Dark matter may be hitting the right note in small galaxies

Scientists have also provided an answer to what dark matter is doing.

Dark matter is a mysterious form of matter that is believed to occupy 85% of the matter in the universe, and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet-undiscovered subatomic particles.

Although its nature is unknown, scientists have claimed that the dark matter is responsible for forming stars and galaxies by its gravitational pull, which led to our existence.

Now, in a new study by scientists from Japan, Germany, and Austria, scientists suggest that Dark matter may scatter against each other only when they hit the right energy. Their idea could explain the reason why galaxies have different shapes from small to large.

When two dark matter particles approach each other, then tend to simply pass each other. (Credit: Kavli IPMU)
When two dark matter particles approach each other, then tend to simply pass each other. (Credit: Kavli IPMU)

Paper author Hitoshi Murayama, a University of California Berkeley Professor and Kavli Institute for the Physics and Mathematics of the Universe Principal Investigator said, “Dark matter is actually our mom who gave birth to all of us. But we haven’t met her; somehow, we got separated at birth. Who is she? That is the question we want to know.”

Astronomers have come to know that the dark matter does not seem to clump together as much as computer simulations suggest. Enlighting the fact that if gravity driving dark matter (only pulling, never pushing) then the dark matter should become very dense towards the center of galaxies.

But when they come at a special speed, they “resonate” and stick with each other for a brief moment, and move out to different directions afterwards, causing them to scatter. This way, dark matter can spread out so that we can understand smooth profile in small galaxies. (Credit: Kavli IPMU)
But when they come at a special speed, they “resonate” and stick with each other for a brief moment, and move out to different directions afterwards, causing them to scatter. This way, dark matter can spread out so that we can understand smooth profile in small galaxies. (Credit: Kavli IPMU)

But, this seems contradictory in small faint galaxies called dwarf spheroidals. In dwarf spheroidals, dark matter does not seem to become as dense as expected toward their centers.

According to astronomers, this mystery could be solved if dark matter scatters with each other like billiard balls, allowing them to spread out more evenly after a collision.

But the problem in this idea is, the dark matter does seem to clump in bigger systems such as clusters of galaxies. Here a question arises- what makes dark matter behave differently in dwarf spheroidals and clusters of galaxies?

In this study, astronomers have provided a possible explanation for this riddle and revealed what the dark matter actually is.

Chinese physicist Xiaoyong Chu, a postdoctoral researcher at the Austrian Academy of Sciences said, “If dark matter scatters with each other only at a low but very special speed, it can happen often in dwarf spheroidals where it is moving slowly, but it is rare in clusters of galaxies where it is moving fast. It needs to hit a resonance.”

Murayama explained, “Resonance is a phenomenon that appears every day. To swirl the wine in a glass to get it more oxygen so that it lets out more aroma and softens its taste, you need to find the right speed to circle the wine glass. Or you dial old analog radios to the right frequency to tune into your favorite station. These are all examples of resonance.”

Using the idea of resonance, the plot demonstrates that we can explain all systems at the same time. (Credit: Xiaoyong Chu, Camilo Garcia Cely, Hitoshi Murayama)
Using the idea of resonance, the plot demonstrates that we can explain all systems at the same time. (Credit: Xiaoyong Chu, Camilo Garcia Cely, Hitoshi Murayama)

The team suspects this is precisely what dark matter is doing.

Murayama further added, “As far as we know, this is the simplest explanation to the puzzle. We are excited because we may know what dark matter is sometime soon.”

However, the team was not convinced that such a simple idea would explain the data correctly. They are bit skeptical that this idea will explain the observational data.

Colombian researcher Camilo Garcia Cely, a postdoctoral researcher at the Deutsches Elektronen-Synchrotron (DESY) in Germany said, “But once we tried it, it worked like a charm!”

Paper authors (from left) Xiaoyong Chu, Camilo Garcia Cely, and Hitoshi Murayama (Credit from left: Xiaoyong Chu, DESY, Kavli IPMU)
Paper authors (from left) Xiaoyong Chu, Camilo Garcia Cely, and Hitoshi Murayama (Credit from left: Xiaoyong Chu, DESY, Kavli IPMU)

“Though, it is no accident that dark matter can hit the exact right note. There are many other systems in nature that show similar accidents: in stars alpha particles hit a resonance of beryllium, which in turn hits a resonance of carbon, producing the building blocks that gave rise to life on Earth. A similar process happens for a subatomic particle called phi.”

Chu said, “It may also be a sign that our world has more dimensions than we see. If a particle moves in extra dimensions, it has energy. For us who don’t see the extra dimension, we think the energy is actually a mass, thanks to Einstein’s E=mc2. Perhaps some particle moves twice as fast in an extra dimension, making its mass precisely twice as much as the mass of dark matter.”

The team’s next step will be to find observational data that backs their theory.

“If this is true, future and more detailed observation of different galaxies will reveal that scattering of dark matter indeed depends on its speed,” says Murayama, who is also leading a separate international group that intends to do precisely this using the under-construction Prime Focus Spectrograph. The US$80 million instruments will be mounted on the Subaru telescope atop Mauna Kea on Big Island, Hawaii, and will be capable of measuring the speeds of thousands of stars in dwarf spheroidals.

The team’s paper was published online on 22 February by Physical Review Letters.

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