“It’s a matter of reference!” I say. You say that the Universe is expanding (at an ever-increasing rate, apparently) and that all atoms within it are a fixed size. I prefer the opposite perspective: that the Universe is a fixed size and all atoms within it are shrinking.
I prefer this perspective because it makes certain hard-to-imagine-but-apparently-true things about our Universe easier to grasp. Things like a fourth spatial dimension. And universes within universes. And what our Universe is expanding into.
What is a reference frame?
I say it is just a matter of reference, but what does that mean? You probably learned about reference frames in terms of moving trains. The gist is that if you are moving, you can make the claim that are you actually stationary and that everything around you is what’s moving.
Sometimes changing your frame of reference feels intuitively wrong: if I am driving northbound on 101 and I claim that, No, actually I am perfectly still and, rather, the Earth is bounding southward beneath my tires, you might call me an egoist.
Other times the switch comes naturally: intellectually, I know the rotation of the Earth causes the Sun to rise and set. But, day to day, I prefer to think of my Earth firmly grounded and the Sun as the one in motion, drawing its slow arc across the sky each day. Sometimes, when I fly, I forget about the forward urgency of the plane and lose myself in the slow procession of the scenery below.
So what does it mean to shift our frame of reference on the entire Universe? Right now you imagine it as a quite huge inflating thing. Now imagine it is a constant size, a sphere you can hold in your hand.
All the little physicists on all the little planets in your miniature Universe peer out, observe hoards of red-shifted galaxies and proclaim their Universe expanding. Still, it rests in your hand, same size as it ever was and ever will be. How can this be!
First, assume that everyone is correct in their observations: that from their perspective, the Universe is rapidly expanding and from yours it is a fixed size. Under what conditions can both facts be true? In fact, it is not so hard. It just requires a bit of weirdness and egoism, like our “stationary” driver.
Ok, let’s do this:
Consider the first illustration below, which shows a universe expanding in two parts as seen by an outside observer. In the transition from A to B, the object-galaxies move apart from each other and the Universe expands to accommodate them. Transition B to C shows an outside observer backing up a bit to better see the now-larger Universe.
Now consider the second illustration, which is the same as the first but with state B omitted. One could somewhat wordily explain the phenomenon we see here as: a picture of an expanding universe from the perspective of an outside observer who is receding from the universe at a pace such that the angular size of the universe remains constant.
That is a lot to wrap your head around, so let us try another explanation: this is a picture of a universe of fixed size. The object-galaxies within it are shrinking from the perspective of the outside observer.
And thus is the crux of my claim: that our Universe, which we understand to be expanding, can be considered a constant size with its constituent atoms shrinking (relative to an observer outside our Universe).
So we are shrinking?
It is worth clarifying what “shrinking” means. Specifically, when something shrinks, are its number of constituent atoms reduced or changed in any way? No, atoms are shrinking only from the perspective of an observer outside our Universe, just as a passing car will shrink as it drives away. In fact, the only observable manifestation of this “shrinking” is the ever-widening void that separates most galaxies in our Universe.
The term…“unhinged”…comes to mind…when I hold a universe in the palm of my hand and watch the galaxies and atoms within it shrink. One could get weird and say “Oh, no, they have not gotten smaller at all: they have merely receded…into the 4th spatial dimension…they only appear shrinking because they are moving ever farther away.” Unhinged, I say, unhinged…
What does it all mean?
So what is the use of imagining the universe at fixed size? Most generally, it provides us a tool for visualizing—directly visualizing, not by metaphor—what 4th-dimensional space looks like.
Back in 2009, Radiolab put out a short called “DIY Universe,” that featured a conversation with Brian Greene talking about synthetic Universes. Back then, the media was abuzz with fears that the Large Hadron Collider could destroy the world by setting off another Big Bang, a Universe within our Universe. Greene and others were quite unconcerned with this, saying:
This Universe that you create would, in essence, create its own space. It wouldn’t encroach on your space by expanding into your domain…It would expand by creating new space, space that hadn’t existed before.
Isn’t this fascinating: that a little miniature Universe can sit on my kitchen table and expand into impossible vastness and not interfere with me at all. All the little physicists on all the little planets in this miniature Universe will proclaim their Universe expanding. Still, it rests here on my table, same size as it ever was and ever will be. They will wonder: “What is our universe expanding into?” And we respond: “You’re looking at it all wrong.”