The universe keeps its secrets well—most of the matter is invisible, and we still don’t know what it is made of. In 1933, Fritz Zwicky, a Swiss astrophysicist, was studying the Coma Cluster of galaxies when he noticed something unusual. The galaxies were moving at enormous speeds that should have torn the cluster apart based on the visible mass alone; but clearly that wasn’t the case. So, he proposed an invisible substance providing the additional gravitational pull needed to keep the cluster intact. He termed this unseen mass to be “Dunkle Materie” (Dark Matter in German). He estimated that this invisible mass was about 400 times more than what was visually observable in the galaxy cluster.
While stars, galaxies, and nebulae captivate our attention with their luminous presence, a far more abundant and mysterious entity pervades this universe. Dark Matter is said to make up 85% of all the matter in existence and the truth is: we know little about it.
Despite its overwhelming presence, this enigmatic substance does not interact with light or any other electromagnetic radiation making it impossible to detect from traditional observational instruments.
We interviewed Dr. Disha Bhatia, a FAPESP Fellow at the Instituto de Física, Universidade de São Paulo in Brazil. She received her PhD from Tata Institute of Fundamental Research in 2018 and has worked as a postdoc at The Institute of Mathematical Sciences in Chennai and at the Indian Association for the Cultivation of Science in Kolkata.
The interview excerpts are as follows:
How would you explain your studies on dark matter to a lay audience?
The only evidence we have for dark matter comes from its gravitational effects. Since dark matter is invisible, its presence is inferred from observed anomalies in the motion of stars, which would otherwise be expected to follow Newton’s law of gravitation.
From our physics lessons, we know that the farther a planet is from the Sun, the weaker the gravitational force it experiences, causing it to move more slowly. This is why Earth orbits faster around the Sun than Jupiter.
However, this same logic fails to explain the motion of stars around the galactic center. These stars, as Zwicky observed, moved at much higher velocities than expected based on their distance from the center. This trend is manifested in the rotation curves of these galaxies. The rotation curve is a plot of the orbital speeds of stars in a galaxy versus their radial distance from that galaxy's centre. As the distance from the galactic centre increases, the velocity is expected to decrease. But the rotation curves are observed to be flat, contrary to the expected decline.
There are two possible explanations for this discrepancy: either Newton's law of gravitation does not apply accurately at such vast distances, or there is more unseen matter exerting additional gravitational force on the stars, thereby increasing their speeds.
This anomaly is not limited to motions of stars within the galaxies alone. The same discrepancies are also observed at the scale of galactic clusters and in measurements of Cosmic Microwave Background (CMB) radiation—often seen as the static or "noise" on old TV screens. The CMB radiation is like the afterglow of the Big Bang– it’s a weak glow of microwave radiation that fills the entire universe. These anomalies are consistent with the idea of missing matter. Modified Newtonian dynamics (MOND), an alternative to dark matter, has so far failed to provide a consistent explanation across all length scales.
Hence there is great agreement with the idea of missing/invisible matter or dark matter.
While we've made observations at the galaxy scale and have a rough estimate of the amount of dark matter in our galaxy, there are still many unknowns. For instance, we don't know its mass, spin, or whether it interacts with visible matter in any way other than gravitation.
These fundamental questions form the foundation of dark matter research. It is highly interdisciplinary, drawing on insights from various branches of physics—astrophysics, cosmology, particle physics, quantum mechanics, statistical physics, and more.
What are the different ways in which dark matter can be detected? What evidence do we have of dark matter as of today?
There are three main methods for detecting dark matter: direct detection, indirect detection, and particle production.
- Direct detection involves attempting to observe dark matter particles directly. Since dark matter exists in our galaxy, it can travel to Earth and collide with detectors. By measuring the recoils from the elastic scattering of dark matter with the detector material, we can identify potential interactions. Experiments like Xenon are designed to detect these signatures.
- Indirect detection focuses on looking for the byproducts of dark matter annihilations in galaxies to the known standard model particles such as gamma rays. The experiments like Fermi-LAT and AMS are used to probe the indirect signatures of dark matter.
