Roughly 400,000 years after the Big Bang, the universe cooled just enough to allow photons to escape from the primordial cosmological soup. Over the next 14 billion years, these ancient photons—the universe’s first light—continued traveling. This relic light is known as the Cosmic Microwave Background.
In a new study, scientists used observational data of this first light—collected from the South Pole Telescope located at the National Science Foundation’s Amundsen-Scott South Pole Station in Antarctica—to explore the theoretical underpinnings of the standard cosmological model that describes the history of the universe over the past 14 billion years. The study was conducted by UC Davis researchers and colleagues in the South Pole Telescope collaboration, which is led by the University of Chicago, and has been submitted to the journal Physical Review D.
The study, based on high-precision measurements of the cosmic microwave background and its polarized light, adds further support to the veracity of the standard cosmological model. It also makes a calculation of the Hubble constant—how fast the universe is expanding—with a new method, offering new insight on an ongoing scientific puzzle known as “the Hubble tension”.
“We have a largely coherent, detailed, and successful model describing these 14 billion years of evolution,” said Lloyd Knox, the Michael and Ester Vaida Endowed Chair in Cosmology and Astrophysics at UC Davis and one of the study’s co-authors. “But we don’t know what actually generated the initial departures from complete homogeneity that eventually led to all the structures in the universe including ourselves.”
“This result is especially exciting, because it represents the first competitive constraints on cosmology using only the polarization of the CMB, making it almost 100% independent of previous results that relied mostly on the total intensity,” said study co-author and University of Chicago research professor Tom Crawford.
A polarizing and winding journey across the universe
In the study, the researchers analyzed two years of polarized light data collected by the South Pole Telescope in 2019 and 2020. The study’s observations cover 1,500 square degrees of sky and the collected data enabled the researchers to create a large-scale map of the mass in the universe.
Most natural light is unpolarized, composed of a random collection of light waves, each oscillating (waving up and down) with no preferred direction. But when light is reflected it can become polarized—meaning the light oscillates in a preferred direction. This happens when sunlight reflects off water, or the ground, and is the reason polarized sunglasses can be so helpful for reducing glare. It also happened as the cosmic microwave background photons underwent their final scattering events in the primordial plasma as it began to disappear 14 billion years ago.
“The light from the cosmic microwave background is partially polarized,” Knox said. “We’re measuring at each location in our sky map the degree to which it’s polarized and the orientation of the polarization.”
After that last scattering, the slightly polarized light streamed across open space. Gravitational forces distort the paths of these light rays. Light from different regions is also distorted differently, resulting in a warped image—an effect called gravitational lensing.