MAY 8, 2019
by Glenn Roberts Jr., Lawrence Berkeley National
Laboratory
The earliest known light in
our universe, known as the cosmic microwave background, was emitted about
380,000 years after the Big Bang. The patterning of this relic light holds many
important clues to the development and distribution of large-scale structures
such as galaxies and galaxy clusters.
Distortions in the cosmic microwave
background (CMB), caused by a phenomenon known as lensing, can further
illuminate the structure of the universe and can even tell us things about the
mysterious, unseen universe—including dark energy, which makes up about
68 percent of the universe and accounts for its accelerating expansion, and
dark matter, which accounts for about 27 percent of the universe.
Set a stemmed wine glass on a
surface, and you can see how lensing effects can simultaneously magnify,
squeeze, and stretch the view of the surface beneath it. In lensing of the CMB,
gravity effects from large objects like galaxies and galaxy clusters bend the
CMB light in different ways. These lensing effects can be subtle (known as weak
lensing) for distant and small galaxies, and computer programs can identify
them because they disrupt the regular CMB patterning.
There are some known issues
with the accuracy of lensing measurements, though, and 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.
And like bugs on a car's
windshield during a long drive, the gas and dust swirling in other galaxies,
among other factors, can obscure our view and lead to faulty readings of the
CMB lensing.
There are some filtering tools
that help researchers to limit or mask some of these effects, but 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, said 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).
"You can be wrong and not
know it," Schaan said. "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.
"Lensing can magnify or
demagnify things. It also distorts them along a certain axis so they are
stretched in one direction," Schaan said.
The researchers found that a
certain lensing signature called shearing, which causes this stretching in one
direction, seems largely immune to the foreground "noise" effects
that otherwise interfere with the CMB lensing data. The lensing effect known as
magnification, meanwhile, is prone to errors introduced by foreground noise.
Their study, published May 8 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 noted, 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.
"So we said, 'Let's rely
only on the shear and we'll be immune to foreground effects,'" Ferraro
said. "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 added, "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.
The new method could benefit a
range of sky-surveying experiments, the study notes, 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 science yield from future galaxy surveys like the Berkeley Lab-led Dark
Energy Spectroscopic Instrument (DESI) project 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. "These days, the
synergies between CMB and galaxy surveys are a big deal," Ferraro said.
In this study, researchers
relied on simulated full-sky CMB data. They used 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, which include
infrared, radiofrequency, thermal, and electron-interaction effects that can
contaminate CMB lensingmeasurements.
The study notes that cosmic
infrared background noise, and noise from the interaction of CMB light
particles (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.
"We couldn't have done
this project without a computing cluster like NERSC," Schaan said. 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.
"This was an outstanding
problem that many people had thought about," Ferraro said. "We're
happy to find elegant solutions."
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