The clearest and most precise images yet of the universe in its infancy—the earliest cosmic time accessible to humans—have been produced by an international team of astronomers.

Measuring light, known as the cosmic microwave background (CMB), that traveled for more than 13 billion years to reach a telescope high in the Chilean Andes, the new images reveal the universe when it was about 380,000 years old—the equivalent of hours-old baby pictures of a now middle-aged cosmos.

The research, by the Atacama Cosmology Telescope (ACT) collaboration, shows both the intensity and polarization of the earliest light after the Big Bang with unprecedented clarity, revealing the formation of ancient, consolidating clouds of hydrogen and helium that later developed into the first stars and galaxies.

The team, which includes researchers from Cardiff University, says analyzing the CMB in high definition has enabled them to confirm a simple model of the universe, ruling out many competing alternatives.

They presented their results at the American Physical Society annual meeting on 19 March 2025 and submitted them to the peer review process for publication in the Journal of Cosmology and Astroparticle Physics.

Professor Erminia Calabrese, Director of Research at Cardiff University’s School of Physics and Astronomy and a lead author of one of the studies being presented, said, “These new images allow us to reconstruct with high precision the processes that seeded the complex cosmic structures that we see in the night sky and our own planet too.

“We’ve been able to measure more precisely than ever before that the observable universe extends almost 50 billion light years in all directions from us, and contains as much mass as 1,900 ‘zetta-suns’, or almost 2 trillion trillion suns.

“Of those 1,900 zetta-suns, the mass of normal matter—the kind we can see and measure—makes up only 100. Three quarters of this is hydrogen and a quarter is helium.

“The elements that humans are made of—mostly carbon, with oxygen, nitrogen, iron and even traces of gold—were formed later in stars and are just a sprinkling on top of this cosmic stew.

“Another 500 zetta-suns of mass is in the invisible dark matter of an as-yet unknown nature, and the remaining 1,300 are the dominating vacuum energy or ‘dark energy’ of empty space.”

A major goal of the work was to investigate alternative models for the universe that would explain the disagreement that emerged in recent years about the Hubble constant, the rate at which space is expanding today.

Measurements derived from the CMB have consistently shown an expansion rate of 67–68 kilometers per second per megaparsec (km/s/Mpc), while measurements derived from the movement of nearby galaxies indicate a Hubble constant as high as 73–74 km/s/Mpc.

Using their newly released data, the ACT team confirmed the lower value for the Hubble constant, and with increased precision.

“We scanned many classes of models which could give a higher value of the expansion rate but they were not favored by the new data,” added Professor Calabrese.

The new measurements have also refined the estimate of the age of the universe, finding it to be 13.8 billion years old, with an uncertainty of only 0.1%.

ACT has been a major research focus for the Cardiff University team with its Astronomy Instrumentation Group involved in the optical layout of ACT since the first instrument design back in 2004.

“Our unique filters have enabled the ACT detectors to operate at the sensitivity required to make these tremendous measurements,” said Professor Carole Tucker, Astronomy Instrumentation Group, Cardiff Hub for Astrophysics Research and Technology.

Work led by Professor Calabrese since 2011 has turned the data into information about fundamental properties of the cosmos.

The final data characterization and interpretation presented at the meeting marks the end of four years’ work together with Cardiff post-doctoral researcher Hidde Jense.

“ACT has been my cosmic laboratory during my Ph.D. studies. It has been thrilling to be part of the endeavor leading to this refined understanding of our universe,” said Jense.

ACT completed its observations in 2022, and attention is now turning to the new, more capable, Simons Observatory at the same location in Chile—the next major CMB project for the Cardiff team.

“It is great to see ACT retiring with this display of results,” added Professor Calabrese.

“The circle continues to close around our standard model of cosmology, with these latest results weighing in strongly on what universes are no longer possible.”