The tell-tale bending of light
If you were to draw a straight line between our planet and the SDP.81 galaxy 12 billion light years away, it would pass perilously close to another galaxy about four billion light-years from Earth. On that 12 billion year journey, the ancient beams of light emitted from SDP.81 are literally bent and distorted by the gravity of that closer galaxy—an unsettling consequence of Einstein’s general theory of relativity. As a result, this decidedly non-ring-shaped galaxy appears as a nearly perfect ring of light due a process called gravitational lensing.
This image of SDP.81, created with data from the massive Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope, is the highest resolution image of this phenomenon out there. SDP.81, here, is the red ring, and the hazy blue in the middle is the foreground galaxy.
That ring is, for sure, a powerful visual verification of gravitational lensing, but according to Yashar Hezaveh, a physicist at Stanford University, it should also be thought of as a natural tool to unravel even more mysteries about the universe. “Once we discover lenses, that’s very exciting,” he said, “But we need to move on and say ‘What did we learn? What can we actually learn from them?’”
A dark matter mystery
If his recent Astrophysical Journal paper is any indication, quite a lot. In that paper, Hezaveh and his colleagues wanted to see if they could exploit the natural magnification of the gravitational lens to test some theories about the structure of the universe and its unseeable components—specifically the clumps of dark matter that are predicted to exist, based on computer simulations, in a spherical halo region around most galaxies.
Most physicists now accept that a majority of the stuff in the universe does not interact with light and therefore has no way of being directly imaged. In simulations of the early universe, Hezaveh said, most scientists use the assumption that this dark matter is “cold dark matter”. Cold dark matter can be thought of as particles like atoms that interact gravitationally like visible matter, but that do not react at all with the electromagnetic spectrum.
In those simulations, he said, “we start the universe right after the Big Bang in the computer model and then we allow it to just evolve and allow the gravity to work.” What the computer shows is the dark matter clumping together to form halos, which ultimately attract more matter, some of it visible, to form galaxies like our own.
At least that’s the idea. The problem here is that another consequence of these models, which seem to capture accurately how galaxies form, suggests that there should be tons of dwarf-galaxy-sized clumps of dark matter in the halos of most galaxies, Hezaveh said. This doesn’t jive with what scientists have been able to observe, though. “We count the number of things in our galaxy, and we find thirty, forty of them and our model is predicting thirty, forty thousand of them,” he explained.
There are two possible reasons for this discrepancy, Hezaveh said. One explanation is that the models we have for cold dark matter are correct, but, for an as-yet-unknown reason the dark matter doesn’t always attract the visible matter into their clumps to form stars and galaxies. “The second hypothesis,” he said, “is that maybe [the clumps] don’t exist at all, and if they don’t exist at all, it just points towards a different property for dark matter all together.”
Using gravitational lenses as a tool
To address this problem, Hezaveh and his colleagues wanted to use the beautifully-lensed SDP.81 to see if they could find invisible dark matter clumps located in the foreground galaxy. The idea was to look for subtle gravitational effects from dark matter in that close galaxy on the already distorted image of the more distant SDP.81 galaxy.
The lensed nature of SDP.81 was integral. If the galaxy were “just sitting by itself somewhere else” without a strong lensing galaxy in front of it, then the effect of the hypothesized dark dwarf galaxy would be “so weak we will never be able to detect it.”
Using incredibly high-resolution data from ALMA, some impressive algorithms, and even the National Science Foundation’s most powerful supercomputer, they unambiguously identified the presence of a dwarf-galaxy-sized mass of dark matter in the foreground galaxy by seeing its otherwise nearly imperceptible effect on the already distorted light of SDP.81.
Hezaveh likens the effect of this newly discovered dwarf galaxy as a drop of water on a wine glass. The base of the glass, which can be thought of as the gravitational lensing of the main foreground galaxy, is already acting to distort the table below it (in our example, SDP.81). The drop of water on top represents the lensing effect of a tiny dark dwarf galaxy, and it further distorts the image below it. The distortions are subtle, but using all that data, the team even identified the location of the dwarf galaxy, which is shown as a white dot on the left side of the SDP.81 image above.
Hezaveh sees this mainly as an exciting verification of a method that can be used to get a more in-depth sample of these dark matter clumps around galaxies in the future. “We have seen a lot of simulations and computer models to see if we can do it,” he said. “Finally, when we had this data fed, we tried it, and it actually works out. That’s what is the exciting thing.”
Using other gravitational lenses in the universe, this method could make some serious progress in understanding how dark matter is distributed in and interacts with galaxies, and it can even shed light on how our own Milky Way galaxy developed. Einstein, who once suggested that humans may never even be able to witness a gravitational lens, would no doubt be ecstatic by the progress we have made.