What exactly does it mean when you say the universe is "expanding"? Does the planck-length get bigger? How can there be more space between objects?

What Does It Mean for the Universe to Be Expanding?

When scientists say the universe is expanding, they mean that the distance between galaxies and other large-scale structures in the universe is increasing over time. This expansion is not about objects moving through space but rather about space itself expanding. Imagine a grid of galaxies: as time passes, this grid stretches, increasing the distance between galaxies13.

Analogy: The Raisin Bread

A helpful analogy is the raisin bread model. As the dough rises, raisins move away from each other, not because they are moving through the dough but because the dough itself is expanding. Similarly, galaxies move away from each other because space is expanding13.

Does the Planck Length Get Bigger?

The Planck length is a fundamental unit of length in physics, approximately (1.62 \times 10^{-35}) meters. It is a theoretical limit beyond which our current understanding of physics breaks down. The expansion of the universe does not affect the Planck length itself; it is a constant scale factor in our understanding of quantum gravity. The expansion refers to the increase in distance between large-scale structures like galaxies, not to changes in fundamental physical constants or units like the Planck length5.

How Can There Be More Space Between Objects?

The concept of "more space" between objects arises from the intrinsic expansion of space itself. It's not that new space is being created in the sense of filling a pre-existing void; rather, the fabric of spacetime is stretching. This stretching increases the distance between objects that are not gravitationally bound together, such as distant galaxies25. Locally, within galaxies or solar systems, gravity holds objects together, preventing them from expanding along with the universe's expansion2.

Does the Universe Expand "Into" Anything?

The universe does not expand into anything; it simply expands. This expansion is a property of spacetime itself and does not imply the existence of an external space into which the universe is expanding25. The expansion can be thought of as an increase in the scale factor of the universe's metric, which describes how distances between objects change over time5.