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

A powerful combination of data from two very different astronomical surveys has allowed researchers to build a "cosmic CT scan" of the universe's evolution.

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.

"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," team co-leader Mathew Madhavacheril of the University of Pennsylvania said in a statement. "It gives us a direct look into how the gravitational influence of matter changed over billions of years."

Following the story of ancient cosmic light

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.

"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," team co-leader paper Joshua Kim, a graduate researcher in the Madhavacheril Group, said in the statement. "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 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."

Though often described as a "cosmic fossil," the CMB hasn't remained entirely unchanged for billions of years. The expansion of the universe has caused its photons to shift to longer wavelengths and lose energy. Its temperature is now uniform at minus 454 degrees Fahrenheit (minus 270 degrees Celsius).

Because mass warps the fabric of spacetime, giving rise to gravity, light from the CMB has warped while traveling past large, dense and heavy structures such as galaxy clusters. This is akin to looking at a grid pattern at the bottom of an empty swimming pool and noting the distortion caused as water is added.

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.

Where is the universe's clumpiness?

While the ACT data captures a snapshot of the CMB in its cosmic baby pictures, DESI provides scientists with a more recent record of a "grown-up" universe.

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.

"What we found was that, for the most part, the story of structure formation is remarkably consistent with the predictions from Einstein's gravity," Madhavacheril said. "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."

The researchers behind this work intend to continue this line of inquiry, but while utilizing more powerful upcoming telescopes, which should provide them with more precise measurements.

The team's research was published on Dec. 10, 2024, in the Journal of Cosmology and Astroparticle Physics.

Originally posted on Space.com.