For generations, astronomers have envisioned a dramatic destiny for our Milky Way Galaxy: a significant collision with Andromeda, our closest substantial galactic neighbor. This cosmic event, anticipated in approximately 5 billion years, is a fixture within astronomy films, textbook discussions, and popular science narratives.

However, a recent study led by Till Sawala from the University of Helsinki, and published in Nature Astronomy, suggests a more uncertain horizon for our galaxy.

By thoughtfully acknowledging uncertainties in present data and considering the gravitational impact of nearby galaxies, the study concluded that there’s merely a 50% probability of the Milky Way merging with Andromeda within the next 10 billion years.

Past Beliefs About a Cosmic Collision

The speculation that the Milky Way and Andromeda are headed for a collision dates back over a century. This was based on Andromeda’s measured radial velocity—its movement along our line of sight—using the Doppler shift.

Proper motion, or the sideways drift of galaxies, is known as transverse velocity. Detecting this sideways movement is notably challenging, especially in galaxies millions of light years away.

Earlier research often presumed Andromeda’s transverse motion was minimal, leading to the notion of an inevitable head-on clash.

The Fresh Take of This Study

This study did not introduce new data but re-evaluated existing observations obtained from the Hubble Space Telescope and the Gaia mission.

Unlike previous investigations, this approach considers measurement uncertainties rather than assuming their most likely values.

The team simulated numerous potential trajectories for both the Milky Way and Andromeda by marginally adjusting initial conditions—parameters like each galaxy’s speed and position.

When initial conditions from prior studies were used, similar outcomes were observed, but this study also explored a broader spectrum of possibilities.

Incorporating the impact of two additional galaxies, namely the Large Magellanic Cloud and M33, also known as the Triangulum Galaxy, added depth to the trajectories explored.

The gravitational influence from M33 nudges Andromeda closer to the Milky Way, increasing the merger likelihood, while the Large Magellanic Cloud diminishes the probability of a collision.

All these elements combined reveal that, in about half the scenarios, the galaxies might not merge within the next 10 billion years.

Potential Outcomes of Merging or Non-Merging

Even if the galaxies merge, catastrophic effects on Earth are improbable as stars are vastly separated, minimizing direct collisions.

Galaxies, under gravity, would eventually merge into a larger, single entity, which is likely an elliptical galaxy rather than the iconic spirals we see today.

Alternatively, if no merger occurs, the galaxies might engage in a long and slow orbit around each other, never quite merging, yet reshaping our comprehension of the Milky Way’s distant trajectory.

Next Steps in Discovering Our Galactic Fate

The greatest uncertainty remains Andromeda’s transverse velocity. Small variations in this sideways motion could differentiate between a merger and a near miss. Upcoming assessments will refine this measurement, ushering us toward clarity.

Presently, we lack certainty regarding our galaxy’s fate, yet the quest for understanding unveils the magnitude of knowledge we’re still uncovering about the cosmos, even right at home.