The structure of the local universe is surprisingly flat, according to a new study, and this cosmic peculiarity could save our Milky Way galaxy from colliding with countless nearby giant galaxies, except for one.
For decades, astronomers have made the puzzling observation that our nearest galactic neighbor, Andromeda, is hurtling towards a possible collision with our galaxy, while other nearby galaxies are moving away from us. Now, new research may finally reveal why. A vast, flat sheet of dark matter is dragging these galaxies into deep space.
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“The observed motion of nearby galaxies and the combined mass of the Milky Way and Andromeda can only be adequately explained by this ‘flat’ mass distribution,” the researchers said in a statement.
Future simulations may further explain how gravity shapes our surroundings and why the local universe looks the way it does.
go with the flow
The motion of galaxies across the expanding fabric of space-time is known as the Hubble flow. This is explained mathematically by Hubble’s law, named after astronomer Edwin Hubble, who discovered the expansion of the universe in the 1920s. The law that bears his name imposes constraints on observed phenomena. In other words, galaxies move away from Earth at a speed proportional to their distance. The farther a galaxy is from our perspective, the faster it appears to be moving away from us.
So why does Andromeda, 2.5 million light-years away, travel at 68 miles per second (110 kilometers per second) when most other large nearby galaxies follow that trend? Interestingly, these retreating galaxies appear to be resisting the enormous gravitational pull of a local group that includes the Milky Way, Andromeda, the Triangular Galaxy, and dozens of smaller, gravitationally bound galaxies.
This universal mystery has persisted for more than half a century. In 1959, astronomers Franz Kahn and Lodewijk Walttier discovered evidence of dark matter around Andromeda and the Milky Way. They calculated that to reverse the initial expansion caused by the Big Bang, the combined mass of these two galaxies would need to be much greater than the mass of all their stars combined.
It turns out that a significant portion of the mass of the Milky Way and Andromeda is contained in the dark matter halo that surrounds each galaxy and drives them closer together.
However, this gravitational pull does not seem to affect nearby galaxies outside the local group, and “matter is actually moving away from the Milky Way faster than the Hubble flow,” study co-author Simon White, director emeritus of Germany’s Max Planck Institute for Astrophysics, said in a statement.
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“Therefore, the closer galaxies are [roughly 8 million light-years] Galaxies that are more distant are moving away from us at a slower rate than predicted by Hubble’s law. [that] It’s actually receding faster than expected,” White told Live Science via email.
build a universe from scratch
To find out why, researchers built their own universe. They ran a number of simulations to investigate the interactions between dark matter, the local galaxy cluster, and receding galaxies just outside it, up to a distance of about 32 million light-years.
The simulation modeled the evolution of the local universe from the beginning of space-time, starting with the mass distribution observed in the cosmic microwave background radiation, the oldest light in the universe, which was emitted when the universe was just 380,000 years old. The researchers then had the model reproduce certain salient features observed in nearby galaxies, including the masses, positions, and velocities of the Andromeda and Milky Way galaxies, as well as the positions and velocities of 31 galaxies located just outside the local cluster.
This reveals that the mass just outside the local group, which includes both dark matter and visible matter, is distributed in vast flat sheets that extend over tens of millions of light-years and extend beyond the boundaries of the simulation.
Nearby galaxies are embedded in this flat sheet of dark matter, so the gravitational pull from our local group is counteracted by the gravitational pull from more distant masses in the sheet, pushing the galaxies away from us.
“If the mass were distributed roughly spherically around the local cluster, rather than in a flat plane, the outer galaxies would move away from us slower than Hubble’s law of cosmic expansion predicts, slowing them down due to the gravity of the Milky Way and Andromeda,” White told Live Science. “Instead, the flat distribution of surrounding material pulls these galaxies outward in a way that almost exactly compensates for their inward pull. [Milky Way] and [Andromeda]. ”
Equally important, there are no galaxies in the regions above and below the sheet. Such sparse regions occurred throughout the Universe, and deep local voids around local populations formed in regions where the initial density of the Universe was slightly lower than average.
“As a result, these areas grew faster than average, and the problem was ‘pushed’ outward,” White said in an email. “To date, these low-density regions make up most of the universe, and gravitational effects have concentrated most of their matter in the ‘walls’ that separate them.”
Harmony of experiments, observations, and models
The location of the void is important. These sparse regions are where existing structures fall into local groups. Any galaxies out there would certainly be moving towards us. Therefore, no other celestial bodies are seen moving toward the Milky Way like Andromeda. Because there are no galaxies out there that do.
Overall, given the vast mass sheets, the simulations accurately modeled the distribution of nearby galaxies and cavities, thereby aligning the experimental results with astronomical observations of galactic motion and with a leading model in cosmology known as lambda cold dark matter.
“We are investigating all possible local configurations of the early Universe that could eventually lead to local groups,” lead study author Ewood Wempe, a cosmologist at the University of Groningen in the Netherlands, said in a separate statement. “It’s great to have a model that matches current cosmological models on the one hand and the dynamics of our local environment on the other.”
Interestingly, researchers report that galaxies even further out in the universe, at high latitudes, have been observed falling toward flat sheets of matter at hundreds of kilometers per hour. Finding additional structures that invade from the direction of the void could further support the results of this study.
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