New research shows that the Milky Way — and indeed our entire galactic neighborhood known as the Local Group of galaxies — appears to lie within a vast, extended “slice” of dark matter with cosmic voids on either side.
The findings are published in a new study natural astronomycould help explain the puzzling motions exhibited by galaxies near us that appear to defy the gravitational influence of nearby fields.
The mystery is related to American astronomer Edwin Hubble’s discovery nearly a century ago that the universe is expanding. The discovery was made after Hubble noticed that nearly every galaxy he observed was moving away from Earth at a rate proportional to its distance.
But there is one notable exception: The Andromeda Galaxy is the closest major galaxy to us, but it is moving towards us. It’s a vexing question because Andromeda, the Milky Way, and dozens of other nearby galaxies are all gravitationally bound to each other, forming what’s known as the Local Group of galaxies. Theoretically, the local group’s immense gravitational pull should draw all of its constituent realms toward each other, not just Andromeda.
To try to solve this long-standing mystery in astronomy, the researchers simulated the evolution of the Local Group from scratch based on observations of the cosmic microwave background, or leftover light from the Big Bang, creating “virtual twins” of the Local Group and dozens of other nearby galaxies beyond it, allowing them to infer the starting conditions of the final galaxy cluster. They then compared the motions of the simulated galaxies with their real-life counterparts and found that they were consistent with each other.
This shows that their virtual twins are profitable. They found that the only way this simulation would make sense was if the entire local swarm lay in a giant “slice” millions of light-years across made of dark matter, an invisible and still hypothetical substance thought to make up about 85 percent of the universe’s total mass.
Astronomers first hypothesized the existence of dark matter by observing that all the visible matter we see in galaxies is not enough to hold it together. Dark matter provides the gravitational scaffolding needed to keep everything in place, remaining invisible and not interacting with ordinary matter. The standard model of cosmology therefore holds that entire galaxies are suspended in giant clumps of matter, called dark matter haloes, that may be trillions of times more massive than our sun.
The problem is, these haloes have traditionally been thought of as giant, lumpy spheres, not the giant “flakes” the researchers propose here. However, they believe this flat geometry provides a clear explanation for runaway galaxies in our neighborhood. “In sheet geometry, the velocity-distance relationship depends not only on the enclosed mass in the spherical case, but also on the mass at greater distances,” the researchers explain in the study. In other words, the mass at the far edge of the dark matter sheet is pulling everything in it slightly outward, and beyond the flat boundary is a void uninhabited by galaxies.
The study’s lead author, Ewoud Wempe of the Kapten Institute in Groningen, the Netherlands, said this is the first assessment of the distribution and velocity of dark matter in the Local Group of galaxies.
“We are exploring all possible local structures in the early universe that could ultimately lead to the formation of local swarms,” Wempe said in a statement about the work. “It is great that we now have a model that is consistent with current cosmological models on the one hand and with the dynamics of our local environment on the other.”
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