- Particle production involves creating dark matter particles in accelerator experiments and looking for missing energy signals that could point to the presence of these elusive particles. Large experiments like the Large Hadron Collider (LHC) are actively studying potential dark matter signatures.
The only evidence we have for dark matter so far comes from its gravitational interactions. These include the rotation curves of stars at the galactic scale, gravitational lensing at the scale of galaxy clusters, and the study of cosmic microwave background (CMB) radiation.
To briefly explain gravitational lensing: Einstein's general theory of relativity tells us that space-time is curved by the presence of mass and energy. As light travels through this curved space-time, it follows the shortest path, which appears bent due to the curvature. The greater the mass, the stronger the curvature, resulting in more bending of light. Gravitational lensing helps identify missing matter, as the observed bending of light is often greater than what can be explained by visible mass alone.
Why do you think dark matter research is especially relevant today?
Dark matter research is crucial because it makes up the majority of the mass in our universe. Galaxies have largely formed due to the presence of dark matter. As a result, understanding dark matter is key to explaining the universe's evolution, from its early stages to its present state. Ongoing research into dark matter will unlock insights into its complex nature and address fundamental questions about its mass, spin, and other properties. While visible matter is composed of quarks, leptons, and gauge bosons, dark matter—roughly five times more abundant than visible matter—must also consist of its own set of particles. Therefore, the research is relevant.
What do you believe are some of the most important future applications of understanding dark matter? How can it potentially help in space exploration?
Dark matter research is still in its early stages. First, we need to determine the particle properties of dark matter before we can explore its potential applications in the future. However, the sensitivity of current dark matter experiments has already reached a level where similarities exist between the technologies used in quantum computing and those employed in dark matter direct detection experiments. In fact, some scientists have recently used data from quantum computing experiments to help constrain the nature of dark matter.
Additionally, dark matter experiments can also serve as probes for studying radioactive decay in rocks and detecting neutrino signatures. These neutrinos can cause recoils in the detector materials in a manner similar to dark matter particles.
As for space exploration, it is to note again that dark matter is the most dominant form of matter, playing a crucial role in galaxy formation during the universe's early stages. While visible galaxies are typically around 10 kpc in size, the dark matter halo—derived from the rotation curves of stars—extends to regions as far as several hundred parsecs. (Note: A parsec is a unit of astronomical distance, equivalent to about 3.26 light-years.) Studies of large-scale light bending due to dark matter, galaxy rotation curves, stars that are composed primarily of dark matter, and dwarf galaxies dominated by dark matter are among the key probes that can help constrain our understanding of dark matter and the universe.
The James Webb Space Telescope (JWST) plays a vital role in constraining the properties of dark matter by studying galaxy formation and gravitational lensing. Through these observations, JWST can map the distribution of dark matter by analyzing how light is bent by massive objects like galaxy clusters.
The Planck satellite, another important tool, was a space-based observatory launched to study the Cosmic Microwave Background (CMB) radiation in high detail. Its measurements provide crucial insights into the early universe and contribute to our understanding of dark matter by revealing its effects on the CMB and large-scale cosmic structures.
There are several other experiments, both direct and indirect, that contribute to constraining the properties of dark matter. Together, these efforts will continue to shape our understanding of this elusive substance and its role in the cosmos.
What are some of the theoretical ideas and mechanisms behind the creation of dark matter? Which model do you personally support the most?
There are two broad categories of dark matter production in the early universe: thermal production and non-thermal production.
Thermal production of dark matter is similar to how Standard Model particles were produced in the early universe. In this scenario, dark matter was in thermal equilibrium with other particles and could be described using thermodynamic quantities such as temperature, pressure, and density. The interactions between dark matter and other particles were frequent enough to maintain this equilibrium. In this state, dark matter particles were continuously being created and annihilated through interactions with standard model particles. However, as the universe expanded, it also cooled. There came a point where these interactions slowed down and eventually froze, resulting in a finite density of dark matter particles.
