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Wednesday, February 19, 2025

Polarized CMB light: A new look into the origins of our universe

UC Davis researchers analyze the polarized light data from the South Pole Telescope, corroborating the standard cosmological model

 

By EKATERINA MEDVEDEVA — science@theaggie.org

 

A long time ago in a galaxy far, far away… Actually, if we look far enough into space, we will not find any galaxies. The light from those places was traveling to Earth for such a long time — meaning that its photons were emitted so long ago — that we have a chance to see what the universe looked like before the formation of galaxies.

The remnants of the first light that was ever emitted when the universe cooled down enough to allow photons to travel freely through space, about 380,000 years after the Big Bang, are known as the Cosmic Microwave Background (CMB). Studying this relic light can give us the key to understanding how the universe came to be.

There are three main sources of data for analysis of CMB: the South Pole Telescope (SPT) in Antarctica, the Atacama Cosmology Telescope (ACT) in Chile and Planck mission in space that was operated by the European Space Agency (ESA) from 2009 to 2013.

The frontier of CMB research for the past few years has been the analysis of its polarized portion of the light. Most light that we encounter in our daily lives is unpolarized. This is when the electric field component associated with a light wave oscillates in various directions perpendicular to its direction of travel. When the light is polarized, however, the electric field component oscillates in a single preferred direction.

In a recent study, a team of UC Davis researchers in collaboration with their colleagues from the South Pole Telescope analyzed this polarization data from the SPT, collected in 2019 to 2020, in an innovatory way to test certain aspects of the standard cosmological model (also known as ΛCDM), which describes how the universe evolved.

“This paper is a combination of really good data and a new state-of-the-art method for analysing it,” Marius Millea, one of the co-authors of the study, said. “The main way in which this analysis is different from previous studies is that it is done using the so-called Bayesian technique. A lot of the older methods pick out only some part of the data to analyze and this method is really able to consider the full data set — the entire [CMB polarization] map that we make and extract all of the information that’s in there.”

The computing for this study was done on a supercluster of Graphics Processing Unit (GPU) nodes called the National Energy Research Scientific Computing (NERSC) center located in Berkeley, California. Using the SPT data, which encompasses 1500 square degrees of the sky, researchers were able to derive the most precise measurements ever from the CMB polarization data, as opposed to a combination of polarization data and other measurements.

One of the most important measurements is the Hubble constant, which is the rate of the expansion of the universe. Currently, there is disagreement about its value that is known as the Hubble tension. The conflict stems from the fact that its value measured locally (by looking at supernovae) significantly differs from the value measured via studying the CMB. The Hubble constant estimates range from 67.4 km/sec/Mpc to 74.0 km/sec/Mpc, according to a NASA article published in 2019. The UC Davis team’s precise measurements, yielding 66.81 ± 0.81 km/s/Mpc, thus reinforce the discrepancy.

“Before our measurements, if you ask, well, ‘What is the expansion rate if you only use the polarization of the CMB?’, the air bars were too large and it could have actually agreed with the value that you get from these local measurements and sort of saying everything is alright,” Millea said. “But we just said no. What we actually find is something totally in line with the whole host of other measurements [of CMB] that disagrees with the local measurement.”

The results of this study reaffirm the standard cosmological model and match the predictions from other major CMB data studies.

“The ΛCDM model provides a good simultaneous fit to the combined Planck, ACT and SPT data, and thus passes a powerful test,” the study reads.

This paper is the first out of a series of the team’s studies that analyze the SPT data, offering exciting prospects in the near future.

“The telescope has been taking data ever since 2020, so we have more than double the amount of data that we used for this analysis, and we have the temperature which we didn’t use in this analysis at all,” Millea said. “This analysis [already] produced the tightest constraints on various things. In the coming analyses we’ll better measure the Hubble constant, the amount of gravitational lensing, the matter density in the universe — so the future is pretty bright.”

 

Written by: Ekaterina Medvedeva — science@theaggie.org

 

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