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From our earthly porch, the cosmos seems prettily jewelled with stars and galaxies. Zoom out, however, and it should become an odyssey of omnidirectional dullness: an endless dark expanse in which galaxies shrink to pale dots, and that looks identical in every direction.
That assumption that the universe is evenly sprinkled with matter and appears the same to all observers, known as the cosmological principle, is now in dispute.
Last week, a PhD student in England revealed that she had discovered a giant necklace of galaxies stretching across 1.3bn light years. This so-called Big Ring of galaxies and galaxy clusters joins an expanding catalogue of mammoth structures that defy scientific expectation. Taken with other perplexing observations, it suggests the Standard Model of physics, in which the cosmological principle plays a supporting role, might not be the final word on how the universe came to be as it is today.
As fascinating as the finding itself, is the mixed reaction among the cosmological cognoscenti. “Lots of people are excited but, having said that, you do get this [resistant] attitude in cosmology that you don’t generally find elsewhere in science,” Alexia Lopez, the University of Central Lancashire student who discovered the Big Ring, told me on Monday. “Good science should be about pushing back and testing our fundamental assumptions but there are clearly people who want to protect the Standard Model.”
Perhaps this is because there is no more fundamental question in science than how the universe began — and anything that rattles our understanding leaves an unsettling scientific and philosophical question mark over our very existence.
The Big Ring was detected as Lopez analysed the gas absorption in the measured light from distant quasars, which are some of the most luminous objects in the universe. One particular gas, a form of ionised magnesium known as MgII, is linked to galaxies and galaxy clusters. “It’s a really obvious, detectable feature in the quasar spectra,” Lopez says of the magnesium absorption, explaining that the method can reveal faraway galaxies that would otherwise go unseen.
Just as a cardboard cut-out will create a silhouette on a backlit screen, the Big Ring materialised into view as the MgII gas around its galaxies absorbed some of the light coming from the quasars behind. If it were visible from Earth, the ring’s diameter would be equivalent to 15 moons.
Lopez — whose collaborators include Central Lancashire colleague Roger Clowes and Gerard Williger, from the University of Louisville, Kentucky — presented her finding at a meeting of the American Astronomical Society in New Orleans last week. She is now writing it up as a scientific paper, where it will be properly peer-reviewed. In 2021, she discovered a similar superstructure called the Giant Arc using the same method. That scythe-shaped curve of galaxies is bigger than the Big Ring, stretching more than 3bn light-years nose to tail.
Those dimensions matter: both exceed 1.2bn light years in size, sometimes referred to as the homogeneity scale. That is about the minimum distance, Lopez explains, over which the universe should look uniform — and now doesn’t. The Big Ring and Giant Arc join other supersized flies in the Standard Model ointment, including “great walls” of galaxy superclusters. The biggest is the Hercules-Borealis Corona Great Wall, lying about 10bn light years away and spanning a whopping 10bn light years across.
Is the homogeneity scale too small, then? Even if it was enlarged, Lopez says, it would still leave a puzzle over how such vast galactic structures could form so quickly in the early universe.
Improbable superstructures, which some researchers suggest could arise simply as a statistical fluke, are not the only deviations from the Standard Model. Another is the surprising inconstancy of the Hubble constant, a number related to the expansion of the universe. As anomalies rack up, the Standard Model begins to look, well, substandard.
Cosmic strings, theorised to be defects in the fabric of space-time, have been floated as one possible fix. They can be thought of as “cracks” opening up in the formation of the early universe, similar to the cracks that appear in ice as water freezes over. These blips might allow oddities such as galactic great walls to form.
As for the Big Ring, it more closely resembles a spiral. As cosmologists struggle to make sense of it all, there is something almost poetic about the Standard Model being loosened by a colossal galactic corkscrew.
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