The suburbia of the Milky Way does not form a ball of matter with the center at its center. Rather, the mass around it is arranged in a wide, flattened form, which alters the sense of gravity back towards home, the galaxies experience a net pull of home, and the galaxies are pulled away, at distances where the Local Group should predominate.
Awkwardly, astronomers have existed with a local exception to an elegant universal law. Space is expanding and the galaxies tend to recede at a rate which is proportional to the distance. However, Andromeda (approximately 2.5 million light-years distant) is approaching the Milky Way at a speed of approximately 110 kilometers per second, and dozens of other large neighbors of the Local Group move past the Milky Way-Andromeda pair without much of a hesitation.
New calculations have shown that these outflowing galaxies are not neglecting gravity, they are immersed in a massive “pancake” of matter where competing gravitational forces are almost canceled. The most important characteristic is a massive sheet of dark matter which is located right outside the Local Group and is tens of millions of light-years long. In such geometry, the mass in the plane may pull the galaxies out of the plane and the Milky Way and Andromeda are counterbalanced by the mass further away in the plane, which pulls them out of it as well.
This can be seen as one reason why Andromeda is the best. Both the Milky Way and Andromeda are contained within a massive dark matter halo and the two galaxies are still acting upon each other, leading to their collision. However, the gravitational background is not spherical, as in the case of other galaxies, which are not a part of the Local Group. The accounting will be different with a flattened arrangement of masses: the distant material in the sheet will give a directional pullage that a simplified model cannot capture.
The modeling computation constructed “virtual twins” of the Local Group by developing conditions of the early universe forward until the simulated current fit the major constraints that were detected: the masses and motions of the Milky Way and Andromeda, and the locations and velocities of 31 galaxies immediately outside the Local Group. In the simulations which most closely recapitulated reality, matter condensed to a plane with huge underdense clumps above and below it– holes which moved at a greater velocity and caused a push of material to the sides of the holes. Having little or no galaxies within those gaps, there are also little nearby objects that would be falling directly toward the Milky Way in those directions, and so Andromeda is the outstanding inbound neighbor in that direction.
That flattened environment also covers an old conflict in weighing the Local Group. Strategies that infer the mass using the Milky Way-Andromeda approach can suggest a heavy aggregate, whereas the ones based on tracer galaxies outside the Local Group may provide different responses assuming that the mass around the group can be roughly spherical. Recession velocities of neighboring tracers can be large, as in the sheet picture, when a big total mass is associated with them, since the outward push of the plane itself partially cancels the inward push of the Local Group itself. In a single report of the simulations, the total halo mass was determined as approximately 3.3 + or -0.6 trillion solar masses, yet generated a rather quiet local expansion.
Another implication is that local gravity must be directional. The material above and below the plane is supposed to flow towards the centre of the sheet and galaxies embedded therein still exhibit almost the Hubble-like recession with only slight deviation. The observational tests with the best chance of success depend on the possibility of uncovering more small, isolated galaxies off the plane, which would tell whether or not the predicted infall pattern is real, or whether the apparent flatness of the local universe was overestimated.
