Ever since humans started gazing at the heavens through telescopes, we have discovered, bit by bit, that in celestial terms we’re apparently not so special. Earth was not the center of the universe, it turned out. It wasn’t even the center of the solar system! The solar system, unfortunately, wasn’t the center of the universe either. In fact, there were many star systems fundamentally like it, together making up a galaxy. And, wouldn’t you know, the galaxy wasn’t special but one of many, which all had their own solar systems, which also had planets, some of which presumably host their own ensemble of egoistic creatures with an overinflated sense of cosmic importance.
This notion of mediocrity has been baked into cosmology, in the form of the “cosmological principle.” Its gist is that the universe is basically the same everywhere we look—homogenized like milk, made of common materials evenly distributed in every direction. At the top of the cosmic hierarchy, giant groups of galaxies clump into sprawling, matter-rich filaments and sheets around gaping intergalactic voids, but past that, structure seems to peter out. If you could zoom way out and look at the universe’s big picture, says Alexia Lopez of the University of Central Lancashire in England, “it would look really smooth.”
Lopez compares the cosmos with a beach: If you plunked a handful of sand under a microscope, the sand grains would look like the special individuals they are. “You would see the different colors, shapes and sizes,” she says. “But if you were to walk across the beach, looking out at the sand dunes, all you would see is a uniform golden beige color.”
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That means Earth (or any of the other trillions of planets that must exist) and its tiny corner of the cosmos appear to hold no particularly privileged place in comparison to everything else. And this homogeneity is convenient for astronomers because it lets them look at the universe in part as a reliable way of making inferences about the whole; whether here in the Milky Way or in a nameless galaxy billions of light-years distant, prevailing conditions should be essentially the same.
This simplifying ethos applies to everything from understanding how dark matter weighs down galaxy clusters to estimating how common life-friendly conditions might be throughout the cosmos, and it allows astronomers to simplify their mathematical models of the universe’s past as well as their predictions of its future. “Everything is based on the idea that [the cosmological principle] is true,” Lopez says. “It is also a very vague assumption. So it’s really hard to validate.”
Validation is especially challenging when significant evidence exists to the contrary—and a host of recent observations suggest indeed that the universe could be stranger and have larger variations than cosmologists had so comfortably supposed.
If that’s the case, humans (and anyone else out there) actually might have a sort of special view of the light-years beyond—not privileged, per se, but also not average, in that “average” would no longer even be a useful concept at sufficiently large scales. “Different observers may see slightly different universes,” at least at large scales, says Valerio Marra, a professor at the Federal University of Espírito Santo in Brazil and a researcher at the Astronomical Observatory of Trieste in Italy.
Astronomers haven’t thrown out the cosmological principle just yet, but they are gathering clues about its potential weaknesses. One approach involves looking for structures so large they challenge cosmic smoothness even at a hugely wide zoom. Scientists have calculated that anything wider than about 1.2 billion light-years would upset the homogeneous cosmic apple cart.
And astronomers have found some. Lopez, for instance, has discovered a beast called the Giant Arc—a curve of galaxies strung across some 3.3 billion light-years. She’s also discovered the Big Ring, a torus of galaxies about 1.3 billion light-years across and four billion around. The two strangenesses are close together and may themselves be connected into an even larger structure.
The study of cosmology itself also gives reason to raise an eyebrow at the cosmological principle. For instance, the light leftover from the big bang, called the cosmic microwave background, has some mysterious large-scale fluctuations that don’t look totally random, notes Dragan Huterer, a cosmologist at the University of Michigan. “This was never satisfactorily explained,” he says.
Some scientists have argued that such potential challenges to the cosmological principle might be explained by another principle, cosmic variance, which refers to the statistical uncertainty inherent in astronomers’ measurements of the universe. We are always limited by what we can see and therefore always mathematically uncertain about what conclusions to draw from a limited sample. Maybe the variations astronomers saw were simply the result of incompleteness rather than a real reflection of the universe’s properties; perhaps what seems to be an anomalous bump in cosmic smoothness would flatten out when compared with an unobserved chunk of the cosmos adjacent to the volume visible to us.
And when it comes to studying suitably large patches of the universe, cosmologists are very limited indeed: the observable universe is only so big. “If you say, ‘I’m going to study the shapes of galaxies,’ well, lucky you: you have billions and billions of galaxies in the universe. You can address your questions with statistics, and your sample variance will be very small,” Huterer says. On larger scales, you only get a few examples because the observable universe only splits into so many big sections.
Marra thought for a while that certain cosmological discrepancies could be a result of cosmic variance. But it’s not enough to explain it anymore, according to his and others’ calculations.
Still, most cosmic observations hold up against the cosmological principle very well. So while scientists have enough information to reasonably question the idea’s validity, they aren’t at all ready to abandon it—least of all because no one has a solid alternative schema to replace it.
“There’s no smoking-gun evidence for the violation of principle,” Huterer says. “However, there are some very interesting anomalies.”
Regardless, it’s a hard problem to decipher because of the nature of cosmology. “Unlike in some lab experiment that you can do over and over and over again,” Huterer says, “you only get one universe.”
