The universe is constantly expanding, a concept that fundamentally reshaped our understanding of cosmology. This cosmic growth implies that the space between galaxies is stretching, causing distant celestial bodies to move away from one another. Given this profound, overarching expansion, it seems counterintuitive that our own galaxy, the Milky Way, remains a stable, non-expanding structure. The stability of the Milky Way is a consequence of the dynamic balance between two competing forces: the mysterious mechanism driving the expansion of space and the powerful, attractive force that binds matter together. This reveals the complex interplay of forces that govern structure formation across different scales.
Understanding Cosmic Expansion
The fundamental observation that the universe is expanding traces back to the work of astronomer Edwin Hubble in the late 1920s. Hubble’s observations revealed that galaxies are receding from the Earth at speeds proportional to their distance, a relationship now codified as Hubble’s Law. This phenomenon is not one of galaxies moving through space, but rather the stretching of the space between them, a concept known as metric expansion. It is similar to dots drawn on the surface of an inflating balloon; the distance between them grows as the material stretches.
The expansion rate is measured by the Hubble constant. For decades, cosmologists assumed that the gravitational pull of all the matter in the universe would cause this expansion to slow down over time. However, observations of distant Type Ia supernovae in the late 1990s showed that the expansion is not only continuing but is actually accelerating.
This acceleration is attributed to a mysterious repulsive force called Dark Energy, which permeates all of space. Dark Energy behaves like a cosmological constant, meaning its energy density remains constant even as the universe expands. The effect of Dark Energy is to generate a gentle, uniform outward push on the fabric of space itself. This outward push has been dominating the overall energy density of the universe for approximately the last four billion years, driving the accelerated expansion observed today.
The Dominance of Local Gravity
The reason the Milky Way is not expanding lies in the immense strength of the local gravitational field, which completely overwhelms the subtle outward push of Dark Energy. Gravity is an attractive force whose influence is determined by the amount of mass present. Within the confines of a galaxy, the density of matter is so high that the gravitational attraction is strong enough to bind all its components together into a stable structure.
The Milky Way is considered a gravitationally bound system. This means that the collective gravitational potential energy of all its components is greater than the kinetic energy required to disperse them. This binding force ensures that the stars, gas, dust, and planets within the galaxy remain locked in their orbits, resisting the underlying expansion of the surrounding space. The local gravitational pull is too powerful to allow this separation to occur.
A significant portion of this local gravitational dominance comes from Dark Matter, which is theorized to constitute about 85% of the total mass of the Milky Way’s halo. This invisible matter provides the necessary gravitational scaffolding to hold the galaxy together. Without the massive, non-expanding halo of Dark Matter, the visible components of the galaxy would not possess enough gravity to counteract the small internal expansive force.
The key difference between gravity and Dark Energy is how their influences scale with distance. Gravity’s strength diminishes rapidly, falling off with the square of the distance between objects. In contrast, Dark Energy’s repulsive effect is nearly uniform and incredibly weak at small, local scales. Where matter density is high, such as inside the Milky Way, the attractive force of gravity is trillions of times stronger than the Dark Energy’s expansive force, ensuring the galaxy’s internal stability.
The Scale of Influence
The contest between gravity and cosmic expansion is entirely dependent on the scale of the system being considered. Locally, gravity is the winner, keeping the Milky Way intact and preventing its internal expansion. This gravitational dominance extends beyond our galaxy to our immediate cosmic neighborhood, a collection of galaxies known as the Local Group.
The Local Group is a gravitationally bound structure spanning approximately three to five million light-years. It encompasses the Milky Way, the Andromeda galaxy, and dozens of smaller dwarf galaxies. Within this region, the collective mass of the galaxies generates a gravitational pull that is stronger than the expansive force of Dark Energy. The Andromeda galaxy is currently moving toward the Milky Way, a motion that directly opposes the general cosmic expansion and is a sign of their mutual gravitational capture.
It is only on scales larger than the Local Group that the expansion of space begins to dominate. At these vast distances, the average density of matter becomes so low that the gravitational attraction between distant galaxy clusters is too weak to overcome the cumulative effect of Dark Energy. Beyond a few million light-years, galaxies are observed to be receding from us, and the expansion predicted by Hubble’s Law becomes clearly visible.