On the other hand, non-thermal production of dark matter refers to scenarios where dark matter is created without being in thermal equilibrium. This can occur through various mechanisms, such as gravitational interactions i.e via fluctuations in the vacuum during the early universe. Dark matter could also be produced through the decay of heavy particles or through the decay of primordial black holes.
Personally, I find the thermal production mechanism more appealing because it is more predictive and aligns with the way Standard Model particles are believed to have been produced in the early universe.
You have done research in different areas of physics such as particle physics, astrophysics and dark matter physics. How do you think such interdisciplinary research has helped you?
Going forward, which particular research area are you planning to focus on?
I believe that interdisciplinary science is crucial in today's era. In virtually every field, progress is driven by the combined efforts of various teams. For example, take the experiment at the Large Hadron Collider (LHC). Building a successful collider like this requires inputs from multiple disciplines, such as particle physics and experimental particle physics. However, constructing the detectors themselves also involves condensed matter physicists and engineers. It's always a collaborative effort.
The same is true for the study of dark matter, which is a problem that spans both large and small scales. On the large scale, we detect the presence of dark matter through its gravitational interactions, such as the motion of stars in galaxies or galaxy clusters. On the small scale, we seek a particle physics explanation for dark matter's nature. To truly make strides in science, having a broad understanding of various fields, coupled with a firm grasp of the specific sector you contribute to, is essential.
Looking ahead, I believe that interdisciplinary research will become even more vital. Already, there are emerging signatures of stochastic gravitational waves through pulsar measurements and scientists are trying to see whether they can be explained by a primordial phenomenon. Additionally, there are strong ongoing efforts to study stars and the cosmos as laboratories that naturally produce highly energetic particles. These cross-disciplinary endeavors will likely unlock new insights into dark matter and other fundamental mysteries of the universe.
Areas such as particle physics and dark matter physics are highly abstract and require the need of expensive experiments to validate the theory. What are some of the challenges that you faced while researching these areas or otherwise?
I think we are really in a very interesting time where several experiments have been sanctioned and are also running. I think the biggest challenge which lies ahead of us is so far null results at all experiments. Since dark matter mass can lie in a very broad range – 10^ {-22} eV to a few solar masses, it becomes hard for experimentalists to build experiments scanning all mass ranges. Some theoretical guesses are made as per where the dark matter mass can be more probable to lie. With the null results, we may have to think out of the box beyond the theoretical guesses.
Having done your Masters in physics at IIT Delhi, how would you describe your time at IITD and how do you think it has benefited you? What is one memorable experience that you cherish?
We had an amazing time at IITD, filled with many unforgettable memories. Our class was fortunate to be taught by Ajoy Ghatak, who very happily agreed to take a special course on Quantum Mechanics. We also had the privilege of learning from K. Thyagarajan, not only about optics but also about origami. Dilip Rangathanan, with his exceptional teaching skills, continually amazed us. And, of course, there was Prof. Ajit Kumar, who brought his own unique style to teaching. We learned not only physics but also several skills from these amazing people. The staff members were also very helpful.
Being a young researcher yourself, what message do you wish to convey to students who wish to pursue research in theoretical physics?
I think doing higher studies and research is very interesting. The most important advice I can give is to focus on understanding the basic concepts first, before jumping into internships and research projects. A strong theoretical foundation in the basics like electrodynamics, quantum mechanics, mathematical physics, statistical mechanics etc will help you in research later on.
Start slow and steady. Attend talks to see what interests you the most. One may not understand the full talk but nonetheless it will give you an overall idea of what's happening in the field.
Talk to faculty members about their research during coffee breaks or whenever you can. This helps you learn more and think about different areas of research. Research takes years of hard work. There will be challenges, so it's important to stay persistent and not lose confidence.
-------
Dark matter stands as a silent testament to how much of the universe remains unknown, even as our understanding deepens. As technology surges forward and our theoretical horizons expand, the hunt for dark matter is slowly taking shape.
The fact that we don’t yet fully understand the majority of matter in our universe proves that this is just the beginning of the story– the story of decoding Dark Matter.
