Tiny distortions in universe’s oldest light reveal clearer picture of strands in cosmic web

In this illustration, the trajectory of cosmic microwave
background (CMB) light is bent by structures known as
filaments that are invisible to our eyes, creating an
effect known as weak lensing captured by the Planck
satellite (left), a space observatory. Researchers used
computers to study this weak lensing of the CMB and produce
a map of filaments, which typically span hundreds of light
years in length. Credit: Siyu He, Shadab Alam, Wei Chen,
and Planck/ESA

Scientists have decoded faint distortions in the patterns of
the universe’s earliest light to map huge tubelike structures
invisible to our eyes – known as filaments – that serve as
superhighways for delivering matter to dense hubs such as
galaxy clusters.


The international science team, which included researchers from
the Department of Energy’s Lawrence Berkeley National
Laboratory (Berkeley Lab) and UC Berkeley, analyzed data from
past sky surveys using sophisticated image-recognition
technology to home in on the gravity-based effects that
identify the shapes of these filaments. They also used models
and theories about the filaments to help guide and interpret
their analysis.

Published April 9 in the journal Nature Astronomy, the
detailed exploration of filaments will help researchers to
better understand the formation and evolution of the cosmic web
– the large-scale structure of matter in the universe –
including the mysterious, unseen stuff known as dark matter
that makes up about 85 percent of the total mass of the
universe.

Dark matter constitutes the filaments – which researchers
learned typically stretch and bend across hundreds of millions
of light years – and the so-called halos that host clusters of
galaxies are fed by the universal network of filaments. More
studies of these filaments could provide new insights about
, another mystery of the universe that
drives its accelerating expansion.

Filament properties could also put gravity theories to the
test, including Einstein’s theory of general relativity, and
lend important clues to help solve an apparent mismatch in the
amount of visible matter predicted to exist in the universe –
the “missing baryon problem.”

“Usually researchers don’t study these filaments directly –
they look at galaxies in observations,” said Shirley Ho, a
senior scientist at Berkeley Lab and Cooper-Siegel associate
professor of physics at Carnegie Mellon University who led the
study. “We used the same methods to find the filaments that
Yahoo and Google use for image recognition, like recognizing
the names of street signs or finding cats in photographs.”

Filament structures in the cosmic web are shown at different
time periods, ranging from when the universe was 12.3 billion
years old (left) to when the universe was 7.4 billion years
old (right). The area in the animation spans 7,500 square
degrees of space. Evidence is strongest for the filament
structures represented in blue. Other likely filament
structures are shaded purple, magenta, and red. Credit:
Yen-Chi Chen and Shirley Ho

The study used data from the Baryon Oscillation Spectroscopic
Survey, or BOSS, an Earth-based sky survey that captured light
from about 1.5 million galaxies to study the universe’s
expansion and the patterned distribution of matter in the
universe set in motion by the propagation of sound waves, or
“baryonic acoustic oscillations,” rippling in the early
universe.

The BOSS survey team, which featured Berkeley Lab scientists in
key roles, produced a catalog of likely filament structures
that connected clusters of matter that researchers drew from in
the latest study.

Researchers also relied on precise, space-based measurements of
the cosmic microwave background, or CMB, which is the nearly
uniform remnant signal from the first light of the universe.
While this light signature is very similar across the universe,
there are regular fluctuations that have been mapped in
previous surveys.

In the latest study, researchers focused on patterned
fluctuations in the CMB. They used sophisticated computer
algorithms to seek out the imprint of filaments from
gravity-based distortions in the CMB, known as weak lensing
effects, that are caused by the CMB light passing through
matter.

Since galaxies live in the densest regions of the universe, the
weak lensing signal from the deflection of CMB light is
strongest from those parts. Dark matter resides in the halos
around those galaxies, and was also known to spread from those
denser areas in filaments.

“We knew that these filaments should also cause a deflection of
CMB and would also produce a measurable weak gravitational
lensing signal,” said Siyu He, the study’s lead author who is a
Ph.D. researcher from Carnegie Mellon University – she is now
at Berkeley Lab and is also affiliated with UC Berkeley. The
research team used statistical techniques to identify and
compare the “ridges,” or points of higher density that theories
informed them would point to the presence of filaments.


Visualizing the cosmic web: This computerized simulation by
the Virgo Consortium, called the Millennium Simulation, shows
a web-like structure in the universe composed of galaxies and
the dark matter around them. Credit: Millennium Simulation
Project

“We were not just trying to ‘connect the dots’ – we were trying
to find these ridges in the density, the local maximum points
in density,” she said. They checked their findings with other
filament and galaxy cluster data, and with “mocks,” or
simulated filaments based on observations and theories. The
team used large cosmological simulations generated at Berkeley
Lab’s National Energy Research Scientific Computing Center
(NERSC), for example, to check for errors in their
measurements.

The filaments and their connections can change shape and
connections over time scales of hundreds of millions of years.
The competing forces of the pull of gravity and the expansion
of the universe can shorten or lengthen the filaments.

“Filaments are this integral part of the cosmic web, though
it’s unclear what is the relationship between the underlying
and the filaments,” and that was a
primary motivation for the study, said Simone Ferraro, one of
the study’s authors who is a Miller postdoctoral fellow at UC
Berkeley’s Center for Cosmological Physics.

New data from existing experiments, and next-generation sky
surveys such as the Berkeley Lab-led Dark Energy Spectroscopic
Instrument (DESI) now under construction at Kitt Peak National
Observatory in Arizona should provide even more detailed data
about these filaments, he added.

Researchers noted that this important step in sleuthing the
shapes and locations of filaments should also be useful for
focused studies that seek to identify what types of gases
inhabit the filaments, the temperatures of these gases, and the
mechanisms for how particles enter and move around in the
filaments. The study also allowed them to determine the length
of filaments.

Siyu He said that resolving the structure can also provide clues to the
properties and contents of the voids in space around the
filaments, and “help with other theories that are modifications
of general relativity,” she said.

Ho added, “We can also maybe use these filaments to constrain
dark energy – their length and width may tell us something
about dark energy’s parameters.”

Explore further:

Researchers capture first ‘image’ of a dark matter web that
connects galaxies

More information: The detection of the imprint of
filaments on cosmic microwave background lensing, Nature
Astronomy
(2018) doi:10.1038/s41550-018-0426-z
, https://arxiv.org/abs/1709.02543

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