NASA’s Roman Space Telescope Will Map Thousands of Cosmic Voids to Decode the Universe’s Expansion
The universe may look crowded when we gaze at the night sky, but on its largest scales, it is a surprisingly structured place. Galaxies are not scattered randomly. Instead, they trace vast, interconnected patterns that resemble the walls of enormous bubbles. Inside these bubbles lie regions with very few galaxies at all. These immense empty spaces are known as cosmic voids, and they are about to become a major focus of study thanks to NASA’s upcoming Nancy Grace Roman Space Telescope.
Roman is expected to identify and precisely measure tens of thousands of cosmic voids, offering scientists a powerful new way to understand how the universe has expanded over billions of years and what roles dark matter and dark energy have played in shaping it.
Understanding the Structure of the Universe
Astronomers estimate that the observable universe contains up to two trillion galaxies. While that number sounds overwhelming, these galaxies are not evenly spread out. Large-scale surveys have shown that galaxies cluster along filaments and sheets, forming the boundaries of enormous structures sometimes described as the cosmic web.
Within this web are the cosmic voids. These voids can span tens to hundreds of millions of light-years and contain very few galaxies compared to their surroundings. Because they are so empty, voids behave differently from dense regions like galaxy clusters, making them especially valuable for studying the universe’s underlying physics.
Why Cosmic Voids Matter
The universe is thought to be made up of three main components: normal matter, dark matter, and dark energy. Normal matter and dark matter both exert gravitational pull, working to slow down cosmic expansion. Dark energy does the opposite, driving the expansion of the universe to accelerate.
Cosmic voids are regions where matter is scarce, which means dark energy dominates their behavior. This makes voids natural laboratories for testing theories about dark energy and its influence over time. By observing how voids form, grow, and change shape, scientists can place strong constraints on cosmological models that describe the universe’s evolution.
What Makes the Roman Space Telescope Special
The Nancy Grace Roman Space Telescope is uniquely suited for this task because of its ability to survey large areas of the sky deeply and efficiently. One of Roman’s core programs, the High-Latitude Wide-Area Survey, will observe regions far from the plane of the Milky Way, where dust and stars would otherwise obscure distant galaxies.
This survey is designed to cover approximately 2,400 square degrees of the sky, an area equivalent to about 12,000 full moons. Within this vast footprint, Roman will detect huge numbers of faint and distant galaxies, far more than many previous missions could observe at once. This dense mapping of galaxies is exactly what astronomers need to reliably identify voids, which are defined by the absence of galaxies rather than their presence.
Detecting and Measuring Thousands of Voids
Using simulations based on Roman’s planned capabilities, researchers have shown that the telescope should be able to detect tens of thousands of cosmic voids, including some as small as 20 million light-years across. Having access to such a large sample is critical, because individual voids can have irregular shapes.
Instead of studying voids one by one, scientists use statistical methods. By stacking thousands of voids together, they can measure an average shape and compare it to predictions from cosmological models. In a universe that is homogeneous and isotropic on large scales, voids should appear spherically symmetric when stacked correctly. Any systematic distortion in that shape can reveal problems with the assumed values of cosmic ingredients like dark energy.
Building a Three-Dimensional Map of Space
To understand voids properly, astronomers need more than just images. Roman will provide two crucial types of data: the positions of galaxies on the sky and their cosmological redshifts. Redshift measures how much a galaxy’s light has been stretched by the expansion of the universe and can be used to estimate its distance.
Converting redshift into a physical distance requires assumptions about how fast the universe is expanding and how dark energy behaves. By testing different assumptions and seeing which ones produce voids with the expected shapes, researchers can refine their understanding of the universe’s composition. In this way, cosmic voids act like a consistency check for cosmological theories.
How Roman Compares to Other Missions
Roman’s work on cosmic voids will complement other major missions, particularly the European Space Agency’s Euclid telescope. While Euclid will survey a wider area of the sky, Roman will observe fainter galaxies and probe deeper into space, resulting in a higher density of detected galaxies in its survey regions.
This higher density is especially important for void studies. Since voids are defined by sparse galaxy populations, being able to detect even the faintest galaxies helps researchers better trace the true boundaries and internal structures of these empty regions.
What Cosmic Voids Can Tell Us About Dark Energy
Because voids expand differently than dense regions, they are sensitive probes of dark energy’s strength and evolution. Changes in void size, shape, and distribution over cosmic time can reveal whether dark energy behaves like a constant force or if it evolves as the universe ages.
Roman’s observations will allow scientists to test these possibilities with unprecedented precision. The results will feed directly into broader efforts to understand cosmic acceleration, alongside other Roman science goals such as supernova studies and weak gravitational lensing.
A New Era for Large-Scale Cosmology
Studying cosmic voids on this scale has not been possible before. Previous surveys lacked either the depth, the area, or the spectroscopic detail needed to build reliable void catalogs in such large numbers. Roman brings all of these capabilities together in a single mission.
By mapping thousands of voids across billions of light-years, Roman will help scientists answer some of the most fundamental questions in cosmology: what dark energy really is, how the universe’s expansion has changed over time, and whether our current models of the cosmos are truly complete.
Research paper:
https://doi.org/10.3847/1538-4357/ae07d9
