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Observing a distant collision and mess between galaxy clusters, astronomers have discovered that dark matter, the most mysterious “stuff” in the universe, passed through the wreckage like a cosmic phantom.
The dark matter was detected as a departure from the normal “normal” matter that includes the stars, planets, moons and everything we see around us in the collision clusters. The Galactic clusters “ghosting” in this research are part of a complex of thousands of galaxies called MACS J0018.5+1626, located about 5 billion light-years from Earth. Clusters like MACS J0018.5+1626 are the largest structures in the universe.
The individual galaxies of the collision clusters escaped unscathed from this cosmic lottery because of the vast space between them, but the dark matter of those galaxies was interfering even more with the event.
To visualize what this collision would look like, study author Emily Silich, an astrophysicist at the California Institute of Technology (Caltech) in Pasadena, suggested that a picture of two dump trucks carrying a sandbar collide.
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“The dark matter is like the sand and flies ahead,” Silich said in a statement.
Scientists have detected dark matter racing ahead of normal matter in similar collisions before, but this new research, which used data collected by NASA’s Hubble and Chandra space telescopes, is the first time ‘researchers managed to directly study the “decoupling” of the velocity of dark matter and “normal” matter.
Silich and colleagues used a multitude of telescopes to observe the MACS J0018.5+1626 collision. In addition to data from Hubble and Chandra, the Caltech Submillimeter Observatory (until recently located on Maunakea in Hawai’i), the WM Keck Observatory, the Planck Observatory, the Atacama Submillimeter Telescope Experiment, and the retired Herschel Space Observatory collected . for the study.
Not only does the data come from a wide range of instruments; It was also collected over many years, and the analysis of the data itself took years.
Bringing up the ghost. How did dark matter give way to normal matter?
The problem of dark matter is based on the fact that it is “anti-social” frustrating when it comes to interacting with light, which makes it close to invisible, and with normal matter.
It is this lack of interaction (or the fact that the interactions are too weak to be seen) that leads scientists to think that dark matter cannot be made up of electrons, protons and neutrons, the baryon particles that make up the atoms that make up the stars, planets, moons and everything else we see around us. That’s because these barons interact with each other and with light.
This might make dark matter sound like a cosmic phantom, leaving you wondering how we know it exists at all. Well, that makes dark matter interacts with gravity, and that influence can affect real matter and light in ways we can detect.
That’s how scientists know that galaxies are surrounded by large halos of dark matter, which prevents them from breaking apart. It’s also how they determined that, despite its apparently insubstantial nature, dark matter makes up 85% of the matter with mass in the universe.
Related: What is dark matter?
One of our best pieces of evidence for the existence of dark matter is the Bullet Cluster, a cluster of two colliding galaxies also known as 1E 0657-56 and located about 3.7 billion light-years away. In the Bullet Cluster, scientists saw dark matter shooting hot gas downwards as the two clusters slid past each other.
It is the lack of interaction with normal matter that allows dark matter to escape the cataclysmic collisions on its way.
The collision underlying MACS J0018.5+1626 is similar to the collision in the Bullet Cluster. What sets it apart is that it is seen at a different angle, tilted at about 90 degrees relative to the Bullet Cluster. As a result, we see MACS J0018.5+1626 in such a way that it appears that one galaxy is racing from Earth and the others are rocketing our way.
This results in a vantage point that allows scientists to measure the velocity of the dark matter and baryonic matter involved in the collision. From there, they can then determine how the two types of content decouple from each other in an event like this.
“With the Bullet Cluster, it’s like we’re sitting in a grandstand watching a car race and we’re able to capture beautiful pictures of the cars moving from left to right right away,” said the chief -study investigator Jack Sayers, a Caltech physics professor. . “In our case, it is more like we are on the way directly with a radar gun, standing in front of a car as it comes to us and able to find its speed.”
The first light of the universe is a cosmic radar gun
The “radar gun” used by the team is a phenomenon known as the “Sunyaev-Zel’dovich (SZ) effect.” This happens when photons that make up the universe’s first light, the cosmic microwave background (CMB), scatter from electrons that are not bound to atoms in hot ionized gas as this gas travels towards Earth.
This is because the photons have a Doppler shift, a change in frequency and wavelength of a wave depending on whether it is moving towards or away from an observer. This results in a change in the brightness of the CMB light that is proportional to the speed at which the scattered electrons are moving. This means that the SZ effect can be used to measure the speed at which hot gas is produced, and therefore the speed at which ordinary matter moves, in MACS J0018.5+1626.
The team then used the Keck Observatory to measure the mass concentration speed of the galaxies in the clusters. Because most of this mass is caused by dark matter, it and galaxies as a whole behave similarly during the collision. So, this showed the researchers by proxy the speed at which the dark matter is moving.
This also revealed something strange to the team about MACS J0018.5+1626: The dark matter and normal matter seem to be moving in opposite directions.
“We had this whole oddball with velocities in opposite directions, and at first, we thought it might be a problem with our data. Even our colleagues who image galaxy clusters didn’t know what was going on,” Sayers explained. . “And then Emily got involved and untangled everything.”
Cosmic accident reconstruction
Aiming to solve the problem of the smashup MACS J0018.5+1626, Silich turned to data from Chandra, which revealed the temperature of the merger’s hot gas and its location. This line of inquiry also showed how much the collisional process “disturbed” this gas.
“These cluster collisions are the most energetic phenomena since the Big Bang,” says Silich. “Chandra measures the extreme temperatures of the gas and tells us about the age of the merger and how recently the clusters collided.”
The team then mapped the dark matter of MACS J0018.5+1626 using the effect of its mass on the fabric of spacetime and, through this, on passing light from background sources, known as “gravitational lensing.”
From here, they were able to simulate the collision of galaxy clusters, a type of cosmic accident reconstruction. They then combined this simulation with a wide range of telescope data to determine the evolutionary phase of MACS J0018.5+1626 and the geometry of the cosmic collision. Work like this showed that the galaxy clusters were racing together at about 7 million mph (11 million kph) just before they collided – about 1% of the speed of light!
Why do dark matter and normal matter seem to travel in opposite directions? The team concluded that this was due to the orientation of the collision and the separation of the two types of material.
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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 will lead to a whole new way of studying dark matter in clusters.”
Although these results do not reveal much new information about dark matter, the team hopes that similar studies that follow them may gradually help to shed some light on this mystery that has puzzled scientists for years.
The team’s study was published last month in The Astrophysical Journal.