Dark Matter Sheet Explains Flat Local Universe and Milky Way's Unique Approach to Andromeda
A recent study indicates the local universe's structure is surprisingly flat, potentially preventing the Milky Way from colliding with numerous massive galaxies, except for Andromeda. For decades, Andromeda has been observed approaching the Milky Way, while other nearby galaxies recede. New research suggests a vast, flat sheet of dark matter may explain this phenomenon.
Dark Matter's Influence
Dark matter attracts visible matter. The gravitational pull from this distant dark matter sheet, located slightly beyond Andromeda and the Milky Way, appears to override the gravitational attraction between our galaxy and other nearby galaxies, drawing them into deep space.
Researchers published these findings on January 27 in the journal Nature Astronomy. The study indicates that the observed motions of nearby galaxies and the combined masses of the Milky Way and Andromeda can only be properly explained by this flat mass distribution.
The Hubble Flow Anomaly
Galaxies typically move away from Earth at speeds proportional to their distance, a phenomenon known as the Hubble flow, described by Hubble's law. However, Andromeda, located 2.5 million light-years away, is moving towards the Milky Way at 68 miles per second (110 kilometers per second). Most other large, nearby galaxies follow the Hubble flow. This discrepancy has been a longstanding enigma.
In 1959, astronomers Franz Kahn and Lodewijk Woltjer found evidence of dark matter around Andromeda and the Milky Way, calculating that their combined mass, far exceeding their visible stars, was necessary to reverse the initial expansion from the Big Bang. A significant portion of the mass of both galaxies is contained in dark matter halos, facilitating their approach.
However, this attraction does not extend to galaxies outside the Local Group. These external galaxies are moving away from the Milky Way faster than the Hubble flow. Galaxies closer than approximately 8 million light-years move away slower than predicted by Hubble's Law, while those farther away recede faster.
Simulations Unveil the Cosmic Sheet
To understand this, researchers developed simulations modeling the evolution of the local universe from the beginning of space-time, using mass distributions observed in the cosmic microwave background. The models reproduced characteristics of nearby galaxies, including the mass, position, and velocity of Andromeda and the Milky Way, alongside 31 galaxies just outside the Local Group.
These simulations revealed that mass slightly beyond the Local Group, encompassing both dark and visible matter, is distributed in a vast, flat sheet extending tens of millions of light-years.
Because nearby galaxies are embedded within this sheet, any gravitational pull from the Local Group is counteracted by the more distant mass in the sheet, which pulls them away.
Regions above and below this sheet are largely devoid of galaxies. These "Local Voids" formed in areas of lower-than-average initial universal density, causing them to expand faster and push matter outwards into separating "walls." The absence of galaxies in these voids explains why no other objects are observed careening towards the Milky Way like Andromeda.
The simulations, by accounting for this vast sheet of mass, accurately modeled the distribution of nearby galaxies and voids. This reconciles experimental results with astronomical observations of galactic motions and the lambda cold dark matter cosmological model. Further observations of high-latitude galaxies falling towards this flat sheet could provide additional support for these findings.