If you imagine the expanding universe like the surface of a balloon, you can see that as the balloon (our stand-in for space-time) expands, galaxies are carried along with it. Because space-time itself is expanding, the universe is not expanding into anything. Credit: Astronomy: Roen Kelly
What is the universe expanding into?
Euan Tobin
Edinburgh, Scotland
This excellent question forces us to confront a region beyond the bounds of our intuition, so please bear with me. When you inflate a balloon, the balloon’s membrane moves outward, closer to the boundaries of whatever room encloses it. You can easily visualize this expansion because one object is inflating relative to a fixed enclosure.
The notion of the expansion of the universe is more problematic because the universe contains everything — it is a closed space-time system. One could say that the universe isn’t expanding into anything because the question implies that something exists “outside” the universe, which is nonsensical, as “outside” is a spatially relative term.
However, such an answer is unsatisfactory. Let’s return to the balloon. This time, let’s draw a series of black dots along the membrane. As the balloon inflates, the dots appear to be moving away from each other. From the perspective of one dot, the more distant dots move away more quickly than those closer to it. As the membrane expands, the dots grow farther apart. In this example, the dots represent the galaxies, while the membrane represents the finite yet unbounded universe. You could travel along the balloon but never come to the “end,” as it is literally unbounded. And though the balloon’s surface might not be infinite in extent, it is not defined by an outer boundary. (Note that a shortcoming of this balloon analogy is that it tries to represent three-dimensional reality along a two-dimensional shell.)
Universal expansion causes galaxies to recede from each other. Galaxies close to us recede at a slower rate than those farther away. As demonstrated by the balloon model, the space-time “fabric” is what is expanding, and the material embedded within it — namely galaxies — moves apart in response to this expansion. We cannot truly say that the universe is expanding “into” anything else. There is no fixed center, nor is there any rigid outer boundary. It is merely expanding, just as the balloon membrane expands — albeit in three spatial dimensions instead of two.
Edward Herrick-Gleason
Astronomy Educator, St. John’s, Newfoundland and Labrador
