June 15, 2021 | David F. Coppedge

Galactic Structure Gives Cosmologists Lumps

How many times have we been told that “new physics” are needed to explain the universe?

Another record-breaking structure adds to the “lumpiness problem” in cosmology – the conundrum that our universe is not the smooth, homogeneous display one might expect from a big bang. Bas den Hond writes Tontologically in New Scientist: “Line of galaxies is so big it breaks our understanding of the universe.” Was your understanding broken, or just theirs?

An arc of galaxies that spans 3.3 billion light years – about 3.5 per cent of the observable universe – presents a big problem for one of the core tenets of cosmology.

Dubbed the Giant Arc by Alexia Lopez of the University of Central Lancashire, UK, who discovered the structure, it isn’t actually visible in the night sky – if it were, it would be as long as 20 full moons side by side.

Lopez detected the structure from spectra of quasars. She figured that if there were 15 Giant Arcs arranged along our line of sight, they would reach from earth to the edge of the observable universe. The observed structure, then, presents a time lumpiness problem as well as a space lumpiness problem. According to the assumed Cosmological Principle, the universe should look homogeneous (composed of similar shapes) and isotropic (the same in all directions) at scales of 1.5 billion light-years. The Giant Arc, though, is estimated to be 3.5 billion light-years wide. It adds to a collection of large-scale structures that don’t fit well with the cosmological principle, den Hond writes.

The new discovery intensifies the challenge that these present to the existing picture of how the universe evolved.

“If the cosmological principle doesn’t hold up, then our standard model of the universe kind of falls through,” says Lopez. “There are alternative theories that can maybe help explain large scale structures, but the standard model is founded on the cosmological principle being true; we have to have homogeneity and isotropy in the universe. It could have some severe implications.”

The standard model was going comfortably well until the Great Attractor was found, then the Great Wall, the Laniakei Supercluster, the Sloan Great Wall and other misfits. A previous article in New Scientist quoted skeptical comments in 2016 that some cosmologists were making about these superstructures. This latest misfit with theory keeps the pressure on.

So change the existing picture. Shouldn’t observations govern theory, not the other way around?

A famous cartoon by Sidney Harris mocks the religion of science.

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