Across the 13.8 billion years since the Big Bang, the universe has undergone extraordinary transformations, shaped by powerful forces acting on matter. Galaxies, clusters, and vast cosmic structures have formed and evolved, creating a web of interconnected systems across the cosmos. New research from Joshua Kim and Mathew Madhavacheril at the University of Pennsylvania, along with collaborators at Lawrence Berkeley National Laboratory, has shed light on a fascinating aspect of this evolution. Their work suggests that the universe has become “messier and more complicated” over time, with the distribution of matter being less clumpy than what was previously expected.
In a groundbreaking study published in the Journal of Cosmology and Astroparticle Physics and on the arXiv preprint server, the team provides a fresh perspective on cosmic structure formation. Their findings indicate a subtle deviation from established models of the universe, hinting at some unaccounted-for factors that may influence how cosmic structures have developed. The researchers made this discovery by cross-correlating data from two major cosmological surveys, combining ancient observations with those of the universe’s more recent history.
Cross-Correlation of Cosmic Datasets
“Our work cross-correlated two types of datasets from complementary, but very distinct, surveys,” says Madhavacheril. The two datasets came from the Atacama Cosmology Telescope (ACT) and the Dark Energy Spectroscopic Instrument (DESI), two advanced instruments that have allowed astronomers to study the universe in different ways. The results revealed that the story of structure formation, for the most part, aligns with the predictions of Einstein’s theory of gravity—but with a hint of something unexpected in the more recent epochs, about four billion years ago. This deviation, the team believes, could be an intriguing area for future research.
ACT’s data offers a view of the universe’s early stages, using the Cosmic Microwave Background (CMB), the faint glow left behind by the Big Bang. Meanwhile, DESI’s data provides a much more recent picture of the cosmos, mapping the distribution of galaxies and offering a direct view of the universe’s more recent evolution.
ACT and the Cosmic Microwave Background
The ACT data, which covers about 23% of the sky, provides a glimpse into the universe’s early history. By observing the CMB, scientists can study the universe when it was just 380,000 years old—a moment often referred to as the universe’s “baby picture.” The CMB is the afterglow of the Big Bang, and its study has provided key insights into the universe’s initial conditions and the formation of its earliest structures.
Joshua Kim, the paper’s first author, explains, “ACT paints a picture of the universe’s infancy by using a distant, faint light that’s been traveling since the Big Bang.” As this ancient light travels across space, it encounters gravitational forces from large structures like galaxy clusters, which distort the CMB in a phenomenon known as gravitational lensing. This lensing effect, first predicted by Einstein over a century ago, provides valuable information about the distribution of matter and the evolution of cosmic structures.
DESI and the Mapping of Galaxies
On the other hand, DESI offers a more contemporary view of the universe. Located at the Kitt Peak National Observatory in Arizona, DESI is mapping the three-dimensional distribution of galaxies, with a particular focus on luminous red galaxies (LRGs). These galaxies serve as reliable cosmic landmarks, helping scientists track the spread of matter over billions of years.
Kim likens DESI’s data to a high school yearbook photo of the universe: a snapshot of cosmic structures as they are today. DESI’s ability to trace the positions of millions of galaxies provides a powerful tool to compare the distribution of matter in the early universe (from the CMB data) to the present-day arrangement of galaxies.
A Cosmic CT Scan: Comparing Ancient and Recent Data
By combining data from both ACT and DESI, the research team was able to compare ancient and more recent measurements directly. This comparison is like a cosmic CT scan, as Madhavacheril puts it, where researchers can slice through different epochs of cosmic history to observe how the distribution of matter and the gravitational influence of structures have evolved over time.
ACT’s gravitational lensing data allowed the team to trace the clumping of matter in the early universe, while DESI’s observations of galaxies helped track how those clumps evolved into the cosmic structures we see today. The overlap of these two datasets provides an unprecedented opportunity to see how matter’s distribution has changed over billions of years.
Discrepancy in Clumpiness: A Hint of New Physics?
Upon analyzing the combined data, the team noticed something intriguing: the expected amount of clumpiness, or density fluctuations, in more recent cosmic epochs didn’t quite match predictions. In particular, they observed that the Sigma 8 (σ8), a key measure of the amplitude of density fluctuations, was lower than expected in the later epochs.
Kim explains that σ8 quantifies how clustered or “clumpy” matter is on large scales in the universe. A lower value of σ8 indicates less clumping than predicted, which could mean that cosmic structures are not growing in the way current models predict. This suggests that the universe’s structural growth may have slowed or followed a different trajectory than earlier models suggested.
The discrepancy is not large enough to conclusively point to new physics, but it is worth considering. Kim adds, “It’s still possible that this deviation is purely by chance, but if it is not, it could be an indication that there’s some missing piece of physics that we have yet to account for.”
Possible Explanations: The Role of Dark Energy
One of the potential explanations for this anomaly is the influence of dark energy, the mysterious force driving the accelerated expansion of the universe. Since the discovery of dark energy in the late 1990s, scientists have understood that it is responsible for the universe’s rapid expansion. However, the exact role dark energy plays in shaping cosmic structures remains unclear.
If dark energy has a stronger influence on the formation of cosmic structures than previously thought, it could explain why the distribution of matter appears less clumpy than expected in the later epochs. This possibility opens up an exciting new avenue of exploration for cosmologists.
Future Directions: New Observatories and Higher Precision
As the team moves forward, they plan to continue refining their measurements using more advanced instruments. One such tool is the Simons Observatory, which will provide even higher precision in measuring the distribution of cosmic structures. These new observatories will enable scientists to better understand the fine details of cosmic evolution and may help clarify whether this small discrepancy is a real signal of new physics or just a statistical fluke.
In addition to providing more accurate measurements, these future telescopes will help expand our understanding of dark energy and its role in shaping the universe. By improving our observational capabilities, scientists hope to unravel the mysteries of how the universe has evolved and whether any new forces or phenomena are at play.
Conclusion: A Complex and Evolving Cosmos
The research led by Joshua Kim and Mathew Madhavacheril marks an important step in our understanding of the universe’s evolution. By combining ancient and recent cosmic data, the team has provided a multidimensional view of the cosmos, revealing insights into how matter has distributed itself over the course of cosmic history.
While the observed discrepancy in clumpiness is subtle, it opens up the possibility that we may be missing a crucial component in our models of the universe’s evolution. Whether this is due to dark energy or some other unknown factor, the discovery serves as a reminder of how much we still have to learn about the cosmos. As new telescopes come online and data collection continues, scientists are poised to answer some of the universe’s most pressing questions, bringing us closer to a deeper understanding of the forces that shape our reality.
References: Joshua Kim et al, The Atacama Cosmology Telescope DR6 and DESI: structure formation over cosmic time with a measurement of the cross-correlation of CMB lensing and luminous red galaxies, Journal of Cosmology and Astroparticle Physics (2024). DOI: 10.1088/1475-7516/2024/12/022
Noah Sailer et al, Cosmological constraints from the cross-correlation of DESI Luminous Red Galaxies with CMB lensing from Planck PR4 and ACT DR6, arXiv (2024). DOI: 10.48550/arxiv.2407.04607