detect only a few examples. Our galaxy, as is known, has about 40 such satellites, but models predict that there should be thousands of similar ones.
"This discrepancy between observed and predicted data on the number of satellites has been a major problem in cosmology for almost two decades, some researchers even called it a“ crisis, ”said team member Neal Dalal of the University of Illinois. "If these dwarf objects are made of dark matter, this may explain the discrepancy, suggesting a new understanding of the true nature of dark matter."
Now there is hope that many other gravitational lenses can be studied to look for distortions caused by other dwarfs with dominance of dark matter, and we hope to explain where did this strange disparity of observations come from compared to theory. If we can do this, then perhaps we can better clarify the dark models of matter and go a step closer to understanding why dark matter makes up 85 percent of the total mass in the Universe.