If the unseen substance is not discovered in tests or particle colliders, we may have to look for it in space
“How do you believe the dark matter issue will be resolved?” Vera C. Rubin enquired immediately after being introduced to me at a Women in Astronomy conference in 2009. I still don’t recall what I replied in return. I was taken aback: the renowned astronomer who had received the National Medal of Science for her work in discovering the first solid evidence for the existence of dark matter was asking me, a twentysomething Ph.D. student, for my thoughts. I’m sure anything I came up with was bad since it was an issue I hadn’t given much consideration to up to that point. Until Rubin asked for my view, it had never occurred to me that I had any right to an opinion on the subject.
She didn’t seem to mind if I had disappointed her with my response. Instead, she invited me to lunch with her and a group of other female astronomers, including former NASA administrator Nancy Grace Roman. Rubin then went on to gush over Roman, who is known as “the mother of the Hubble Space Telescope.” It was a surreal experience for me to have an old lady who had solved one of the major scientific puzzles of our time enthusiastically introduce us to her own hero.
Rubin’s fame was confirmed in the 1960s, when she analyzed stars within galaxies and discovered something strange: stars on the fringes of galaxies were moving faster than expected, as if there was an unseen mass exerting a gravitational pull. Her results paralleled those of Fritz Zwicky’s early 1930s galaxy cluster research, which led him to propose the possibility of Dunkle Materie, German for “dark matter.” Throughout the 1970s, Rubin and astronomer Kent Ford released evidence that supported this conclusion, and by the early 1980s, scientists had reached a consensus that physics faced a dark matter dilemma.
The majority of laboratory efforts to find dark matter have fallen into three types. Direct detection investigations seek evidence of dark matter particles interacting with normal matter particles—for example, the element xenon—via one of the nongravitational basic forces, the weak force, as well as proposed new forces. Collider experiments, such as those at the Large Hadron Collider in Geneva, adopt the opposite strategy, smashing together two normal particles in the goal of producing dark matter particles. Meanwhile, “indirect detection” investigations hunt for evidence of dark matter interacting with itself and creating detectable particles as a consequence of the collision.
So yet, none of these methods have yielded any results. We still don’t sure whether dark matter can communicate with conventional stuff in any manner other than gravity. It may be difficult to make or detect in the accelerators and experiments that we can build. As a result, astronomical observations—cosmic probes of dark matter—remain one of our best bets. These probes enable us to explore for dark matter signals in settings that are difficult to create on Earth, such as within neutron stars. More generally, such searches investigate the behavior of dark matter under gravity in a variety of locales.
Despite its potential for investigating dark matter, this technique has sometimes been caught in a crossfire between the astronomy and physics sectors. Physicists prefer to prioritize colliders and laboratory experiments above linkages to astrophysical studies. Dark matter is often dismissed by astronomers as a particle physics issue. This divergence has financial ramifications. We have a chance to alter it in 2022. The beginning of the 2020s signaled the commencement of an important procedure known as the Snowmass Particle Physics Community Planning Exercise. This initiative, which occurs about once every decade, gathers together physicists to describe potential scientific undertakings to a congressionally required commission that determines scientific priorities. For the first time, cosmic probes of dark matter will be considered separately. Although Snowmass does not provide any policy recommendations, it is likely that judgments on what science to highlight will be made at each level of the organizational structure.
Cosmic probes enable us to search for dark matter signals in settings that are impossible to create on Earth, such as within neutron stars.
DARK MATTER CANDIDATE UNIVERSE
We still don’t know much about dark matter, but we’ve gone a long way since Rubin’s research in the 1970s and 1980s. We now have strong evidence that each galaxy has its own bubble of dark matter, known as a dark matter halo, that spreads well beyond the visible section of the galaxy. The quantity of dark matter in these galaxy-halo systems is greater than the amount of matter in stars, planets, and gas. In other words, all of the particles identified in laboratories and colliders—referred to collectively as the Standard Model of particle physics—contribute just around 20% of the universe’s naturally gravitating matter. Taking into consideration dark energy and the reality that matter and energy are basically equal, we only comprehend around 4% of the universe. The Standard Model is both a magnificent accomplishment and an apparently unfinished hypothesis. To fix the issue, we need a new particle or particles.
