Imagine rewinding the clock to the very beginning of the universe. What would you see? Not the glittering galaxies captured by Hubble, but a universe shrouded in a dense, primordial fog. To peer into this ancient past, you’d need to look at the cosmic microwave background (CMB)—the oldest light in existence, a faint afterglow left behind by the Big Bang.
About 380,000 years after the Big Bang, the universe transitioned from an opaque, seething plasma into a transparent expanse, allowing this light to travel freely for the first time. Encoded within it are the fingerprints of the cosmos’ earliest structure—subtle ripples that would eventually form the first atoms and later seed galaxies, stars, and planets.
Now, researchers working with the Atacama Cosmology Telescope (ACT) have released possibly the sharpest pictures yet of this early era. And with it, scientists are piecing together an increasingly precise history of how our universe grew from a fireball into the vast cosmic structure we see today.
Newborn universe
In the first several hundred thousand years after the Big Bang, the universe was essentially opaque. It was filled with a primordial plasma so hot that light just couldn’t propagate freely. So you can’t “see” the universe at that time because there was no light. You can only use CMB to study these early stages. Effectively, the universe’s baby picture is a CMB map.
For decades, satellite missions like COBE, WMAP, and Planck have mapped this ancient radiation. But ground-based experiments like the ACT have a different strength: higher resolution at smaller scales.
“We are seeing the first steps towards making the earliest stars and galaxies,” says Suzanne Staggs, director of ACT and Henry deWolf Smyth Professor of Physics at Princeton University. “And we’re not just seeing light and dark, we’re seeing the polarization of light in high resolution. That is a defining factor distinguishing ACT from Planck and other, earlier telescopes.”
Unlike Planck, which surveyed the entire sky, ACT focuses on smaller patches with much finer detail. However, the ACT has five times better resolution than Planck, making even faint signals visible.
“Before, we got to see where things were, and now we also see how they’re moving,” says Staggs. “Like using tides to infer the presence of the moon, the movement tracked by the light’s polarization tells us how strong the pull of gravity was in different parts of space.”
What this map tells us
This early map gives a remarkably clear view of the density and velocity of the gases that filled the young universe. This is so precise it can even be used to finesse our models of how the universe evolved. The new results confirm a simpler model called the ΛCDM (Lambda Cold Dark Matter) model and rule out the majority of competing models.
The ΛCDM model says that the cosmos is made up of normal matter (stars, planets, and gas), dark matter (an invisible substance that shapes galaxies), and dark energy (a mysterious force causing the universe to expand faster over time). The “Λ” (Lambda) represents dark energy, while “CDM” stands for cold dark matter, which moves slowly compared to the speed of light.
According to this model, the universe began with the Big Bang, expanded rapidly, and over billions of years, gravity pulled matter together to form galaxies, stars, and planets. Despite its success in predicting how the universe came to be, some mysteries remain. For starters, we have no idea what dark matter and dark energy really are. We can see their effects, but we can’t see them or understand their nature.
Then, there’s the “Hubble tension”. This refers to the rate at which the universe expands, a value called the Hubble constant. This value seems to differ based on how it is measured… and it shouldn’t.
Measure the Hubble constant using the movement of nearby galaxies, and you end up with a value of 73 to 74 kilometers per second per megaparsec (km/s/Mpc). That is, the universe expands with 73-74 kilometers every second per megaparsec (3.26 milllion light years). But if you measure it with the CMB, you end up with a value of 67-68 km/s/Mpc.
This 5 km/s/Mpc discrepancy—small yet statistically significant—has fueled speculation that new physics might be at play. Some theorists propose exotic solutions: new neutrino species, early dark energy, or modifications to gravity. This new map confirms previous CMB measurements, but the jury is still out on what causes this discrepancy.
“It was slightly surprising to us that we didn’t find even partial evidence to support the higher value,” says Staggs. “There were a few areas where we thought we might see evidence for explanations of the tension, and they just weren’t there in the data.”
Weighing the universe
You can’t take a baby picture without weighing and measuring the newborn; this is also what researchers did here.
By analyzing how the CMB’s light is subtly bent by massive structures, they calculated the total amount of baryonic (normal) matter, cold dark matter, and dark energy in the cosmos.
Yet again, their results confirm the ΛCDM model’s composition. According to their results, the universe is made out of:
- about 5% normal matter; this is everything we would normally call “matter,” all the stars and the planets and everything in between.
- 27% dark matter; an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation.
- 68% dark energy; an invisible and hypothetical form of matter that makes the universe expand faster.
“We’ve measured more precisely 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,” says Erminia Calabrese, professor of astrophysics at the University of Cardiff and a lead author on one of the new papers. Of those 1,900 zetta-suns, the mass of normal matter—the kind we can see and measure—makes up only 100. Another 500 zetta-Suns of mass are mysterious dark matter, and the equivalent of 1,300 are the dominating vacuum energy (also called dark energy) of empty space.
Out of this “normal” matter, three-quarters of the mass is hydrogen and almost a quarter is helium. Everything else (the oxygen, carbon that makes you and me, and all the other elements) makes up only around 2% of this matter.
“Almost all of the helium in the universe was produced in the first three minutes of cosmic time,” says Thibaut Louis, CNRS researcher at IJCLab, University Paris-Saclay and one of the lead authors of the new papers. “Our new measurements of its abundance agree very well with theoretical models and with observations in galaxies.”
The new data also confirm that the age of the universe is 13.8 billion years, with an uncertainty of only 0.1%.
Did something strange happen before the CMB form?
The CMB tells us a lot about the young universe. After 380,000 years after the Big Bang, light started propagating through the universe and we have a better chance of observing what happened. But for everything that happened in between the Big Bang and the 380,000 years, the universe was essentially unobservable, hidden behind the dense plasma.
Some researchers suspect that in that very early period, some “strange physics” may have happened.
One hypothesis involves “early dark energy”, a mysterious force that could have temporarily boosted cosmic expansion before the CMB formed. If true, this could help explain the ongoing Hubble constant tension—why the early universe seems to predict a slower expansion rate than modern measurements suggest.
“A younger universe would have had to expand more quickly to reach its current size, and the images we measure would appear to be reaching us from closer by,” explains Mark Devlin, the Reese W. Flower Professor of Astronomy at the University of Pennsylvania, and ACT’s deputy director. “The apparent extent of ripples in the images would be larger in that case, in the same way that a ruler held closer to your face appears larger than one held at arm’s length.”
For now, we just don’t have the answers. The Atacama Cosmology Telescope has provided one of the clearest images of the early universe yet, but it’s not the final word. Next-generation experiments—including the James Webb Space Telescope (JWST), Simons Observatory, and future CMB missions—will push even further. Scientists hope to detect signals from even epochs, ultimately answering one of the biggest questions in cosmology: What really happened in the first seconds of the universe?
Then and only then, once we’ve answered that question, can we truly claim to understand the universe.
The results were published in three papers:
- The CMB maps: Næss, Guan, Duivenvoorden, Hasselfield, Wang et al, 2025.
- The CMB power spectra and fitting to LCDM: Louis, La Posta, Atkins, Jense et al, 2025 .
- Constraints on extensions to LCDM: Calabrese, Hill, Jense, La Posta et al, 2025.
