A new filter to better map the dark universe

The oldest known light in our universe, known as the Cosmic Microwave Background, was released about 380,000 years after the Big Bang. The structure of this relic light contains many important clues for the development and distribution of large scale structures such as galaxies and clusters of galaxies.

The distortions in the Cosmic Microwave Background (CMB), caused by a phenomenon called lens, can further illuminate the structure of the universe and can even tell us things about the mysterious and invisible universe, including l & # 39; Dark energy, which accounts for about 68% of the universe and explains its accelerated expansion, and dark matter, which represents about 27% of the universe.

Place a walking wine glass on a surface and you will see how the lens effects can simultaneously enlarge, pinch and stretch the view of the surface underneath. In the CMB lens, the effects of gravity from large objects such as galaxies and galaxy clusters curl CMB light in different ways. These lens effects can be subtle (known as weak lenses) for distant and small galaxies, and computer programs can identify them as they disrupt the regular structure of CMB.

There are, however, known problems with the accuracy of lens measurements, and particularly with temperature-based CMB measurements and associated lens effects.

The lens can be a powerful tool for studying the invisible universe, and could even potentially help us unravel the properties of ghostly subatomic particles like neutrinos, but the universe is an inherently messy place.

And like insects on the windshield of a car during a long drive, the gas and dust that swirl in other galaxies can, among other factors, obscure our vision and lead to erroneous readings of the CMB lens.

Some filtering tools help researchers limit or obscure some of these effects, but these known obstructions continue to be a major problem in many studies based on temperature-based measurements.

The effects of this interference with temperature-based CMB studies can lead to erroneous lens measurements, said Emmanuel Schaan, Postdoctoral Researcher and Postdoctoral Researcher Owen Chamberlain of the Physics Division of the Lawrence Berkeley National Laboratory (Berkeley Lab) Department. of energy.

"You can go wrong and not know it," Schaan said. "The existing methods do not work perfectly, they are really limiting."

To solve this problem, Schaan collaborated with Simone Ferraro, a member of the Berkeley Lab Physics Division, to develop a way to improve the clarity and accuracy of CMB lens measurements by separately considering different types of lenses. # 39; lens effects.

"Lenses can magnify or demagnify objects, they also deform them along a certain axis to stretch them in one direction," Schaan said.

The researchers discovered that a certain lens signature called shear, which causes this stretch in one direction, appears largely immune to the "noise" effects of foreground that otherwise interfere with CMB lens data. . The lens effect known as magnification, meanwhile, is subject to the errors introduced by the foreground noise. Their study, published May 8 in the journal Letters of physical examinationnote a "dramatic reduction" of this margin of error when one focuses only on shear effects.

The sources of the lens, which are large objects that separate us from CMB light, are usually groups of galaxies and clusters that have an approximately spherical profile on temperature maps, noted Ferraro. The latest study has shown that the emission of various forms of light from these objects "in the foreground" only seems to mimic magnification effects in the lens, but not the shear effects.

"So we said:" Let's just rely on shearing and we'll be immune to the leading effects, "Ferraro said. "When you have a lot of generally spherical galaxies and you calculate them on average, they only contaminate the enlarged part of the measurement, and for shear, all the errors are almost gone."

He added, "This reduces the noise, which allows us to get better maps, and we are safer that these maps are correct," even when the measurements involve galaxies far apart as a result. lenses in the foreground.

The new method could be useful for a variety of sky-monitoring experiments, note study notes, including the POLARBEAR-2 and Simons Array experiments, in which participants from Berkeley Lab and UC Berkeley participate; the Advanced Atacama Cosmology Telescope project (AdvACT); and the South Pole Telescope – 3G Camera (SPT-3G). This could also assist the Simons Observatory and the proposed new multi-site CMB experiment, named CMB-S4. Berkeley Lab scientists are involved in planning these two efforts.

This method could also improve the scientific performance of future galaxy surveys, such as the Black Energy Spectroscopic Instrument (DESI) project led by the Berkeley Laboratory under construction near Tucson, Arizona, and the large telescope project. Synoptic (LSST) under construction in Chile analyzes data from these sky surveys and CMB lens data.

Increasingly large sets of data from astrophysical experiments have led to greater coordination in the comparison of data between experiments to provide more meaningful results. "These days, the synergies between CMB and galactic surveys are very important," said Ferraro.

In this study, researchers used simulated CMB full-sky data. They used the resources of the Berkeley Lab National Center for Computer Science (NERSC) to test their method on each of the four leading noise sources, including infrared, radio frequency, thermal, and electron interactions that could contaminate CMB lens measurements.

The study notes that background noise in the cosmic infrared and noise from the interaction of CMB light particles (photons) with high energy electrons have been the most troublesome sources of problematic to solve using standard filtering tools in CMB measurements. Some existing and future CMB experiments seek to mitigate these effects by taking precise measurements of the polarization, or orientation, of the CMB's light signature rather than its temperature.

"We could not have done this project without a computer cluster like NERSC," Schaan said. NERSC has also proved useful for serving other simulations of the universe in order to prepare for future experiments, such as DESI.

The method developed by Schaan and Ferraro is already used in the analysis of data from current experiments. One of the possible applications is to develop more detailed visualisations of the filaments and nodes of dark matter that seem to connect matter in the universe via a complex and changing cosmic network.

The researchers reported a positive reception of their new method.

"It was a thorny problem that many people had thought of," said Ferraro. "We are happy to find elegant solutions."