Giant filaments crisscross the universe, connecting galaxy clusters like superhighways between cities. Their shape and structure contain vital information about the universe’s contents and history. However, their complex shape makes them extremely difficult to measure. Now, researchers have presented a new method for measuring the mass of these filaments, which could be used to unlock the secrets of dark matter and dark energy.
At its largest scales, our Universe resembles a web. It’s fitting to call it the cosmic web, and it’s the largest structure found in nature. Within the cosmic web are small, compact clusters of galaxies, known as galaxy clusters. Between these clusters are voids—vast spaces containing virtually nothing. Threads connect the clusters.
Each filament forms the core of a supercluster and can extend for tens of millions of light years and contain up to hundreds of thousands of galaxies.
The detailed shape of the cosmic web and its complex structures have a long history. The cosmic web emerged more than 13 billion years ago at the dawn of the universe, when stars and galaxies first illuminated space. The modern structure of the cosmic web tells us about this evolution and is closely linked to the properties of ordinary matter, dark matter, and dark energy. A change in any of these components creates a new pattern in the cosmic web.
Dark matter filaments. ESA/Springel et al., Virgo Consortium
For decades, astronomers have used simple statistics about the cosmic web, such as the average distance between galaxies, to gain insight into these fundamental cosmological components. But because we still don’t fully understand the nature of dark matter and dark energy, cosmologists are eager to learn more about the details of the cosmic web.
The filaments represent a key to understanding the nature of dark matter and dark energy. Unlike galaxy clusters, the filaments have not yet completely collapsed. However, they still contain much more material than empty space. Therefore, changing the amount of dark matter and dark energy could alter the average length, width, and density of the filaments.
But weighing the filament is currently very difficult. This is because most of the filament’s mass is in the form of dark matter, invisible to our observations. We can only map visible galaxies and use that to determine the filament’s true mass and structure.
Recently, in a paper accepted for publication in The Astrophysical Journal, researchers proposed a method for doing just that. They calibrated their methods against cosmological models that track the evolution of dark matter, stars, and galaxies. The researchers discovered a useful relationship between a quantity called “galaxy redshift dispersion” and the mass of dark matter.
On average, galaxies are moving away from us in the expanding universe. However, galaxies can experience additional motion due to their local gravitational environment. For example, if we look at a filament, some galaxies may be moving slightly in our direction, while others are moving slightly away. Galaxies moving toward us will have a slight blue shift in their wavelengths, while galaxies moving away from us will experience a slight red shift.
The researchers took sections of each filament and constructed a map of the distribution of these redshifts to obtain an idea of the spread of galaxy velocities. They then related this spread of galaxy velocities to the mass of dark matter within that section of the filament. They found a close relationship between these two quantities, allowing them to reconstruct the mass of hidden dark matter within the filament using only the observed properties of the galaxies.
However, this is only the first step. The researchers will continue to study the relationship between galaxy velocities and dark matter mass to develop an efficient and reliable method for estimating the mass. Then, using simulations, they will be able to see how these quantities change under various cosmological scenarios. The final step will be to apply this methodology to real galaxy surveys to discover what the filaments can tell us about the hidden corners of the universe.
In early 2024, astronomers detected dark matter for the first time, hanging from massive filaments that stretch throughout the universe and form a “cosmic web” that envelops galaxies like a spider web enveloping morning dew.
Researchers from Yonsei University in Seoul, South Korea, used the Subaru Telescope—an 8.2-meter optical-infrared telescope located near the summit of Maunakea in Hawaii—and the effect of gravity on light to indirectly observe dark matter embedded in the filaments of the cosmic web in the Coma cluster. The team’s findings were published in January 2024 in the journal Nature Astronomy.
This is the first-ever detection of dark matter in the cosmic web and could help confirm how this structure – with threads stretching for tens of millions of light-years – influenced the evolution of the Universe.
The Coma Cluster, also known as Abell 1656, is a collection of over a thousand galaxies located approximately 321 million light-years away in the constellation Coma Berenices. Due to its colossal size and relative proximity, the Coma Cluster is an ideal location for scientists searching for dark matter within the strands of the cosmic web.
The diagram shows how light from a background source is bent by its mass, creating an effect called gravitational lensing. NASA, ESA, and L. Calzada
The cosmic web is a network of filaments composed of matter that feeds galaxies with gas, fueling their growth. This web also helps bind galaxies together, leading to galaxy clusters.
The main strands of the cosmic web are themselves the walls of galaxy superclusters, with the wall corresponding to the Coma cluster known as the “Great Wall.” The Great Wall was, in fact, the first super-large structure discovered in the Universe.
Galaxy clusters are thought to congregate at the intersections of the filaments, but these filaments are believed to terminate between galaxies, forming so-called “intrascopic filaments.” Dark matter is thought to move along these filaments of the cosmic web, hanging from these intrascopic filaments.
Although the cosmic web, the largest structure in the Universe, has been known for decades, astronomers have only seen the faint glow of its gaseous filaments when they are illuminated by bright regions at the centers of galaxies powered by supermassive black holes. These active black holes are called quasars.
In 2023, the Keck Cosmic Web Imager, also atop Maunakea, captured for the first time direct light emanating from the web’s fine filaments, intersecting and stretching through the darkest reaches of space. These filaments are located in isolated locations between galaxies, in the largest and most hidden parts of the cosmic web.
However, “seeing” the location of dark matter around these strands of the cosmic web is a different story entirely.
