In cosmology, there is one principle to rule them all: the aptly named cosmological principle. It insists that there are no special places in the cosmos – on large-enough scales, everything is roughly the same in all directions, from all observational perspectives. At first glance, this seems like it must be false: obviously some places in the universe are different from other places. The centre of the sun is nothing like a nebula floating in deep space; my office is highly unlike the event horizon of a black hole.
For it to be useful, we must zoom out – which is why modern statements of the cosmological principle tack on “at sufficiently large scales” to the front and then, mostly, call it a day. Sure, my office isn’t a black hole, and the sun isn’t a nebula (yet), but any supercluster of galaxies is just about the same as any other one, and the pattern of filaments and voids that these mammoth structures trace is the same in all directions. It is a cornerstone of our best model of the cosmos, called the standard model, also known as lambda-CDM.
Even so, a multitude of cosmologists have called this fundamental rule into question. And while some of those questions have been answered, not all of them can be brushed away so easily.
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The ones that are more easily resolved are the questions of isotropy, the idea that the view out into space in every direction, from every orientation, is equivalent. There is really only one big challenge to that rule: the cosmic microwave background (CMB) dipole. The CMB is relic radiation from the big bang, glowing faintly throughout the entire universe with a nearly uniform brightness. But it does seem to have what physicists call a dipole – one direction is colder than the other. Generally, that’s explained by attributing it to the motion of our galaxy through space, which would distort the light coming from the areas ahead of and behind us as the Milky Way moves.
Exceptions to the idea of homogeneity, which demands that everything is smooth rather than lumpy when viewed on large-enough scales, tend to be more threatening to the cosmological principle. Those threats generally come in the form of structures of unusual size, leading to names such as the Sloan Great Wall, the Huge Large Quasar Group, the Giant Arc or the Big Ring, all of which measure billions of light years across. It’s tough to consider our universe to be smooth when it does seem to have such enormous lumps in it. The “at sufficiently large scales” that gets inserted in the cosmological principle can only stretch so far, right?
In the early 2000s, astronomers started finding structures that were just a little bit too big to fit within the standard model. Not everyone agrees on what the limit ought to be, but the proposals are generally a couple hundred megaparsecs, or several hundred million light years – anything bigger than that is a problem, and there do definitely seem to be structures that are bigger. So, again and again, homogeneity across the universe has been called into question.
Personally, I’m not all that worried about the potential breakage of the cosmological principle itself. Some researchers say a little bit of clumpiness is actually OK within the standard model, some say the universe has expanded so much since its formation that we can’t actually expect the cosmological principle to hold anymore, and some simply say we have to look at an even larger scale. Any of those are plausible solutions, but the real question for me is: if I have to zoom out to such a degree to make a paradigm fit the real universe, is that still a useful paradigm?
Think about the example I gave at the beginning. My office isn’t a black hole – that is useful information to me every morning when I go into my office with zero concern about being spaghettified by extraordinary gravitational forces. In fact, not only is it useful that my office isn’t a black hole, it’s useful that my office isn’t my kitchen or my bathroom. Distinctions matter.
When it comes to the universe, those distinctions might not have such day-to-day significance – what do I care that our solar system happens to be in the Milky Way galaxy and not some other one? But they matter in terms of the big questions: how does the cosmos function, and are we alone in it? The cosmological principle lets us make one huge and necessary assumption: that where we are isn’t special, that what we observe from Earth is true everywhere. Without that assumption, the limits of our understanding are claustrophobically tight. If we want to crack on with studying the entire universe, we have to assume some level of homogeneity and isotropy. But just because an assumption is useful, or even necessary, that doesn’t make it true.
In science, we assume things all the time that aren’t necessarily true, largely to make calculations easier. Countless research papers are based on one-dimensional simulations of three-dimensional objects – and they do give us a deeper understanding, even if it’s not comprehensive. Every distance we measure to any object in space (and even the distance scale for the cosmological principle itself) assumes a value for the expansion rate of the universe, even though we don’t quite know what it is. We can accept something as close enough to truth to be useful, while knowing it’s not quite all the way correct.
So, do I believe in the cosmological principle, this foundation upon which our understanding of the universe is built? No, not really. The cosmos is too huge and too full of variety for me to completely buy into the idea that there are no unique, special places. It brings to mind a few lines from Hamlet: “What a piece of work is a man! how noble in reason! how infinite in faculty!” What a piece of work, too, is the cosmos and its contents, and how seemingly infinite in faculty.
It’s infinite enough, at least, that the cosmological principle can be both true enough and untrue. There is life in our corner of the universe, so there is probably life elsewhere as well, because in some ways, the universe is the same everywhere, in every direction. Nevertheless, the universe is also different everywhere. My office isn’t a black hole, and the aliens probably aren’t reading Hamlet, no matter how noble in reason they may be. There are special places in the cosmos, and we are in one of them, and only by holding both that and the cosmological principle in our heads at once can we truly understand our place in the universe.
