Every major galaxy is speeding away from the Milky Way, except one — and we finally know why

Illustration of the night sky over a dark, mountainous horizon. The sky shows a large spiral galaxy at an angle on the left and a milky white cloud of stars cross the sky vertically on the right.

A view of the potential merger between the Milky Way and Andromeda as it may appear in Earth's night sky in 3.75 billion years. (Image credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger)

The structure of the local universe is surprisingly flat, according to new research, and this cosmic quirk may save our Milky Way from colliding with countless other massive, nearby galaxies — except one.

For decades, astronomers have made the puzzling observation that our nearest galactic neighbor, Andromeda, is speeding toward a possible collision with our galaxy, while other nearby galaxies are moving away from us. Now, a new study may finally reveal why: A vast, flat sheet of dark matter is drawing those 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 statement.

Future simulations could further explain how gravity sculpts our surroundings and why the local universe looks the way it does.

Going with the flow

The motion of galaxies throughout the expanding fabric of space-time is known as the Hubble flow. It's mathematically described by Hubble's law, named after astronomer Edwin Hubble, who discovered the expansion of the universe in the 1920s. His eponymous law constrains an observational phenomenon: Galaxies are moving away from Earth at speeds that are proportional to their distance. The farther a galaxy is from our vantage point, the faster it seems to be receding.

So why is Andromeda, located 2.5 million light-years away, hurtling toward us at 68 miles per second (110 kilometers per second), while most other large, nearby galaxies are following the flow? Curiously, these receding galaxies appear to resist the immense gravitational attraction of our Local Group, which includes the Milky Way, Andromeda, the Triangulum Galaxy and dozens of gravitationally bound, smaller galaxies.

This universal enigma has endured for more than half a century. In 1959, astronomers Franz Kahn and Lodewijk Woltjer found evidence of dark matter situated around Andromeda and the Milky Way. They calculated that to reverse the initial expansion imparted by the Big Bang, these two galaxies would require a combined mass much greater than all their stars put together.

It turns out that a significant portion of the mass of the Milky Way and Andromeda is contained in dark matter halos that surround each galaxy and facilitate the galaxies' rapid approach toward each other.

However, this attraction does not seem to affect nearby galaxies outside the Local Group, where "material is actually moving away from the Milky Way faster than the Hubble flow," study co-author Simon White, director 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 slower than predicted by Hubble's Law, whereas galaxies farther than [that] are actually receding faster than predicted," White told Live Science via email.

Composite of two images. Each image shows two bright white and red dots in the center. In the left image, the dots are surrounded by dark blue clouds that fade to purple and pink further from the dots. A cluster of neon blue dots surround the white dots. Overlaying the entire image are arrows pointing toward the viewer. The image on the right is similar, except the blue, purple, pink clouds are concentrated horizontally in the center and the arrows are pointed up in the bottom half of the image and pointed down in the top half. — The average distribution of dark matter in the local universe, showing Andromeda and the Milky Way as the two bright-orange blobs at center and the 31 nearby galaxies outside the Local Group as cyan dots. The left image looks down on the flat sheet of dark matter and galaxies, while the right image views it from the side. (Image credit: Max Planck Institute for Astrophysics)

Building a universe from scratch

To find out why, the researchers built their own universe. They ran a multitude of simulations to explore the interactions among dark matter, our Local Group, and the receding galaxies just outside it, to a distance of around 32 million light-years.

The simulations modeled the evolution of the local universe from the beginning of space-time, starting with the mass distributions observed in the cosmic microwave background, the oldest light in the cosmos, emitted when the universe was just 380,000 years old. The researchers then had the model reproduce certain salient characteristics 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 just slightly beyond the Local Group, including both dark matter and visible matter, is distributed in a vast, flat sheet that stretches for tens of millions of light-years and continues beyond the boundaries of the simulation.

Because 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 more distant mass in the sheet, drawing them away from us.

"If the mass were distributed approximately spherically around the Local Group, rather than being flat, then the external galaxies would be moving away from us slower than predicted by Hubble's law for the cosmic expansion, because they would be slowed down by the gravitational pull of the Milky Way and Andromeda," White told Live Science. "Instead, the flattened distribution of the surrounding matter pulls these galaxies outwards in a way which almost exactly compensates for the inward pull of the [Milky Way] and [Andromeda]."

Equally important, the regions above and below the sheet are devoid of galaxies. Such sparse regions occur throughout the cosmos, and the deep Local Voids around our Local Group formed in areas where the initial density of the universe was a bit lower than average.

"As a result these regions expanded faster than average, and their matter was 'pushed' outwards," White said via email. "By the present day these low-density regions fill most of space and gravitational effects have concentrated most of their material into the 'walls' that separate them."

Reconciling experiments, observations and models

The location of the voids is essential. These sparse regions are where any existing structures would fall toward the Local Group; any galaxies there would indeed be moving toward us. So we don't see any other objects careening toward the Milky Way, as Andromeda is doing, because there simply aren't any galaxies there to do so.

Overall, when accounting for the vast sheet of mass, the simulations accurately modeled the distribution of nearby galaxies and the voids, thereby reconciling experimental results with astronomical observations of galactic motions as well as with the leading model of cosmology, known as lambda cold dark matter.

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