A computer model of galaxies surrounded by filaments of gas that make up the cosmic web. Yannick Bache
This is because, despite dark matter making up approximately 85% of all matter in the Universe, it is invisible because it does not interact with light the way ordinary matter, which is what makes up stars and dust, does.
Dark matter’s dominance over ordinary matter also means that it dominates the threads of the cosmic web, forming the invisible scaffolds along which the structure of the universe is formed.
However, although dark matter does not interact with light, it does interact with gravity, and this interaction affects the motion of everyday matter and light that we can see.
A team of researchers used this concept to detect dark matter in the filaments of the cosmic web that thread through the Coma cluster.
See also... New method for observing individual atoms confirms century-old quantum mechanical theory |
Albert Einstein’s 1915 theory of gravity, known as general relativity, proposes that objects with mass cause a curvature of the fabric of spacetime. The theory, in turn, explains that what we perceive as gravity arises from this curvature. Furthermore, when light from a background source passes through this curvature, its trajectory is altered.
This can cause background sources to appear displaced in the sky, amplified, or, in some extreme cases, even appear at multiple points in the same image. This is called gravitational lensing.
Gravitational lensing essentially uses gravity as a magnifying glass. First, you place a large gravitational well (such as dark matter) you want to study between you and a light source (such as a star or galaxy). Then, you observe how light travels toward you from this source. The light refracts and twists around the gravitational well, and by this behavior, you can infer information about the object at its center, such as its mass.
And if it really seems that there is nothing between you and the light source, but this light is refracted by something, then you are facing some kind of dark matter.
This is precisely what scientists discovered while studying the Coma cluster—a supercluster of galaxies 321 million light-years away where astronomers predicted several strands of the cosmic web would converge, and where, as a result, abundant filaments of dark matter would be found. When the scientists pointed a telescope in this direction, they saw precisely the gravitational lensing signals they had hoped to detect. They were faint, but still present.
This discovery is not only exciting because it further confirms the existence of the cosmic web in its original form, but the team also hopes it will give us a little more insight into how the universe evolved over time. Perhaps one day we will truly understand the vastness of our universe.
Thus, using light from galaxies and stars behind the Coma cluster, and thanks to the high sensitivity, high resolution, and wide field of view of the Subaru Telescope’s Hyper Suprime-Cam (HSC), the team detected for the first time a weak lensing effect on the dark matter component of intracluster filaments.
This first-ever detection of dark matter at the ends of the cosmic web helps further confirm the existence of large-scale structure extending throughout the Universe.
A team of Australian researchers has detected shock waves in the cosmic web for the first time. These shock waves may help us understand the role of magnetic fields. The researchers hope that studying the energetic glow of these shock waves will help answer questions about the evolution of the universe.
Magnetic fields on a cosmic scale have long remained a mystery to all of humanity. But the discovery of shock waves in thinner regions of the cosmic web could help researchers better understand how these fields influence the overall shape of the observable universe. It could also help us understand how the cosmic web grows.
In a 2023 study published in Science Advances, the International Centre for Radio Astronomy Research (ICRAR), based in Australia, demonstrates the first detection of shock waves between galaxy clusters. These waves are accelerated particles moving through cosmic clusters and provide evidence of the pervasive nature of magnetic fields.
“When matter coalesces in the Universe, it creates a shock wave that accelerates particles, strengthening intergalactic magnetic fields,” said Tessa Wernstrom of the University of Western Australia in a press release. “Magnetic fields permeate the Universe—from planets and stars to the vast spaces between galaxies. However, many aspects of cosmic magnetism are still poorly understood, especially at the scales observed in the cosmic web.”
Think of the cosmic web as the observable universe on a grand scale—an intertwined network of filaments and clusters filled with gas, galaxies, and dark matter, weaving through cosmic voids millions of light-years across. The ICRAR team wanted to understand the role of magnetic fields on such scales, a task long proven to be challenging.
These magnetic fields, which permeate the cosmic web, Wernstrom writes in The Conversation, are created by moving energetic particles. She also explains that the theory behind this project predicts that “when gravity pulls on the thread, shock waves are generated that strengthen the magnetic field and create a glow visible to radio telescopes.” These shock waves have been detected for the first time.
To detect these elusive signals, the team used a process called stacking, which involves superimposing multiple observations on top of each other to remove background noise and amplify the weak signal enough to be detected. This method revealed the signatures of shock waves the team was looking for, previously observed only in galaxy cluster collisions.
“These shock waves emit radio emissions that should cause the cosmic web to ‘glow’ in the radio spectrum,” says Wernstrom, “but it has never been reliably detected because the signals are so weak.”
But superposition alone proved insufficient. The key was combining this method with observations of the polarization of these fields. Large-scale shock waves, according to Wernstrom’s work, “force magnetic fields to order” and make them highly polarized, meaning they move in the same direction.
Combining the ideas of stacking and polarization, Wernstrom wrote that the team detected “an excess of highly polarized light between clusters, much larger than would be expected from galaxies alone. We can interpret this as light from shock waves in the connecting filaments.”
A new study offers new insights into the role of magnetic fields in the strands of the cosmic web. A deeper understanding of the strength and significance of magnetic fields will help researchers better understand how the universe grows.
“Detecting and studying this glow could not only confirm our theories about how the large-scale structure of the Universe formed,” Wernstrom writes, “but also help answer questions about cosmic magnetic fields and their meaning.”
See also... Complete list of HTML/Javascript event handlers with examples List of required documents for a website in the Russian Federation |
