Mapping Dark Matter and Dark Energy with a cosmic filter

The cosmic microwave background — light left over from immediately after the big bang — could be used to map out the structure of the universe, thus uncovering the secrets of dark matter and dark energy.

The patterning of the earliest known light in our universe — the cosmic microwave background (CMB)— holds many important clues to the development and distribution of large-scale structures such as galaxies and galaxy clusters.

Distortions in the CMB — emitted 380,000 years after the big bang — caused by a phenomenon known as lensing, can reveal the fine structure of the universe. This also means it can potentially tell us things about the mysterious, unseen ‘dark universe’ — dark energy, which makes up about 68% of the universe and accounts for its accelerating expansion, and dark matter — which accounts for about 27% of the universe.

The Universe from the bottom of a swimming pool

Imagine the Universe as a grid pattern printed on the bottom of a swimming pool. The gravitational effects of matter and energy are added in much like water filling the pool. We view the bottom through the water — stretched and squeezed by disturbances in the surface.

Gravitational effects from large objects like galaxies and galaxy clusters bend the CMB light in different ways. These lensing effects can be subtle — weak lensing — for distant and small galaxies, and computer programs can identify them because they disrupt the regular CMB patterning.

R.Lambourne (2012)

There are some known issues with the accuracy of lensing measurements, though — particularly with temperature-based measurements of the CMB and associated lensing effects.

While lensing can be a powerful tool for studying the invisible universe, and could even potentially help us sort out the properties of ghostly subatomic particles like neutrinos, the universe is an inherently messy place.

The gas and dust swirling in other galaxies, among other factors, can obscure our view and lead to faulty readings of the CMB lensing.

A set of cosmic microwave background images with no lensing effects (top row) and with exaggerated cosmic microwave background lensing effects (bottom row). (Wayne Hu and Takemi Okamoto/University of Chicago)

Though there are some filtering tools that help researchers to limit or mask some of these effects — these known obstructions continue to be a major problem in the many studies that rely on temperature-based measurements.

The effects of this interference with temperature-based CMB studies can lead to erroneous lensing measurements, says Emmanuel Schaan, a postdoctoral researcher and Owen Chamberlain Postdoctoral Fellow in the Physics Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

He says: “You can be wrong and not know it. The existing methods don’t work perfectly — they are really limiting.”

To address this problem, Schaan teamed up with Simone Ferraro, a Divisional Fellow in Berkeley Lab’s Physics Division, to develop a way to improve the clarity and accuracy of CMB lensing measurements by separately accounting for different types of lensing effects.

Schaan adds: “Lensing can magnify or demagnify things. It also distorts them along a certain axis so they are stretched in one direction.”

The researchers compare this to looking at the surface of a table through the stem of a wine glass.

What the team found, is that a certain lensing signature — shearing — which causes this stretching in one direction, seems largely immune to the foreground “noise” effects that otherwise interfere with the CMB lensing data.

These images show different types of emissions that can interfere with CMB lensing measurements, as simulated by Neelima Sehgal and collaborators. From left to right: The cosmic infrared background, composed of intergalactic dust; radio point sources, or radio emission from other galaxies; the kinematic Sunyaev-Zel’dovich effect, a product of gas in other galaxies; and the thermal Sunyaev-Zel’dovich effect, which also relates to gas in other galaxies. (Emmanuel Schaan and Simone Ferraro/Berkeley Lab)

The lensing effect known as magnification, meanwhile, is prone to errors introduced by foreground noise. Their study, published in the journal Physical Review Letters, notes a “dramatic reduction” in this error margin when focusing solely on shearing effects.

The sources of the lensing, which are large objects that stand between us and the CMB light, are typically galaxy groups and clusters that have a roughly spherical profile in temperature maps, Ferraro notes, and the latest study found that the emission of various forms of light from these “foreground” objects only appears to mimic the magnification effects in lensing but not the shear effects.

Ferrano says: “We said, ‘Let’s rely only on the shear and we’ll be immune to foreground effects’.

“When you have many of these galaxies that are mostly spherical, and you average them, they only contaminate the magnification part of the measurement. For shear — all of the errors are basically gone.”

He continues: “It reduces the noise, allowing us to get better maps. And we’re more certain that these maps are correct. Even when the measurements involve very distant galaxies as foreground lensing objects.”

Benefits to a range of experiments

The study notes that the new method could benefit a range of sky-surveying experiments — including the POLARBEAR-2 and Simons Array experiments, which have Berkeley Lab and UC Berkeley participants; the Advanced Atacama Cosmology Telescope (AdvACT) project; and the South Pole Telescope — 3G camera (SPT-3G). It could also aid the Simons Observatory and the proposed next-generation, multilocation CMB experiment known as CMB-S4 — Berkeley Lab scientists are involved in the planning for both of these efforts.

The method could also enhance the data gained from future galactic surveys like the Berkeley Lab-led Dark Energy Spectroscopic Instrument (DESI) project — currently under construction near Tucson, Arizona, and the Large Synoptic Survey Telescope (LSST) project under construction in Chile, through joint analyses of data from these sky surveys and the CMB lensing data.

Increasingly large datasets from astrophysics experiments have led to more coordination in comparing data across experiments to provide more meaningful results. As Ferrano points out: “These days, the synergies between CMB and galaxy surveys are a big deal.”

The world’s leading supercomputing centre for open science enables researchers to perform simulations of quantum computers (Berkeley)

In this study, researchers relied on simulated full-sky CMB data — using resources at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) to test their method on each of the four different foreground sources of noise. This includes infrared, radiofrequency, thermal, and electron-interaction effects that can contaminate CMB lensing measurements.

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

Schaan adds: “We couldn’t have done this project without a computing cluster like NERSC.”

NERSC has also proved useful in serving up other universe simulations to help prepare for upcoming experiments like DESI.

The method developed by Schaan and Ferraro is already being implemented in the analysis of current experiments’ data. One possible application is to develop more detailed visualizations of dark matter filaments and nodes that appear to connect matter in the universe via a complex and changing cosmic web.

The researchers reported a positive reception to their newly introduced method.

Ferrano concludes: “This was an outstanding problem that many people had thought about.

“We’re happy to find elegant solutions.”

Original research: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.181301