The High Altitude Water Cherenkov Observatory (HAWC) is a fair hike from civilization. It is located 4,100 meters above sea level, near the Sierra Negra, a largely barren volcano that looms over most of the surrounding peaks in Puebla, Mexico. In the rarified mountain air, a team of scientists from educational institutes in the United States, Europe, and Mexico mill around an array of 300 corrugated steel tanks. To the untrained eye, these tanks look large and unwieldy, and rather ordinary, but each one contains close to two hundred thousand liters of water and four light sensors sensitive to ultraviolet wavelengths.
The idea is simple. When a gamma ray reaches the Earth’s atmosphere, it produces a shower of charged particles that cascade through the air. When these particles pass through water in these tanks, they produce a flash of light – a tiny blip that sets off the sensors anchored to the floor. Scientists around the globe use the pattern and timing of these blips to trace the direction and intensity of gamma rays and create pictures of their sources – monumentally large astrophysical events and objects such as supernovas, the nuclei of galaxies, the jets escaping black holes, or mergers of stars. This makes HAWC a high-energy telescope, capable of observing two-thirds of the sky every 24 hours.
HAWC is just one of several observatories that are helping advance the quest for dark matter. These particles have eluded detection for decades, taking astrophysicists from the flank of an extinct volcano in Mexico to the depths of the Antarctic ice floor. Scientists first speculated that something was missing from their astrophysical observations of a cluster of galaxies in the 1930s, but most of the scientific community did not give credence to this theory until astronomer Vera Rubin began systematically studying the rotation curves of dozens of galaxies and galaxy clusters in the 1970s. What she found did not fit within the standard model of galactic physics. At very large distances from the centers of galaxies, stars were moving much faster than expected. The only possible explanation (that did not invoke some kind of completely new force) was that something invisible was exerting a gravitational effect strong enough to change their velocity. Physicists began to call this dark matter.
Over the years, the allure of this mysterious matter grew. Researchers began to find other phenomenon littered throughout the universe that pointed to its existence. For instance, when light passed a massive object such as a cluster of galaxies, it veered off course. To the observer beyond this large object, the source of the light appeared magnified or distorted. This gravitational lensing could not be explained given only the luminous regular matter in the path of the light, and provided additional evidence for the presence of dark matter.
So what is tugging at the universe? What passes through matter without interacting with it, but exerts a gravitational force strong enough to bend light and steer galaxies? The scientific community is perched on the edge of this discovery. Still, its detection remains tantalizingly out of reach.
When Dr. Mehr un Nisa, a post-doctoral researcher on astrophysical neutrinos and gamma rays at Michigan State University, first travelled to Mexico to visit HAWC, Donald Trump had just been elected as the U.S. President.
“It was actually a very good way of taking my mind off of the elections. The HAWC site is beautiful. It’s majestic,” she says.
For Mehr, the election of an avowedly anti-immigrant administration was personal. She was born and raised in Pakistan, and earned her undergraduate degree in Physics at the Lahore University of Management Sciences (LUMS), where she first studied general relativity and became fascinated by what she calls “the big picture stuff.” She moved to the U.S. to pursue a PhD in Physics at the University of Rochester. As a result, Mehr’s palpable excitement at visiting the observatory was tinged with some apprehension and amusement at what was happening back in the U.S.
“I remember joking with my team about whether we would be let back across the wall,” she says. “But I loved this visit because it was my first time physically seeing how we took these little blips in water tanks and transformed them into pictures of the sky.”
These blips are instrumental in her work. She studies the unknown particles showering down from space, and tries to nail down their sources. Dark matter particles are not directly observable; they do not emit light or interact with standard model particles upon collision. However, they can annihilate or decay to produce standard model particles, such as gamma rays or cosmic rays or neutrinos. When these particles pass through a medium like water faster than the speed of light, they produce a bluish light, known as Cherenkov radiation. Most people recognize this as the blue glow emitted by under-water nuclear reactors. Gamma ray and cosmic ray showers traveling to the earth set off HAWC’s sensors with the same conical bluish glow in the water.
“We’re looking for all of these particle signatures of dark matter, but because we don’t have a precise idea of what we are looking for in uncharted territory, it’s more a matter of eliminating possibilities, or imposing constraints,” says Mehr. “For example, one of the things we try to constrain is called the dark matter-proton cross-section, which tells us how strongly dark matter can interact with normal matter (composed of protons).”
Because dark matter interacts so weakly with protons, researchers would need a very large pool of protons to look for these rare interactions.
“Building that sort of thing on earth is very expensive. So theorists came up with the idea to use the sun as their laboratory. It is also basically a huge vat of protons,” she says.
Like other scientists before them, Mehr and her collaborators began to look at whether dark matter could be captured in the heart of the sun. If so, the process of annihilation and production of secondary particles like neutrinos and gamma rays would occur deep inside its blazing core. And if those particles escaped the atmosphere of the sun, the signal received on earth would give the team an idea about how strongly dark matter was interacting with the protons in the sun.
What made things infinitely more complicated was this escape from the surface of the sun. The star at the center of our solar system has such a dynamic and thick atmosphere that gamma rays produced inside its core would be quickly absorbed or extinguished. This brought physicists to a special model of dark matter annihilation.
“The scenario we’re looking at is that dark matter annihilates into a pair of long-lived mediators. A mediator is just another particle in the dark sector, one we cannot directly observe.” When this happens, the mediator is not immediately extinguished. It can escape the surface of the sun, and decay outside to produce gamma rays that reach the planet.
