Why Are There Galactic Collisions in an Expanding Universe?

One day, Andromeda and the Milky Way will collide and eventually merge into one giant galactic structure. This phenomenon occurs in a universe that is often described as expanding in all directions following the 'Big Bang' explosion. However, the reality is far more complex. Matter in the universe is not uniformly distributed, forming filaments around vast voids.

The Actual Structure of the Universe

The 'Big Bang' is often described as a uniform expansion, akin to a cake with raisins expanding evenly. In reality, the distribution of matter in the universe is far from uniform. Clusters of matter are concentrated along filaments, with vast empty regions in between. Even on the largest scales, expansion is not uniform; gravitational forces dominate on smaller scales, pulling galaxies together.

For example, the Milky Way and Andromeda are moving toward each other, with Andromeda expected to collide with the Milky Way in about 4.5 billion years. This collision is a result of the gravitational interaction between these two massive systems.

Gravity Assist and Galactic Collisions

A similar concept to 'gravity assist' in spacecraft mechanics applies to galactic collisions. When two massive bodies with strong gravitational fields pass each other, their gravitational interactions can cause them to deviate from their original paths, leading to significant changes over time.

This effect, known as 'galactic slurping,' is relatively rare between two large galactic systems but can occur frequently between large bodies and smaller bodies like stars or asteroids. For instance, massive stars or galaxies can fling asteroids and smaller bodies in various directions, altering their trajectories.

The Expansion Process and Initial Conditions

The process of galactic collisions began early in the universe's history, when the universe was only 400 million years old. At this time, matter was less dense and more compact. This means that even in an expanding universe, galactic collisions can still occur due to the initial conditions and the significant gravitational interactions between massive objects.

The standard explanation for the non-uniform structure of the universe is that in the first phase of the Big Bang, there were small quantum uncertainties that were amplified through a period of faster-than-light expansion. This left behind regions of high and low density, creating the filamentary structure we observe today.

Conclusion

Despite the general expansion of the universe, galactic collisions can still occur. The filaments and voids in the universe, coupled with strong gravitational forces, contribute to these dramatic encounters. From the early stages of the universe to the present day, gravitational interactions have played a crucial role in shaping the cosmos and leading to the inevitable collisions between galaxies.

The 'Big Bang' theory, while a cornerstone of cosmology, does not tell the whole story. The intricate dance of matter within the universe, guided by fundamental forces like gravity, continues to fascinate scientists and awe observers as we watch the universe evolve and collide.