Throughout history, as humans peered into the sky with telescopes, we gradually realized that in cosmic terms, we’re not particularly extraordinary. Initially, Earth was believed to be the universe’s center. This notion was debunked, and it turned out that Earth wasn’t even the center of our solar system. To add to the humility, our solar system isn’t central in the universe. Indeed, our system is just one among many that make up a galaxy, which itself is nothing unique but one among countless others, each with its own solar systems and possibly planets inhabited by beings who also consider themselves uniquely important.

This understanding of our ordinariness has become a cornerstone of cosmology, encapsulated in the “cosmological principle.” This principle posits that the universe is essentially uniform wherever we look—much like homogenized milk, with common materials spread evenly in every direction. At the highest levels, massive galaxy clusters coalesce into vast, matter-dense filaments and sheets that surround enormous intergalactic voids. Beyond this, the universe appears to taper off in complexity. From a distant perspective, the universe would appear remarkably smooth, according to Alexia Lopez of the University of Central Lancashire in England.

Lopez likens the universe to a beach: Zooming in on a handful of sand under a microscope, you’d notice each grain’s unique color, shape, and size. However, when viewed from a distance, walking along the beach, the sand melds into a consistent golden beige.

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Thus, Earth—or any of the countless other planets that likely exist—and its little cosmic neighborhood do not occupy any special status compared to the vast universe. This uniformity is advantageous for astronomers because it enables them to use parts of the universe to make reliable deductions about the whole. Whether in our Milky Way or a distant, unnamed galaxy billions of light years away, the prevailing conditions are expected to be roughly the same.

This principle simplifies many aspects of astronomy, from understanding how dark matter influences galaxy clusters to estimating how widespread life-supporting conditions might be across the cosmos. It facilitates the simplification of mathematical models about the universe’s history and future predictions. “Everything hinges on the assumption that the cosmological principle holds true,” Lopez remarks, “though it’s quite a vague hypothesis and incredibly difficult to verify.”

Verification becomes particularly challenging when significant evidence suggests otherwise—and recent observations hint that the universe might be weirder and more varied than previously assumed by cosmologists.

If true, humans—and potentially any other observers out there—might actually possess a somewhat unique view of the universe, not privileged but distinct, since “average” might not be a useful concept on vast cosmic scales. “Different observers might see slightly different universes,” especially at larger scales, suggests 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 aren’t ready to discard the cosmological principle just yet, but they are actively investigating its potential flaws. One method involves identifying structures so immense that they disrupt the assumed cosmic uniformity on a grand scale. Calculations suggest that any structure larger than about 1.2 billion light-years would challenge the notion of a homogenous universe.

And indeed, some such structures have been found. Lopez has identified an enormous formation known as the Giant Arc, a chain of galaxies stretching about 3.3 billion light-years. She also discovered the Big Ring, a torus of galaxies approximately 1.3 billion light-years in diameter and four billion light-years around. These two vast structures are in close proximity and might even be interconnected into a larger configuration.

Cosmology itself also provides reasons to question the cosmological principle. For instance, the cosmic microwave background—the leftover radiation from the big bang—displays large-scale anomalies that aren’t entirely random, according to Dragan Huterer, a cosmologist at the University of Michigan. “This has yet to be satisfactorily explained,” he notes.

Some scientists argue that such deviations might be accounted for by another concept known as cosmic variance, which deals with the statistical uncertainties inherent in astronomical measurements of the universe. Limited by what we can observe, we’re always somewhat uncertain about drawing conclusions from a restricted sample. Perhaps the variations seen by astronomers are merely due to this incompleteness rather than an actual characteristic of the universe; any perceived irregularities might disappear when compared with unobserved portions of the cosmos adjacent to what we can see.

Studying expansive areas of the universe presents significant challenges, as the observable universe has its limits. “If you decide to study galaxy shapes, you’re fortunate because there are billions of galaxies to analyze,” Huterer explains. However, on larger scales, the sample size dwindles because the observable universe is divided into only so many vast segments.

Marra once thought that certain cosmological discrepancies might be explained by cosmic variance, but recent calculations suggest this explanation is insufficient.

Yet, the majority of cosmic observations still align well with the cosmological principle. Thus, while scientists have sufficient data to question its validity, they are far from ready to abandon it, especially since no robust alternative theory has been presented.

“There’s no definitive evidence of a violation of the principle,” Huterer states. “However, there are some intriguing anomalies.”

Nonetheless, it’s a complex issue to unravel, given the nature of cosmology. “Unlike a lab experiment that can be repeated multiple times,” Huterer says, “we only have one universe to study.”