Tracing Shadows: Unlocking the Secrets of Dark Matter
By Shivank Kancharla
Only about 5 percent of the observable universe consists of the matter we all know – the atoms that form the stars, the planets, and everything else familiar. The rest? It’s split between two great cosmic unknowns: 27 percent is dark matter, an invisible substance that holds galaxies together with its gravity; and 68 percent is dark energy, the mysterious force driving the accelerating expansion of the Universe. Dr. Adrienne Erickcek, a professor at the Department of Physics and Astronomy at The University of North Carolina Chapel Hill, is working to shed light on one of these conundrums. “Dark matter is everywhere, but we can only see it through its gravitational pull,” she explains. Her research explores how the early universe’s expansion shaped the distribution of dark matter, offering a glimpse into the origins of the universe and this enigmatic substance.
Dark matter doesn’t emit or reflect light, making it impossible to observe directly. Instead, scientists detect it by how it influences things we can see, such as the movement of stars or how light bends around distant galaxies. Without dark matter, the delicate dance of stars and galaxies in the cosmos wouldn’t hold. Dr. Erickcek’s work dives into the origins of these structures by studying a crucial yet elusive window in time – the early universe. Her research focuses on the consequences of an early matter-dominated era (EMDE), a moment after the Big Bang when heavy, unstable particles might have dominated the universe before decaying. During this period, dark matter density fluctuations grew more rapidly than previously thought, setting the stage for the formation of what is now known as micro haloes—tiny, dense clusters of dark matter that formed later in the universe’s evolution.
These microhaloes, though small in mass, are much larger than a planet like Earth and are far more diffuse. If they exist, their density makes them an exciting target for research: they would increase the likelihood of dark matter particles colliding and annihilating, potentially producing detectable gamma rays. Dr. Erickcek’s research suggests that some micro haloes might still orbit galaxies like the Milky Way, relics of the universe’s earliest moments. Observing gamma rays from these structures with advanced instruments like the Fermi Gamma-Ray Space Telescope could provide a critical clue to the origins of dark matter.
Dr. Adrienne Erickcek
Dr. Erickcek’s work doesn’t just aim to uncover what dark matter is but also how it fits into the grander narrative of the universe’s evolution. Her research bridges two seemingly disconnected realms: the vast cosmic web of galaxies we observe today and the chaotic, high-energy conditions of the universe's first moments. “There’s so much we still don’t know about what happened right after cosmic inflation,” she says, referring to the period immediately following the Big Bang. “That ignorance profoundly limits our understanding of dark matter.” This knowledge gap pushes scientists like her to explore new ideas, including the possibility that dark matter could exist in a “hidden sector,” interacting with ordinary matter only through gravity. Since experiments like the Large Hadron Collider have yet to detect any new particles beyond the Standard Model, researchers are turning to more creative solutions, such as particles like axions or sterile neutrinos.
Looking to the future, Dr. Erickcek is excited about advancements in gravitational lensing, a technique that reveals dark matter by measuring how it bends light from distant galaxies. With improved instruments, scientists hope to map the distribution of dark matter across the universe in unprecedented detail. “The more we learn about how dark matter is arranged, the closer we get to understanding what it is,” she says. However, she acknowledges the challenges ahead. The biggest limitation is that the only way we have to understand the behavior of dark matter is through gravity. Without a way to observe it directly, progress remains slow and uncertain.
When asked if she thinks we will find a definitive explanation for dark matter anytime soon, Dr. Erickcek gave a thoughtful response: “I think we’re getting closer, but it could take decades. It’ll require not just better instruments but also creativity and maybe even some luck.” Throughout her work, Dr. Erickcek exemplifies the intersection of patience, curiosity, and persistence that defines modern cosmology. Her research not only brings us closer to solving the dark matter puzzle but also deepens our understanding of the early universe and its lasting influence on the cosmos we inhabit today. As the search continues, scientists like Dr. Erickcek remind us that the journey to understanding the universe’s greatest mysteries is as important as the answers themselves.
Image of simulations showing the formation of the first halos that form after something an EMDE. The lighter regions show areas of higher dark matter density, and the lower left panel shows a zoom-in on the white box in the lower right panel.
Source: : Delos, M. S.; Erickcek, A. L.; Bailey, A. P.; Alvarez, M. A. Are Ultracompact Minihalos Really Ultracompact? Physical review. D/Physical review. D. 2018, 97 (4). https://doi.org/10.1103/physrevd.97.041303.
References:
Erickcek, A. L.; Law, N. M. ASTROMETRIC MICROLENSING by LOCAL DARK MATTER SUBHALOS. The Astrophysical Journal 2011, 729 (1), 49. https://doi.org/10.1088/0004-637x/729/1/49.