The cosmos’s homogeneity and isotropy make it simple enough to analyze. Matter is evenly distributed along every line of sight, indicating that the universe is “isotropic.” Moreover, telescopes pointed in different directions have all seen similar scenes. (Think of the expanding universe as a rising fruitcake with galaxies evenly spread like fruit pieces, each one flying away from its neighbors as the batter between them expands.) This suggests that the cosmos is homogeneous, with matter sprinkled smoothly throughout. As powerful telescopes peered deeper into the darkness, they saw more-distant galaxies appearing in similar numbers. Over the past century, astronomical surveys solidified what’s become known as the cosmological principle in two ways. Not only is Earth not special, but nothing anywhere is special. “We’re not special,” said Andrew Howell, a cosmologist at Las Cumbres Observatory and the University of California, Santa Barbara. The following century, they spotted countless galaxies beyond our own. Astronomers proved in the 1800s that the sun is just a normal star. His insight that Earth orbits the sun rather than the other way around set off a series of humbling shifts in perspective. The cosmological principle grew out of the Copernican principle, Nicolaus Copernicus’ 1543 realization that Earth is not the fixed center of creation. We are “trying to poke as many holes as we can,” said Seshadri Nadathur, a cosmologist at University College London, “while being very skeptical of someone else saying they’ve poked a hole.” From Copernicus to Einstein So far, though, each new claim of a too-big structure or other anomaly has failed to make a dent. Most everyone agrees that the cosmological principle is worth scrutinizing. Others are maverick cosmologists unsettled by the consensus view that most of the stuff in the cosmos hides from our instruments in the form of “dark matter” and “dark energy” they wonder whether theorists may have conjured phantasms to patch up an overly simplistic theory of cosmology. Some challengers, like Lopez and her colleagues, are astrophysicists puzzled by striking celestial conglomerations. “If that turns out to be wrong, then we have to redo many of our measurements or reinterpret many of our measurements,” said Ruth Durrer, a cosmologist at the University of Geneva.Īs a load-bearing strut of modern cosmology, the cosmological principle has increasingly become a target. This assumption, enshrined as the “cosmological principle,” has let researchers draw sweeping conclusions about the whole universe based only on what we see from our corner of it. From a zoomed-out perspective, no matter where you are or which way you look, you should see roughly the same number of galaxies pinwheeling around. Lopez’s “Giant Arc” seemed to clash with an idea that has guided astronomy for centuries: that the universe has no conspicuous features. “It’s so big that it’s hard to explain with our current beliefs,” Lopez, one of the astrophysicists at the University of Central Lancashire who identified the galaxy chain, said during the presentation. Spanning an estimated 3.3 billion light-years of space, the smile-shaped structure joined a controversial club: unexpectedly big things. The arc of distant galaxies, which Alexia Lopez presented at the American Astronomical Society’s meeting in June, sprawls so far across the sky that it would take 20 full moons to hide it. The latest attempt to rattle the foundations of cosmology appeared as a smattering of dots pulled upward into a cosmic sneer.
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