For almost a century, observations have shown that most galaxies recede from the Milky Way, supporting the picture of an expanding universe and the Big Bang. However, the motion of nearby galaxies has posed a puzzle, because most large neighbors appear to follow the general Hubble Lemaitre expansion and do not seem to respond strongly to the gravity of the Local Group, even though our own neighbor Andromeda is moving toward us at about 100 kilometers per second.
A team led by PhD graduate Ewoud Wempe of the Kapteyn Institute at the University of Groningen, working with colleagues in Germany, France and Sweden, set out to solve this discrepancy. They constructed detailed simulations in which the early universe matter distribution is based on measurements of the cosmic microwave background, then evolved these initial conditions forward in time with a powerful computer.
The simulations were tuned so that at the present day they reproduce key properties of the Local Group, including the masses, positions and velocities of the Milky Way and Andromeda. They also match the positions and motions of 31 galaxies that lie just outside the Local Group, in the nearby volume where the puzzling velocity pattern is observed. The resulting models form a set of virtual twins of our cosmic neighborhood.
In these virtual universes, the nearby galaxies that lie in the sheet trace a structure in which the dark matter and visible matter are concentrated in a thin plane. The galaxies embedded in this sheet move away from us with velocities that closely follow the Hubble Lemaitre law, despite the substantial mass of the Local Group at the center. Above and below the sheet, the simulations reveal large void regions that are almost completely devoid of galaxies.
The researchers identify two main reasons why the local expansion appears so regular. For galaxies within the plane, the gravitational pull of the Local Group is largely counterbalanced by the mass distributed more widely in the sheet, so their motions are dominated by the overall cosmic expansion rather than by infall toward the Milky Way or Andromeda. In the surrounding voids, where matter would be expected to move toward overdense regions, there are effectively no galaxies to observe, so any deviations from the smooth expansion remain hidden.
According to Wempe, this work represents the first detailed assessment of the distribution and velocity of dark matter in the region around the Local Group. He notes that the simulations explore all possible local configurations of the early universe that can evolve into a system resembling our present day environment, and that the resulting models remain consistent with the standard cosmological framework while reproducing the dynamics of nearby galaxies.
Co author Amina Helmi highlights that astronomers have tried for decades to understand why local galaxies adhere so well to the Hubble Lemaitre relation despite the mass of the Local Group. She emphasizes that the new models, derived purely from galaxy motions, reveal a mass configuration that lines up with the observed positions of galaxies both inside and just beyond the Local Group.
The results, published in Nature Astronomy, suggest that our cosmic neighborhood is shaped by a delicate balance between the gravity of the Local Group and the extended dark matter sheet in which it is embedded. This configuration naturally explains why nearby galaxies move almost as if the Local Group were not there, while still remaining compatible with the broader cosmological picture of structure growth in an expanding universe.
Research Report:The mass distribution in and around the Local Group
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