Every major galaxy is speeding away from the Milky Way, except one — and we finally know why
The structure of the local universe is surprisingly flat, new research shows, and this cosmic quirk could prevent our Milky Way from colliding with countless other nearby massive galaxies except one.
For decades, astronomers have made the curious observation that our closest galactic neighbor, Andromeda, is speeding toward a possible collision with our galaxy, while other nearby galaxies move away from us. Now, a new study may finally reveal why: a vast, flat layer of dark matter is pulling these galaxies into deep space.
“The observed motions of nearby galaxies and the joint masses of the Milky Way and the Andromeda Galaxy can only be properly explained with this ‘flat’ mass distribution,” the researchers said in a report. statement.
Future simulations could further explain how gravity shapes our environment and why the local universe looks the way it does.
Go with the flow
The movement of galaxies through the expanding fabric of space-time is known as Hubble Flow. It’s mathematically described by Hubble’s law, named after the astronomer Edwin Hubblewho discovered the expansion of the universe in the 1920s. His eponymous law constrains an observational phenomenon: galaxies move away from the Earth at speeds proportional to their distance. The farther a galaxy is from our observation point, the faster it appears to be receding.
So why is Andromeda located 2.5 million light years awayhurtling toward us at 68 miles per second (110 kilometers per second), while most other large nearby galaxies follow the flow? Interestingly, these receding galaxies appear to resist the immense gravitational pull of our local group, which includes the Milky Way, Andromeda, Triangular galaxy and dozens of gravitationally bound smaller galaxies.
This universal enigma has lasted for more than half a century. In 1959, astronomers Franz Kahn and Lodewijk Woltjer discovered traces of dark matter around Andromeda and the Milky Way. They calculated that to reverse the initial expansion caused by the Big Bang, these two galaxies would need a much greater combined mass. that all their stars united.
It turns out that a significant part of the mass of the Milky Way and Andromeda is contained in dark matter halos that surround each galaxy and facilitate the rapid movement of galaxies toward each other.
However, this attraction does not appear to affect nearby galaxies outside the Local Group, where “matter is actually moving away from the Milky Way faster than the Hubble flow,” co-author of the study. Simon Whitedirector emeritus of the Max Planck Institute for Astrophysics in Germany, said in a statement.
“Thus, galaxies closer than [roughly 8 million light-years] are moving away from us more slowly than predicted by Hubble’s law, while galaxies farther away than [that] are actually retreating faster than expected,” White told Live Science via email.
Building a universe from scratch
To find out why, researchers built their own universe. They performed a multitude of simulations to explore the interactions between dark matter, our local group, and the receding galaxies just outside, at a distance of about 32 million light-years.
The simulations modeled the evolution of the local universe since the beginning of space-timestarting with the mass distributions observed in the cosmic microwave backgroundthe oldest light in the cosmos, emitted when the universe was only 380,000 years old. The researchers then had the model reproduce some salient features observed in nearby galaxies, including the mass, position and velocity of Andromeda and the Milky Way, as well as the positions and velocities of 31 galaxies located just outside the local group.
This revealed that the mass slightly exceeded the local group, including both dark matter and visible matter, is distributed in a vast flat sheet that spans tens of millions of light years and extends beyond the limits of the simulation.
Since nearby galaxies are embedded in this flattened sheet of dark matter, any gravitational pull from our local group is counteracted by the gravitational pull from the mass farthest from the sheet, pulling them away from us.
“If the mass were distributed approximately spherically around the local group, rather than being flat, then the outer galaxies would move away from us more slowly than Hubble’s law for cosmic expansion predicts, because they would be slowed by the gravitational pull of the Milky Way and Andromeda,” White told Live Science. “Instead, the flattened distribution of surrounding matter pulls these galaxies outward in a way that almost exactly offsets the inward attraction of matter. [Milky Way] And [Andromeda]”.
Just as important, the regions above and below the sheet are devoid of galaxies. Such sparse regions occur throughout the cosmosand the deep local voids around our local group formed in areas where the initial density of the universe was a little lower than average.
“As a result, these regions grew faster than average, and their business was ‘pushed’ outwards,” White said by email. “Nowadays, these low-density regions fill most of the space, and gravitational effects have concentrated most of their matter in the ‘walls’ between them.”
Reconciling experiences, observations and models
Locating voids is essential. These sparse regions are where all existing structures would fall on the side of the local group; all the galaxies there would actually be moving towards us. So we don’t see any other objects moving towards the Milky Way, as Andromeda does, because there simply aren’t any galaxies to do so.
Overall, by taking into account the vast mass layer, the simulations accurately modeled the distribution of nearby galaxies and voids, thereby reconciling the experimental results with astronomical observations of galactic motions as well as with the leading model of cosmology, known as lambda cold dark matter.
“We are exploring all possible local configurations of the early universe that could ultimately lead to the local group,” said the study’s lead author. Ewoud Wempecosmologist at the University of Groningen in the Netherlands, said in a different declaration. “It’s great that we now have a model consistent with the current cosmological model on the one hand and with the dynamics of our local environment on the other.”
Interestingly, researchers report that high-latitude galaxies, further away in the cosmos, are falling toward the flat layer of matter at several hundred kilometers per hour. The discovery of additional structures from the void directions could further support the results of this study.
