New Cosmology Model Explains Galaxy Movements & Andromeda’s Approach

For decades, the behavior of nearby galaxies has presented a persistent dilemma for astronomy. While almost all large galaxies in the local environment are moving away from the Milky Way following the overall expansion of the universe, one exception stands out clearly: Andromeda.

Our closest galactic neighbor is approaching at high speed and maintains a trajectory that will culminate in a future collision. This contrast challenged fundamental models, simulations, and assumptions about how mass is distributed in the nearby cosmos.

Now, new research proposes a compelling and elegant answer: the local universe isn’t organized spherically, but rather within a vast, flat sheet dominated by dark matter.

The finding stems from work combining detailed astronomical observations with high-precision cosmological simulations. The result describes a surprisingly ordered galactic environment, where the geometry of invisible matter plays a decisive role.

Instead of surrounding the stars of the Local Group uniformly, most of the mass is concentrated in a flattened structure extending for tens of millions of light-years. This configuration explains, simultaneously, the mutual attraction between the Milky Way and Andromeda and the recession of other nearby galaxies.

This model resolves a historical contradiction between the standard cosmological model and observations of the so-called Hubble flow in the immediate vicinity. It also offers a more coherent picture of how gravity shapes local space and why certain regions of the cosmos appear almost empty, while others concentrate most of the matter.

The expansion of the universe was one of the great discoveries of the 20th century. According to Hubble’s law, galaxies move away from each other at speeds proportional to their distance. Generally, the farther away a galaxy is, the faster it appears to recede. However, this relationship presents deviations on local scales, where the gravity of large concentrations of mass alters the expected motion.

The Local Group, comprised of the Milky Way, Andromeda, the Triangulum Galaxy, and dozens of dwarf galaxies, represents one of those cases. Andromeda is located about 2.5 million light-years away and is moving towards our galaxy at about 110 kilometers per second. This movement contrasts with that of other large nearby galaxies, located outside the Local Group, which are receding even faster than predicted by the average cosmic expansion.

This behavior puzzled astronomers for over half a century. As early as the late 1950s, the calculations of Franz Kahn and Lodewijk Woltjer suggested that the visible mass of the Milky Way and Andromeda was insufficient to explain their mutual attraction. This discrepancy became one of the earliest pieces of evidence for the existence of dark matter, a form of invisible matter that interacts primarily through gravity.

Over time, it was confirmed that both galaxies are enveloped in extensive halos of dark matter, whose mass far exceeds that of their stars and gas. This structure explains why Andromeda is approaching, but doesn’t clarify why other galaxies seem to escape the combined attraction of the Local Group.

The new study published in Nature Astronomy addressed this problem from a comprehensive approach. The researchers started from a key premise: the distribution of dark matter outside the main halos could be as important as the mass of the galaxies themselves.

As the authors noted, “Our galaxy, Andromeda, and their dwarf companion galaxies form the Local Group. It is believed that most of the mass within and around it is dark matter rather than gas or stars, so its distribution must be inferred from the effect of gravity on the motion of visible objects.”

The model departs from initial conditions of the early universe, inferred from the cosmic microwave background, and evolves to reproduce the current positions and velocities of observed galaxies.

The result was consistent and revealing. The mass surrounding the Local Group isn’t distributed spherically, but within a flat and extensive structure. Both dark matter and visible matter are concentrated in this structure, and its gravitational influence dominates the motion of nearby galaxies.

The existence of this dark matter sheet changes how the galactic environment is interpreted. Galaxies embedded in this structure experience a gravitational attraction towards more distant regions of the sheet, which almost completely compensates for the attraction exerted by the Milky Way and Andromeda. These galaxies follow the Hubble flow or even recede faster than expected.

In contrast, Andromeda shares the same plane as the Milky Way and is within the mutual gravitational domain of both halos. This location explains why it’s the only large galaxy moving towards us.

The study also illuminated the role of large cosmic voids. Above and below the sheet, space appears remarkably devoid of galaxies. These regions formed in areas of the early universe where the initial density was slightly below average. They expanded faster and expelled their matter into the surrounding areas.

This pattern explains why no other galaxies are observed heading towards the Milky Way from those directions. Simply put, there aren’t any massive objects in those voids that could cause them to do so. The geometry of the environment acts as a dynamic filter that orders possible movements.

The authors acknowledged that previous models faced persistent difficulties. “Modeling efforts have long struggled to reproduce the quiet Hubble flow around the Local Group, as they require unrealistically small mass beyond the two main galaxy halos. Here we revisit this using simulations of Local Group analogues with initial conditions constrained to match the observed dynamics of the two main halos.”

The new approach reconciled those tensions within the framework of the standard cosmological model, known as Lambda-CDM. According to the work, “The observations are reconcilable within ΛCDM, but only if the mass is strongly concentrated in a plane of up to 10 Mpc, with the surface density increasing away from the Local Group and with deep voids above and below.”

This configuration not only reconciles the dynamic mass estimates of the Local Group with the observed field of velocities, but also reflects structures already known in the distribution of nearby galaxies. The resulting Hubble flow presents an ordered behavior, although anisotropic, a characteristic that remained partially hidden due to the scarcity of observable galaxies in certain directions.

Beyond the immediate environment, the researchers detected additional clues that reinforce the model. Galaxies located at greater distances and at high latitudes appear to move towards the flat sheet at speeds of several hundred kilometers per hour. Future identification of more structures falling from the empty regions could provide independent confirmation of this cosmic architecture.

The impact of the study goes beyond solving a local mystery. By showing that the observed dynamics fit fully within the standard cosmological model, the work strengthens the idea that dark matter not only governs the formation of individual galaxies, but also the large-scale organization of nearby space.

In the words of the authors, the coherence achieved between simulations, observations, and theory offers a more complete picture of the galactic neighborhood. The local universe ceases to be a chaotic scenario and reveals itself as a structure carefully sculpted by gravity, where even the invisible imposes a precise order.

the future collision between the Milky Way and Andromeda no longer appears as an anomaly, but as a natural consequence of our position within a cosmic sheet of dark matter. A silent and imperceptible structure that, from the shadows, defines the destiny of galaxies.