In case you think about the increasing universe just like the floor of a balloon, you possibly can see that because the balloon (our stand-in for space-time) expands, galaxies are carried together with it. As a result of space-time itself is increasing, the universe will not be increasing into something. Credit score: Astronomy: Roen Kelly
What’s the universe increasing into?
Euan Tobin
Edinburgh, Scotland
This wonderful 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’ll be able to simply visualize this enlargement as a result of one object is inflating relative to a hard and fast enclosure.
The notion of the enlargement of the universe is extra problematic as a result of the universe comprises all the things — it’s a closed space-time system. One might say that the universe isn’t increasing into something as a result of the query implies that one thing exists “outdoors” the universe, which is nonsensical, as “outdoors” is a spatially relative time period.
Nonetheless, such a solution is unsatisfactory. Let’s return to the balloon. This time, let’s draw a sequence 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 symbolize the galaxies, whereas the membrane represents the finite but unbounded universe. You could possibly journey alongside the balloon however by no means come to the “finish,” as it’s actually unbounded. And although the balloon’s floor may not be infinite in extent, it’s not outlined by an outer boundary. (Notice {that a} shortcoming of this balloon analogy is that it tries to symbolize 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 “cloth” is what’s increasing, and the fabric embedded inside it — specifically galaxies — strikes aside in response to this enlargement. We can not really say that the universe is increasing “into” the rest. There isn’t a mounted middle, neither is there any inflexible outer boundary. It’s merely increasing, simply because the balloon membrane expands — albeit in three spatial dimensions as an alternative of two.
Edward Herrick-Gleason
Astronomy Educator, St. John’s, Newfoundland and Labrador