Physicists now have a plethora of dark matter options to choose from. Most physicists prefer “cold dark matter” candidates—particles that travel slowly (meaning, at nonrelativistic speeds much slower than that of light). One of the classic models of cold dark matter is the weakly interacting heavy particle (WIMP). WIMPs are thought to have generated spontaneously in the early cosmos, and scientists expect that they interact with ordinary matter through the weak force. The most prevalent WIMP theories belong to a class of particles known as fermions, which also contains electrons and quarks.
For a long period, WIMPs were the most popular dark matter candidates, especially in the United States. However, since evidence for WIMPs has failed to appear at the Large Hadron Collider or in any of the direct and indirect detection experiments in recent years, public opinion has evolved.
An axion is a hypothetical dark matter candidate that has recently piqued the interest of the particle physics community. Axions are expected to have lower masses than WIMPs and are not fermions. Axions, on the other hand, are part of a family of particles known as bosons, which also contains photons, or light particles. Axions, being bosons, have fundamentally different characteristics than WIMPs, which offers up an exciting option for the structures they may create. Axions are what brought me into the area of dark matter study in the first place.
ALTERNATIVES THAT ENTICE WIMPS
It had been five years since my encounter with Vera Rubin and my first effort at addressing the issue she had posed to me. It was 2014, and I was a postdoctoral researcher at the Massachusetts Institute of Technology, first at the Kavli Institute for Astrophysics and Space Research and subsequently at the Center for Theoretical Physics (CTP), seeking for anything fascinating to work on. It was there that Mark Hertzberg, a postdoctoral researcher at CTP at the time, and I first discussed a physicists’ debate: Could axions create an unusual state known from atomic physics as a Bose-Einstein condensate?
This possibility occurs as a result of a fundamental distinction between bosons and fermions. Fermions must satisfy the Pauli exclusion principle, which states that two fermions cannot be in the same quantum state at the same time. Because electrons orbiting an atom cannot occupy the same quantum state, they must spread out in different patterns with different amounts of energy, which is why electron orbitals in chemistry can be so complicated: because electrons cannot occupy the same quantum state, they must spread out in different patterns with different amounts of energy called orbitals. Axions, on the other hand, have the ability to share a quantum state. This implies that when they are sufficiently cooled, they may all occupy the same low-energy state and behave collectively as a single superparticle—a Bose-Einstein condensate. In my opinion, the potential that this may happen organically in space is pretty thrilling.
Axions were hypothesized in the 1970s by Hertzberg’s Ph.D. mentor at M.I.T., Frank Wilczek, who was among the first to see that one result of a model provided by Helen Quinn and the late Roberto Peccei was a particle, which Wilczek dubbed “axion” after a brand of laundry detergent. As a result, Hertzberg was already well-versed in axions. I, on the other hand, was fresh to the concept. I had spent the most of my career focusing on other issues, and I needed to catch up. Along the process, I learnt to differentiate between the classical axion and the class of particles known colloquially as axionlike particles.
The Peccei-Quinn extension to the theory of quantum chromodynamics (QCD), which defines another of the four basic forces, the strong force, gives birth to the classic axion. Although QCD is a very effective model, it also predicts events that have yet to be witnessed. This challenge is solved by Peccei and Quinn’s approach, which also provides a method for creating dark matter. However, another notion known as string theory suggests a sequence of particles with the same mathematical structure as the original axion; these particles are referred to be axionlike. The standard QCD axion is projected to have a mass of about 10–35 kilogram, which is many orders of magnitude lighter than the electron, while the larger class of axions from string theory may be significantly lighter, down to 10–63 kilogram.
The study Hertzberg and I undertook with our postdoctoral mentor Alan Guth led us to question a widely held belief about how axions generate Bose-Einstein condensates. In 2009, a famous physicist, Pierre Sikivie of the University of Florida, sparked widespread interest when he postulated that QCD axions might create huge condensates in the early cosmos. His calculations revealed that they might result in ringlike galaxy halos rather than the spherical halos predicted by WIMP models. If this is the case, we may be able to discern what dark matter is comprised of just by looking at halo forms.
But when Mark, Alan, and I sat down to investigate how Sikivie’s group arrived at this forecast, we came to a completely different conclusion. Although we agreed that axion Bose-Einstein condensates would occur in the early cosmos, we predicted that they would be far smaller—the size of asteroids. In addition, our model provided no indicators of what type of axion structures may be found billions of years in the future in the present-day cosmos. Attempting to properly predict how—and whether—we go from tiny asteroid-sized condensates to today’s galactic-scale dark matter halos remains a big computational issue.
