Does the expanding Universe stretch space or create new space?

It’s been nearly a full century, almost 100 years, since humanity first reached a revolutionary conclusion about the nature of our Universe: space itself cannot and does not remain static, but rather evolves with the passage of time. One of the most counterintuitive predictions of Einstein’s general relativity is that any Universe — so long as it’s uniformly (or almost uniformly) filled with one or more species of matter, radiation, or energy — cannot remain both static and stable over time. Instead, it must either expand or contract, something initially derived independently by three separate people: Right at around the same time that Friedmann conducted his spectacular work, starting in 1923, observations began to show that the spirals and ellipticals in our sky were actually galaxies: “island universes” that were well outside of our own Milky Way.

With new, more powerful measurements, we could determine that the farther away a galaxy was from us, the greater the arriving light in our instruments was redshifted, or observed at longer wavelengths, compared to the light that was initially emitted. It was as though the very act of journeying through space altered the wavelength of that traveling light. But what, exactly, happens to the fabric of space itself while this process occurs; what’s the best way to interpret these facts?

Is the space itself stretching, as though it’s getting thinner and stretched out? Is more space constantly being created, as though it were “filling in the gaps” that the expansion creates? This is one of the toughest things to conceive of in modern astrophysics.

If we think hard about it, however, we find that we are indeed capable of wrapping our heads around it. Let’s explore what’s going on. An animated look at how spacetime responds as a mass moves through it helps showcase exactly how, qualitatively, it isn’t merely a sheet of fabric.

Instead, all of 3D space itself gets curved by the presence and properties of the matter and energy within the Universe. Space doesn’t “change shape” instantaneously, everywhere, but is rather limited by the speed at which gravity can propagate through it: at the speed of light. The theory of general relativity is relativistically invariant, as are quantum field theories, which means that even though different observers don’t agree on what they measure, all of their measurements are consistent when transformed correctly.

The starting point of our conversation has to be general relativity: our modern theory of gravity first put forth by Einstein. General relativity, at its core, is a framework that relates two things that might not obviously be related: If we inhabited a completely empty Universe, or a Universe that contains nothing within it at all — no matter, radiation, or energy of any form — you’d get the flat, Newtonian-like space you’re intuitively used to: something that’s static, uncurved, and unchanging. But what if we then considered the absolute simplest case of a non-empty Universe?