One of the Holy Grails in cosmology is a look back at the earliest epochs of cosmic history. Unfortunately, the Universe’s first few hundred thousand years are shrouded in an impenetrable fog. So far, nobody’s been able to see past it to the Big Bang. As it turns out, astronomers are chipping away at that cosmic fog by using data from the Atacama Cosmology Telescope (ACT) in Chile.
ACT measured light first emitted in the baby Universe some 380,000 years after the Big Bang. According to the Consortium director Suzanne Staggs, that measurement opened the window to a time when the first cosmic structures were starting to assemble. “We are seeing the first steps towards making the earliest stars and galaxies,” she said. “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.”
The clearer data and images from ACT are also helping scientists understand just when and where the first galaxies began to form. If the ACT data are confirmed, they represent the earliest baby picture of the Universe, showing scientists what the seeds of galaxies looked like only a few hundred thousand years after the Big Bang.
How ACT Provided a Cosmic Baby Picture
Stagg and others in the ACT Collaboration focused on very subtle variations in the density and velocity of gases in the very young Universe. According to ACT deputy director Mark Devlin, it was a lengthy process. “To make this new measurement, we needed a 5-year exposure with a sensitive telescope tuned to see millimeter-wavelength light,” he said, pointing out that the observations required highly sensitive detectors and computer support.
The collaboration measured the polarization of light from the Cosmic Microwave Background (CMB). That’s the faint microwave glow that fills space. It’s the oldest light in the Universe and represents an epoch when light was first able to move freely through the expanding infant Universe. Before that time, space was filled with a so-called “primordial plasma”. It was too hot to allow light to propagate. So, essentially, everything and every place was dark. The CMB is the faint glow of light that was finally able to travel freely. It shows tiny temperature fluctuations in different regions which indicates variations in the density of gas and how it moved through space. Think of those variations as the “seeds” of future stars and galaxies.
A small part of the light from the CMB got polarized when it interacted with the earliest “density structures” in the infant Universe. Essentially, it vibrates in a different direction than the rest of the light. Light waves vibrate in all directions, but they can be shifted into a very specific direction when they hit a surface. Here on Earth, the easiest way to understand this is to put on a pair of polarized sunglasses. They block horizontally polarized light waves that bounce off surfaces such as water. In space, when a light wave hits a cloud of gas, that polarizes it and changes its vibration direction. The polarization can reveal information about the object that redirected the light wave. In this case, it was caused when the earliest light bounced off of the density structures that existed back then.
The Atacama Cosmology Telescope in Chile (now shut down) was used to observe the Cosmic Microwave Background radiation in great detail. The data helped astronomers redefine the age of the Universe and understand the structure that dominated some 380,000 years after the Big Bang. Credit: Debra Kellner
Digging into Polarized Light from the CMB
ACT is not the first telescope to study this long-ago era of cosmic history. The Planck satellite, for example, also measured the faint light of the CMB. ACT did one better, according to team member Sigurd Naess. “ACT has five times the resolution of Planck and greater sensitivity,” said Naess, a researcher at the University of Oslo and a lead author of one of several papers related to the project. “This means the faint polarization signal is now directly visible.”
The polarization images obtained by ACT reveal the detailed movement of the hydrogen and helium gas in the early Universe. “Before, we got to see where things were, and now we also see how they’re moving,” said 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.”
ACT images of polarized light from the CMB show very subtle variations in the density and velocity of the gases that filled the young Universe. What look like hazy clouds in the light’s intensity are more and less dense regions in a sea of hydrogen and helium. Those regions extended across millions of light-years. Eventually, gravity pulled the denser areas together to form stars and galaxies. Their detailed appearance at such an early epoch of cosmic time helps scientists answer some tricky questions about the birth of the Universe. “By looking back to that time when things were much simpler, we can piece together the story of how our universe evolved to the rich and complex place we find ourselves in today, ” says Jo Dunkley, the Joseph Henry Professor of Physics and Astrophysical Sciences at Princeton University and the ACT analysis leader.
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The colored band in this illustration shows the time period in the history of the universe that the new ACT images capture. Diagram by Lucy Reading Ikkanda, Simons Foundation
Revealing More
The data from ACT also contains information about other objects in space, including the Milky Way, other galaxies, and galaxy clusters. In a sense, it’s tracing the evolution of the Universe from its infancy to modern times. But, that data also points toward something else, according to Erminia Calabrese, a lead author one of several papers about the ACT observations.
“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,” said Calabrese. “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.”
ACT’s new data also helped scientists refine the age of the Universe to a much more precise limit of 13.8 billion years. They also can help scientists understand more about how fast it’s growing in modern times. These new measurements will help scientists as they prepare to transition to the new Simons Observatory in Chile. Like ACT, it too, will be focused on studies of the CMB and will observe large swathes of the sky at multiple frequencies.
For More Information
The Atacama Cosmology Telescope: DR6 Constraints on Extended Cosmological Models
New High-definition Pictures of the Baby Universe