Another group was investigating additional intriguing implications of axionlike particles the same year our work was published. Hsi-Yu Schive of National Taiwan University led a team that released computer simulations of some axionlike particles known as “ultralight axions” or “fuzzy dark matter,” so termed because they have a very low mass and behave like blurred-out waves rather than pointlike particles. They demonstrated that these particles may create wavelike dark matter halos with Bose-Einstein condensates at their centers. Schive’s study sparked renewed interest in ultralight axions and boosted expectations that astrophysical investigations might reveal evidence of the wavelike halo structures we anticipate to see.
Working in science entails more than simply calculations, observations, and experiments; it also entails collaborating with others, especially policymakers.
Axions and axionlike particles, together with WIMPs, are currently some of our best suggestions for what dark matter may be. A model known as self-interacting dark matter is another that is gaining acceptance (SIDM). This theory anticipates fermion dark matter particles interacting with one another—a self-interaction—beyond gravity. These self-interactions inside a halo might result in more fascinating patterns and structures than a smooth, spherical blob. The specifics of the structures, on the other hand, are difficult to predict and rely on the mass and other properties of the particles. Interestingly, axions can interact with one another in ways that self-interacting fermions cannot.
Neutrinos are an alternative to WIMPs, axions, and SIDM. Although Standard Model neutrinos are now known to be too light in mass to account for all of the missing matter, these neutrinos are real and difficult to detect, making them a relatively minor component of the dark matter known as the cosmic neutrino background. In addition, a new sort of neutrino, the sterile neutrino, has been proposed as a companion to the Standard Model neutrino. Sterile neutrinos are distinguished by the fact that they interact predominantly gravitationally, with relatively little interactions via Standard Model forces. Furthermore, they are perhaps the most popular warm—or, at the very least, midway between hot and cold—dark matter suggestion.
Another concept that physicists are just now beginning to investigate is the possibility of a whole sector of dark matter rather than a single particle. Perhaps dark matter is made up of a mix of classic axions, axionlike particles, WIMPs, sterile neutrinos, and SIDM. Another intriguing theory is that dark matter is made up of stellar-mass black holes that originated in the early cosmos. This alternative has grown in popularity after the 2017 finding of gravitational waves revealed that black holes in this mass range are more prevalent than previously thought.
THERE ARE CLUES IN THE SKY
We are rather passive observers in astronomy. We may pick our equipment, but we can’t build a galaxy or a stellar process and then sit back and watch it happen. Cosmic events seldom occur on human-friendly time scales—galaxy creation takes billions of years, and cosmic processes that may release dark matter particles take tens to hundreds of years.
Even yet, astrophysical dark matter probes can teach us a lot. The NASA Fermi Gamma-ray Space Telescope, for example, has served as a dark matter experiment by seeking for gamma-ray fingerprints that could only be explained by dark matter. WIMPs, for example, are projected to be their own antimatter partners, which means that if two WIMPs interacted, they would destroy one other in the same way that matter and antimatter do when they encounter. These explosions should create a lot of gamma-ray radiation where there’s a lot of dark matter, particularly in the centers of galaxies where there’s a lot of dark matter.
The Fermi telescope, in fact, detects an overabundance of gamma-ray photons near the core of the Milky Way. These findings sparked heated dispute among observers and theorists. According to one theory, the pyrotechnics are caused by dark matter colliding with itself. Another theory is that the signal is coming from neutron stars at the heart of the Milky Way, which release gamma-ray photons throughout their normal lifespan. Some astrophysicists believe the signal is caused by a neutron star, while others believe it is caused by dark matter. The fact that there is disagreement is natural, and I, too, struggle to decide what I believe. I am moved by physicists Tracy Slatyer and Rebecca Leane’s careful study demonstrating that a dark matter explanation makes sense, but only examination of more comprehensive evidence can convince the community about either theory. Future data from the Fermi telescope, as well as prospective experiments like NASA’s All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X), have the potential to resolve the argument.
