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By observing a distant and complex collision of galaxy clusters, astronomers have discovered that the most mysterious “stuff” in the universe, dark matter, is lurking among the debris like a cosmic ghost.
Dark matter is detected in colliding clusters as it rapidly moves away from traditional “normal” matter, which includes stars, planets, moons, and everything else we see around us. The galactic clusters “shadowed” in this study are part of a complex of thousands of galaxies, collectively known as MACS J0018.5+1626, located about 5 billion light-years from Earth. Clusters like MACS J0018.5+1626 form the largest structures in the universe.
The galaxies in the colliding clusters escaped the cosmic collision unscathed, thanks to the vast space between them; but their dark matter content was even less affected.
To imagine what this collision might look like, study lead author Emily Silich, an astrophysicist at the California Institute of Technology (Caltech) in Pasadena, suggested imagining two dump trucks carrying sand colliding with each other.
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“Dark matter is like sand and flies ahead,” Silich said in a statement.
Scientists have previously detected dark matter overtaking normal matter in similar collisions, but this new study, using data collected by NASA’s Hubble and Chandra space telescopes, represents the first time researchers have been able to directly examine the “decoupling” of the speed of dark matter and “normal” matter.
Silich and his colleagues used a number of telescopes to observe the collision of MACS J0018.5+1626. In addition to data from Hubble and Chandra, the Caltech Submillimeter Observatory (until recently located on Maunakea in Hawaii), the W. M. Keck Observatory, the Planck Observatory, the Atacama Submillimeter Telescope Experiment, and the now-retired Herschel Space Observatory collected data for the study.
Not only does the data come from a wide variety of tools, it has also been collected over decades and analysis of the data takes years.
Ghost surrender. How dark matter evaded ordinary matter
The problem with dark matter is that it’s frustratingly “antisocial” when it interacts with light, something that makes it nearly invisible, and ordinary matter.
It’s this lack of interaction (or the interactions being too weak to see) that leads scientists to think that dark matter can’t be made up of electrons, protons, and neutrons—the baryonic particles that make up the atoms that make up stars, planets, moons, and everything else we see around us. This is because these baryons interact with each other and with light.
This can make dark matter seem like a cosmic ghost and make you wonder how it exists. Dark matter It interacts with gravity, and this influence can affect baryonic matter and light in ways that we can detect.
That’s how scientists know that galaxies are enveloped in vast halos of dark matter that keep them from being torn apart by gravity. It’s also how they’ve determined that dark matter, despite its seemingly insignificant nature, makes up 85% of the mass of things in the universe.
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Some of the best evidence we have for the existence of dark matter is the Bullet Cluster, a cluster of two colliding galaxies about 3.7 billion light-years away, also known as 1E 0657-56. In the Bullet Cluster, scientists observed dark matter passing near hot gas as the two clusters passed each other.
The reason dark matter is able to avoid cataclysmic collisions as it moves is because it does not interact with ordinary matter.
The collision that forms the basis of MACS J0018.5+1626 is similar to the collision of the Bullet Cluster. What makes it different is that it is viewed from a different angle, tilted about 90 degrees to the Bullet Cluster. As a result, we see MACS J0018.5+1626 as one galaxy hurtling away from Earth while the other hurtles toward us.
This results in a vantage point that allows scientists to measure the speed of both dark matter and baryonic matter involved in the collision. From there, they can determine how the two types of matter are separated from each other in such an event.
“With Bullet Cluster, it’s as if we were sitting in the stands watching a car race and we could get beautiful snapshots of cars moving left to right on a straight line,” says Jack Sayers, a Caltech physics professor and the study’s principal investigator. “In our case, it’s more like we were standing in front of a car coming toward us on a straight line with a radar gun and getting its speed.”
The first light of the universe is a cosmic radar gun
The “radar gun” the team used is a phenomenon called the “Sunyaev-Zel’dovich (SZ) effect.” It occurs when photons that make up the first light in the universe, the cosmic microwave background (CMB), scatter off electrons that are not bound to atoms in hot ionized gas as the gas travels toward Earth.
This causes photons to experience a Doppler shift, which is a change in the frequency and wavelength of a wave depending on whether it is traveling toward or away from an observer. This causes a change in the brightness of the CMB light proportional to the speed at which the scattered electrons are traveling. This means that the SZ effect can be used to measure the speed at which hot gas is being produced in MACS J0018.5+1626, and hence the speed at which normal matter is moving.
The team then used the Keck Observatory to measure the speed of the mass density of galaxies in the clusters. Since most of this mass is accounted for by dark matter, dark matter and galaxies as a whole behave similarly during the collision. Therefore, this gave the researchers an indirect indication of the speed at which dark matter is moving.
This also showed the team something else strange about MACS J0018.5+1626: Dark matter and ordinary matter appear to be moving in opposite directions.
“We had something completely strange with opposing velocities, and at first we thought it might be a problem with our data. Even our colleagues who simulate galaxy clusters didn’t know what was going on,” Sayers explained. “And then Emily came in and figured it all out.”
Cosmic accident reconstruction
Aiming to solve the puzzle of the MACS J0018.5+1626 collision, Silich turned to data from Chandra, which revealed the temperature and location of the merger’s hot gas. This line of inquiry also revealed how much of that gas was “shocked” by the collision process.
“These cluster collisions are the most energetic events since the Big Bang,” Silich says. “Chandra measures the extreme temperatures of the gas and tells us the age of the merger and how recently the clusters collided.”
The team then mapped MACS J0018.5+1626’s dark matter using the effect of its mass on the fabric of space-time and, through it, on light from background sources, called “gravitational lensing.”
From there, they were able to simulate the collision of galaxy clusters, a kind of reconstruction of a cosmic accident. They then combined this simulation with a wealth of telescope data to determine the evolutionary stage of MACS J0018.5+1626 and the geometry of the cosmic collision. Such studies showed that just before the collision, the galaxy clusters were racing at about 7 million mph (11 million km/h)—about 1% the speed of light!
Why do dark matter and normal matter appear to move in opposite directions? The team determined that this is due to the orientation of the collision and the separation of the two forms of matter.
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“It took us a long time to put all the pieces of the puzzle together, but now we finally know what’s going on,” Sayers concluded. “We hope this leads to a whole new way of looking at dark matter in clusters.”
Although these findings do not reveal much new information about dark matter, the team hopes that future similar studies can gradually help solve this mystery that has puzzled scientists for decades.
The team’s work was published last month in The Astrophysical Journal.