A cosmic 'CT scan' shows the universe is far more complex than expected (2025)

These snapshots reveal that, as forces like gravity have reshaped the universe, the universe has in turn become less clumpy. In other words, the universe grew more complicated than expected. The team behind these findings used the sixth and final data release from the Atacama Cosmology Telescope (ACT) in combination with Year 1 data from the Dark Energy Spectroscopic Instrument (DESI) to reach these conclusions.

This powerful combination of data allowed the researchers to layer cosmic time, akin to stacking ancient cosmic photographs over recent images of the universe, creating a multidimensional perspective of the cosmos.

In order for the team to build this so-called CT scan of the universe, they needed to turn to light that has existed almost as long as the cosmos itself.

With such ancient light, it's possible to track the changes the universe underwent as gravity reshaped it over around 13.8 billion years.

You may like

• Scientists find hints of the dark universe in 3D maps of the cosmos
• Hubble tension is now in our cosmic backyard, sending cosmology into crisis

The CMB is light left over from an event that happened shortly after the Big Bang called the "last scattering." This occurred when the universe had expanded and cooled enough to allow electrons and protons to form the first neutral atoms of hydrogen. The disappearance of free electrons meant that photons, aka particles of light, were free to travel without being endlessly scattered. In other words, the universe suddenly went from being opaque to being transparent.

Today, that first light is seen as the CMB, also known as the "surface of last scattering."

This process is known as "gravitational lensing." Albert Einstein first suggested it as part of his theory of gravity, general relativity.

By noting how the CMB has warped and distorted over time, scientists can learn a great deal about the evolution of matter over billions of years.

DESI does this by mapping the universe’s three-dimensional structure, achieved by mapping the distribution of millions of galaxies, particularly luminous red galaxies (LRGs). Using these galaxies as "cosmic landmarks," scientists can reconstruct how matter has dispersed over cosmic time.

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

Putting together ACT CMB lensing maps and DESI LRG data is like browsing through a photo album showing the development of an infant to an adult, but for the cosmos.

Browsing this cosmic photo album, the team noticed a small discrepancy. The "clumpiness" of matter the team calculated in later eras of the cosmos doesn't match theoretical predictions.

Though the discrepancy isn't quite large enough to suggest entirely new physics are at play, it does suggest that cosmic structures haven’t quite evolved in the way early-universe models would suggest. The results also hint that the universe’s structural growth may have slowed in ways current models don’t fully explain.

Related Stories:

Robert Lea

Senior Writer

RobertLeais a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.