The Fermi telescope has also been used by scientists to seek for evidence of axions. According to theories, when axions come into contact with magnetic fields, they sometimes disintegrate into photons. We expect that by peering across large distances, we will detect evidence of this light, proving the existence of axions. And neutron stars, the potentially perplexing signal in the core of the Milky Way, are an excellent spot to seek for dark matter on their own. According to certain ideas, these massive spinning stars create axions when protons and neutrons clash in their centers. We may be able to see these axions decay into photons and flee from stars. And, when neutron stars emit dark matter over tens to hundreds of years, they cool down in a manner that we may be able to see if we search long enough. Another major area of research right now is whether nonaxion dark matter accumulates in neutron stars, influencing the star’s structure. As a member of the NASA Neutron Star Interior Composition Explorer (NICER) team, I am in charge of a research project that uses data from NICER, a small telescope on the International Space Station that will be renewed later this year. Our effort is seeking for proof that dark matter exists inside or around neutron stars.
We may also learn more about the nature of dark matter by investigating the greatest evidence for its existence that we have so far—the cosmic microwave background (CMB) radiation. This light is a radio signal that started in the early cosmos and is inextricably present all around us. It captures a moment in cosmic history, and the patterns we see in the frequencies of its light reflect the composition of the cosmos when it was formed. It turns out that we can only explain the patterns we observe in the CMB by assuming the presence of dark matter—if there was no dark matter, the CMB data would be meaningless. The data patterns tell us what proportion of the overall mass and energy dark matter contributed, and they even assist confine the probable masses of the dark matter particles. As I write, the CMB-Stage 4 cooperation is preparing to conduct the most comprehensive measurements of the CMB yet using a combination of telescopes in Chile’s Atacama Desert and near the South Pole.
THE VIEW FROM THE HORIZON
Rubin and Roman have both died since the 2009 Women in Astronomy meeting, but their legacies continue on via efforts that strive to better comprehend our cosmos. The NASA Nancy Grace Roman Space Telescope will launch in the mid-2020s, and although its primary mission will be to research cosmic acceleration (the “dark energy issue”) and exoplanets, it will also provide light on dark matter. Simultaneously, here on Earth, the Vera C. Rubin Observatory in the Atacama Desert will assist study into a variety of topics, including the hunt for dark matter, which made Rubin famous.
To put it another way, we have a lot to look forward to in the next years. One reason for this is because practically every large-scale astronomical observation may teach us something about dark matter. For example, a team headed by Alma X. Gonzalez-Morales and Luis Arturo Urea-López in Mexico demonstrated that we may utilize gravitational lensing, a phenomena in which enormous masses distort spacetime so much that it behaves like a funhouse mirror, to restrict the mass of fuzzy dark matter. Gonzalez-Morales and Urea-López are both active members of the Rubin Observatory’s Legacy Survey of Space and Time program, where they study on gravitational lensing and are members of the dark matter working group. We are considering observations that will capture more comprehensive information regarding dark matter halos, which will subsequently be compared to computer simulations of possible dark matter candidates within the group. Similarly, Roman telescope studies of large-scale structure will provide light on the behavior of dark matter on cosmic scales.
Proposed x-ray observatories, such as NASA’s Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), may let us take a closer look at neutron star structure in ways that will improve our knowledge of dark matter’s probable features in the future. Other future projects, such as NASA’s All-sky Medium Energy Gamma-ray Observatory, or AMEGO (not to be confused with AMEGO-X), will perform the same thing at a different wavelength.
I will be an active participation not just as a scientist, but also as one of three conveners for the Snowmass Cosmic Frontier’s theme Dark Matter: Cosmic Probes, with Alex Drlica-Wagner and Hai-Bo Yu. It is our obligation to communicate to funding decision-makers the thrill and prospects of astrophysical searches for dark matter. The paper I will contribute to may have an impact on the direction provided to the National Science Foundation and the United States Department of Energy for the research we do over the coming decade.
A comparable procedure, known as the 2020 Decadal Survey on Astronomical and Astrophysics, was just completed by the astronomy community. The resultant report avoided directly addressing the dark matter issue, but it did strongly promote initiatives to better map the CMB, neutron star instrumentation, and x-ray observatories—three targets that will help us comprehend dark matter.
Working in science entails more than simply calculations, observations, and experiments; it also entails collaborating with others, especially policymakers. The amount of progress we achieve will be determined in part by the level of support we get from politicians. Of course, thinking about this is stressful. The good news is that there is a cosmos to marvel about, and attempting to comprehend dark matter is a terrific diversion.