What’s the universe increasing into?

Euan Tobin
Edinburgh, Scotland

This glorious query forces us to confront a area past the bounds of our instinct, so please bear with me. Once you inflate a balloon, the balloon’s membrane strikes outward, nearer to the boundaries of no matter room encloses it. You possibly can simply visualize this enlargement as a result of one object is inflating relative to a set enclosure.

The notion of the enlargement of the universe is extra problematic as a result of the universe comprises every thing — it’s a closed space-time system. One may say that the universe isn’t increasing into something as a result of the query implies that one thing exists “exterior” the universe, which is nonsensical, as “exterior” is a spatially relative time period.

Nevertheless, such a solution is unsatisfactory. Let’s return to the balloon. This time, let’s draw a collection of black dots alongside the membrane. Because the balloon inflates, the dots look like shifting away from one another. From the attitude of 1 dot, the extra distant dots transfer away extra rapidly than these nearer to it. Because the membrane expands, the dots develop farther aside. On this instance, the dots characterize the galaxies, whereas the membrane represents the finite but unbounded universe. You can journey alongside the balloon however by no means come to the “finish,” as it’s actually unbounded. And although the balloon’s floor won’t be infinite in extent, it isn’t outlined by an outer boundary. (Word {that a} shortcoming of this balloon analogy is that it tries to characterize three-dimensional actuality alongside a two-dimensional shell.)

Common enlargement causes galaxies to recede from one another. Galaxies near us recede at a slower fee than these farther away. As demonstrated by the balloon mannequin, the space-time “material” is what’s increasing, and the fabric embedded inside it — specifically galaxies — strikes aside in response to this enlargement. We can’t actually say that the universe is increasing “into” the rest. There isn’t any fastened middle, neither is there any inflexible outer boundary. It’s merely increasing, simply because the balloon membrane expands — albeit in three spatial dimensions as a substitute of two.

Edward Herrick-Gleason
Astronomy Educator, St. John’s, Newfoundland and Labrador