However, while HAWC can spot these gamma rays, it is not equipped to observe neutrinos. After her PhD, Mehr expanded her research to astrophysical neutrinos using data from the IceCube Neutrino Observatory, stationed at the South Pole. It is an engineering marvel. IceCube’s team of physicists and engineers drilled 80 shafts into the Antarctic ice floor, using high-pressure nozzles that directed boiling water at the earth’s core, until they reached depths of two and a half kilometers. Then, physicists quickly dropped strings of spherical optical sensors into these shafts before the ice refroze. Once stabilized, these sensors sat permanently frozen and poised to detect neutrinos from a range of potential sources including dark matter.
Just like the sensors placed in HAWC’s water tanks, those placed within the ice can detect the Cherenkov radiation produced by the secondary particles from neutrino interactions with the ice. The refractory properties and clarity of water and ice make them ideal for observing neutrinos and cosmic rays.
With the data coming in from HAWC and IceCube, the team of scientists Mehr works with can witness hadronic processes that produce both neutrinos and gamma rays. If they can find both kinds of secondary particles coming from the same source, they will have a good “cross-check,” and will be able to narrow down some of the most mysterious objects in the universe. With each new observation, these teams circle to more stringent constraints, allowing them to mark off what is not dark matter.
Around the globe, physicists are carrying out intense searches for dark matter that broadly fit into three categories.
Direct detection experiments search for signals between atomic nuclei and Weakly Interacting Massive Particles (WIMPs). These are hypothetical particles thought to constitute dark matter. Such experiments are conducted deep underground.
Indirect detection experiments like Mehr’s observe familiar particles borne out of dark matter annihilation, such as neutrinos and gamma rays.
Finally, experiments at CERN in Switzerland look for dark matter in high-energy collisions, orchestrated in the particle accelerator at the Large Hadron Collider (LHC). At LHC, high-energy particle beams are made to travel in opposite directions in tubes spread over tunnels spanning 26.7 km. These are kept at temperatures colder than outer space, in an ultra-high vacuum. A strong magnetic field guides them around the tunnel before they collide and produce a new, heavier particle that escapes detectors unnoticed, so physicists look for missing energy and momentum instead.
Despite these efforts, there is currently no evidence for dark matter or any other type of new particle outside the standard model. Scientists continue to collect higher quality data as the search progresses, and Mehr remains hopeful that they will be able to find this evidence soon.
“Our research is also focused on looking at new and exotic phenomenon like gamma rays from evaporating black holes, or the joint detection of gravitational waves, gamma rays and neutrinos,” she says.
“IceCube has only one confirmed source of extraterrestrial neutrinos so far, so we would be pretty happy to detect any neutrino source at this point,” she continues, laughing.
Mehr credits Segev BenZvi, an experimental physicist at the University of Rochester, for introducing her to the search for dark matter. Over the course of her first year as a PhD student at the university, she became enthralled with the subject. BenZvi later became her PhD advisor.
This curiosity was never latent. “As a teenager, I read anything I could get my hands on,” she says. Books like Stephen Hawking’s A Brief History of Time and Carl Sagan’s Cosmos clearly had an impact. With his talk of the symmetries of the universe and black holes, Hawking made her realize that there was a lot more to physics than memorizing the formulas and equations taught in classrooms to solve problems. But most people in Pakistan did not appreciate the value of pursuing pure sciences as a career.
“My dad yelled at me when I said I wanted to do a physics major. He would have preferred engineering or medicine,” she says. “It was when I got into a graduate program that he started being supportive and proud of me.”
Sagan’s book about the cosmos had an additional appeal. “I remember first learning about Hypatia through Sagan,” she says, referring to the Alexandrian philosopher, astronomer, and mathematician from the 4th century AD. “He mentions a fair number of historical figures from different parts of the world and the broader impact of science on their societies. This was encouraging for me because it meant that you don’t have to belong to a specific culture to tackle pure science.”
Despite this, being a woman in STEM came with its own set of annoyances. The first talk Mehr gave was in front of a bunch of white men and just one woman – a friend who had come to support her. While this did not faze her, there were times when she wished for more representation. She has only worked with women a few times in her career, but has noticed an immediate jump in her productivity on those few occasions, partly because of the amount of support and empathy women displayed even when criticizing her work.
The recent push for more female inclusion has become a double-edged sword. On the one hand, it encourages more smart, driven women to pursue the field. On the other, people sometimes assume that they benefitted from positive discrimination.
“Sometimes the questions people ask me give me the impression that they think I only got this job because I’m a woman. That annoys me because obviously I find that very patronizing,” she says. “This trope of the eccentric, white, male scientist should be going obsolete.”
Nonetheless, Mehr considers herself lucky to have received so much support from so many quarters. And so, in November 2016, when she stood next to HAWC’s array of water tanks and joked about confronting Trump’s wall on the way back to the U.S., the teams’ collaboration and support served as a heartening confirmation of Sagan’s message of inclusiveness, and the pursuit of science as a global, collective effort.
“One of my collaborators pointed out that this is the sort of thing we can build when people from two countries come together, at a fraction of the cost of the wall,” she says. “Ultimately, we are able to look at something that is much bigger than us.”
Syeda Amna Hassan has a master’s degree in journalism from the U.C. Berkeley Graduate School of Journalism. She has previously worked as an investigator for criminal courts seeking evidence in terrorism cases. She currently works at ITU as a faculty member, and as project coordinator for a child sexual abuse prevention program in Kasur and Sheikhupura.