Powerful forces have made the structure of the universe very "messy"

Across cosmic history, powerful forces have shaped matter into the intricate structures that define the universe. From the earliest moments after the Big Bang to the present-day cosmic web, the distribution of galaxies, clusters, and filaments has evolved in ways largely consistent with Einstein’s theory of gravity.

However, new research suggests that the universe’s large-scale structure has developed in an unexpected way – becoming “messier and more complicated” over time.

Led by Joshua Kim and Mathew Madhavacheril from the University of Pennsylvania, along with collaborators from Lawrence Berkeley National Laboratory, the study reveals that the distribution of matter in the cosmos is less clumpy than expected.

The findings add a new layer of complexity to our understanding of how the universe has grown over the past 13.8 billion years.

Studying cosmic history

To probe the structure of the universe across time, the research team combined two powerful datasets: the final data release from the Atacama Cosmology Telescope (ACT) and the first-year results from the Dark Energy Spectroscopic Instrument (DESI).

By merging these complementary sources, they created a unique, multidimensional view of cosmic evolution.

“Our work cross-correlated two types of datasets from complementary, but very distinct, surveys. What we found was that, for the most part, the story of structure formation is remarkably consistent with the predictions from Einstein’s gravity,” explained Madhavacheril.

“We did see a hint for a small discrepancy in the amount of expected clumpiness in recent epochs, around four billion years ago, which could be interesting to pursue.”

By integrating these two sources of data, the team effectively layered snapshots of the universe taken at different epochs, much like stacking transparencies of ancient cosmic photographs over more recent ones.

This method allowed them to observe changes in the distribution of matter over billions of years, revealing surprising new details about the way cosmic structures have formed and evolved.

A picture of the universe in its infancy

One of the key components of this research relies on ACT’s data, which provides a glimpse into the early universe through a faint remnant of the Big Bang known as the Cosmic Microwave Background (CMB).

This radiation, which has been traveling through space for nearly 14 billion years, offers an unparalleled view of the universe when it was just 380,000 years old.

“ACT, covering approximately 23% of the sky, paints a picture of the universe’s infancy by using a distant, faint light that’s been traveling since the Big Bang,” said Kim.

“Formally, this light is called the Cosmic Microwave Background (CMB), but we sometimes just call it the universe’s baby picture because it’s a snapshot of when it was around 380,000 years old.”

The journey of this ancient light has not been a straight one. As it traveled through space, the CMB was distorted by the gravitational pull of massive cosmic structures such as galaxy clusters.

This phenomenon, known as gravitational lensing, alters the path of the CMB photons in much the same way that a pair of spectacles distorts the view of an image.

By analyzing these distortions, cosmologists can infer valuable information about the large-scale structure of the universe, including how matter is distributed across cosmic time.

Tracing the universe’s structure with galaxies

While ACT provides insight into the early universe, DESI offers a more recent perspective by mapping the three-dimensional distribution of galaxies across vast stretches of space.

Located at the Kitt Peak National Observatory in Arizona and operated by Lawrence Berkeley National Laboratory, DESI is specifically designed to study the arrangement of luminous red galaxies (LRGs), which serve as cosmic landmarks.

“The LRGs from DESI are like a more recent picture of the universe, showing us how galaxies are distributed at varying distances. It’s a powerful way to see how structures have evolved from the CMB map to where galaxies stand today,” explained Kim.

By mapping the positions of millions of galaxies, DESI provides a precise record of how matter has spread out over the past several billion years.

When combined with the lensing data from ACT, this creates an unprecedented overlap between early and late-universe observations, allowing researchers to directly compare how structures have changed over time.

Cosmic CT scan of the universe

One of the most exciting aspects of this research is its ability to function like a cosmic CT scan, providing a layered view of the universe’s structure at different points in time.

By examining the way matter has clumped together across cosmic history, the team was able to track the influence of gravity over billions of years.

“This process is like a cosmic CT scan, where we can look through different slices of cosmic history and track how matter clumped together at different epochs. It gives us a direct look into how the gravitational influence of matter changed over billions of years,” said Madhavacheril.

Clumpiness of the universe’s structure

Despite overall agreement with established models, the researchers identified a small but notable discrepancy. The expected level of clumpiness, a measure of how matter is distributed in the universe, appears to be slightly lower than predicted in the later epochs of cosmic history.

This discrepancy is captured by a key metric known as Sigma 8 (σ8), which quantifies the amplitude of matter density fluctuations. A lower-than-expected value of σ8 suggests that cosmic structures may not have formed in complete accordance with early-universe predictions.

“This slight disagreement with expectations isn’t strong enough to suggest new physics conclusively – it’s still possible that this deviation is purely by chance,” said Kim.

However, if this deviation turns out to be real and not a statistical anomaly, it could point to unknown physical processes influencing the way cosmic structures form and evolve.

One leading hypothesis is that dark energy, the mysterious force responsible for the accelerating expansion of the universe, may be affecting structure formation in ways that current models do not fully capture.

Looking toward the future

While this research from the University of Pennsylvania raises new questions about cosmic evolution, upcoming projects promise to refine these measurements with even greater precision.

The Simons Observatory, a next-generation telescope currently under development, will provide more detailed observations of the CMB and its lensing effects, helping scientists determine whether the observed discrepancy is real or simply a fluctuation within expected parameters.

By combining increasingly powerful observational tools with sophisticated theoretical models, researchers hope to gain a deeper understanding of how the universe’s large-scale structure has developed over billions of years.

Whether this study ultimately confirms existing models or points toward new physics, it represents an important step toward unraveling the mysteries of the cosmos.

As scientists continue to push the boundaries of our understanding, one thing remains clear – the story of the universe’s evolution is far from over. Each new discovery brings us closer to answering some of the biggest questions in cosmology, revealing the intricate and ever-changing nature of the cosmos.

The study is published in the Journal of Cosmology and Astroparticle Physics (JCAP) and on the preprint server arXiv.

Image Credit: Unsplash/CC0 Public Domain

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