Tuesday, March 5, 2024
Tuesday, March 5, 2024
HomeScienceWebb can directly test a theory about dark matter

Webb can directly test a theory about dark matter


What about galaxies and dark matter? Most, if not all, galaxies are surrounded by halos of this mysterious, unknown but ubiquitous material. And it also played a role in the formation of galaxies. The nature of that role is something astronomers are still figuring out. Today, they are exploring the nascent Universe, searching for the smallest, brightest galaxies. That’s because they could help tell the story of dark matter’s role in galactic creation.

An international team of astronomers led by Smadar Naoz of UCLA is running simulations of early galaxy formation. Their computer programs track the circumstances of galactic births shortly after the Big Bang. These “hot off the press” computer models include some new wrinkles. They take into account previously ignored interactions between dark matter and the primordial “stuff” of the Universe. They would be hydrogen and helium gas. The result of the simulations: tiny, bright galaxies that formed more quickly than in computer models that did not include these movements. Now astronomers just need to find them, using JWST, in an effort to see if their theories about dark matter hold up.

Interactions of dark matter with supersonic baryonic matter

How would interactions between baryonic matter and dark matter make a difference? Here’s a likely story: In the early Universe, gas clouds moved at supersonic speeds through clumps of dark matter. It bounced off the dark matter. Finally, after millions of years, the gaseous material came back together to form stars in an explosion of stellar birth. The team’s simulations track the formation of those galaxies just after the Big Bang.

A composite model of matter distribution in the Universe (with dark matter overlay) in a simulation of galaxy formation carried out by the TNG Collaboration.

Naoz’s team believes that the existence of these smaller, brighter and more distant galaxies could confirm the model of so-called “cold dark matter.” It suggests that the Universe was in a hot, dense state containing only gases after the Big Bang. Over time, it evolved into an uneven distribution of galaxies (and eventually galaxy clusters). Stars and galaxies formed along the way, but the first steps probably depend on gravitational interaction with dark matter. If the supersonic interactions that Naoz’s team modeled really occurred, then those small galaxies would be the result.

Simulating the formation of galaxies and the influence of dark matter

JWST has seen some pretty early galaxies during its time in operation. It hasn’t detected the earliest ones… yet. However, the images HA provided are tantalizing clues to what might exist in earlier times and could provide insight into the role of dark matter. So it makes sense that astronomers would want to push their vision back in time as far as they can. And that means looking for bright spots of light that existed a few hundred million years after the Big Bang.

Artist's conception of galaxies with star formation in the early universe.  Stars and galaxies appear in bright white points of light, while more diffuse dark matter and gas appear in purples and reds.  The first clouds of gas bounced around clumps of dark matter, only to clump together again under the gravity of the dark matter, causing stars to form.  Credit: Aaron M. Geller/Northwest/CIERA + IT-RCDS
Artist’s conception of galaxies with star formation in the early universe. Stars and galaxies are shown in bright white points of light, while dark matter and gas are shown in purples and reds. The first clouds of gas bounced around clumps of dark matter, only to clump together again under the gravity of the dark matter, causing stars to form. Credit: Aaron M. Geller/Northwest/CIERA + IT-RCDS

“The discovery of patches of small, bright galaxies in the early universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two types of matter can produce the type of galaxy we are looking for. “Naoz said. “If dark matter does not behave like standard cold dark matter and the transmission effect is not present, then these bright dwarf galaxies will not be found and we will have to go back to the drawing board.”

In an article by team member and first author Claire Williams (published in Letters from astrophysical journals) team suggests that scientists using JWST start looking for galaxies that are much brighter than expected. If they exist, that will likely prove that the interactions occurred early in cosmic time. If none can be found, then perhaps scientists still don’t understand dark matter interactions. The big question to answer is, if they exist, how did they form so quickly and why are they so bright?

Transmitting through dark matter corridors

Let’s examine that by looking at the role of dark matter. The standard cosmological model says that the gravitational pull of dark matter clumps in the early Universe attracted ordinary matter. Over time, this caused stars to form, followed by galaxies. Dark matter is believed to move more slowly than light. Thus, astronomers predicted that the processes of star and galaxy formation occurred very gradually. At least that’s what previous simulations suggest.

But what if something else was happening more than 13 billion years ago? How would that change things? It was a time before the formation of the first galaxies. But it was a time when ordinary matter in the form of large overdensities of hydrogen and helium was circulating through the expanding Universe. It bounced off slower-moving clumps of dark matter and overcame its gravitational pull, at least for a while. The baryon matter then accumulated again, under the influence of the dark matter. That’s when the star birth fireworks began.

This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen, among other things, from excited hydrogen atoms: Lyman-alpha emission.  Scientists are looking at this and other young galaxies to understand the role dark matter plays in early cosmic history.
This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen, among other things, from excited hydrogen atoms: Lyman-alpha emission. Scientists are looking at this and other young galaxies to understand the role dark matter plays in early cosmic history.

“While streaming suppressed star formation in smaller galaxies, it also boosted star formation in dwarf galaxies, causing them to eclipse non-streaming areas of the universe,” Williams said. Basically, the accumulated gas began to disintegrate after millions of years. This caused a huge burst of star formation. Many young, hot, massive stars began to shine, outshining the stars of other small galaxies. Ultimately, what this means is that since dark matter is impossible to “see,” those bright galaxy blobs could be indirect proof of its existence. And they would demonstrate the role that dark matter played in the creation of galaxies.

Find those shiny patches

No one has seen exactly what Naoz and the team are looking for… yet. Once they do, it will go a long way toward providing insight into the role of cold dark matter. “The discovery of patches of small, bright galaxies in the early universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two types of matter can produce the type of galaxy we are looking for. “Naoz said.

Of course, JWST is a perfect telescope to help see these galaxies. It should be able to observe regions of the Universe where small young galaxies are brighter than astronomers expect. That extreme luminosity will help JWST detect them, showing them how they looked at a time when the Universe was only a few hundred million years old. Since dark matter is impossible to study directly, searching for such bright spots of young galaxies in the early Universe could offer a powerful test for theories about dark matter and its role in the formation of the first galaxies.

For more information

Bright galaxies test dark matter
The Supersonic Project: Illuminating the dim end of the JWST UV luminosity function



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