"Long before it's in the papers"
January 11, 2013

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Astronomers identify structure so huge it disrupts cosmic uniformity

Jan. 11, 2013
Courtesy of the Royal Astronomical Society
and World Science staff

As­tro­no­mers have found a clump of bright ga­lax­ies so huge, it jolts their long­stand­ing be­lief that the uni­verse is pret­ty un­iform on large scales.

And the sci­en­tists warn that more such dis­cov­eries may be on the way.

The green cir­cles mark the po­si­tions of quasars in the "Large Qua­sar Group" that as­tro­no­mers say is now the larg­est known struc­ture an­y­where. The red "plus" signs mark the mem­bers of an­oth­er, small­er Large Qua­sar Group. The map co­vers around 29.4 by 24 de­grees on the sky in the ce­les­tial equiv­a­lents of lon­gi­tude and lat­i­tude—right as­cen­sion and dec­li­na­tion.

Called the “large qua­sar group,” the clump is said to be the uni­verse’s larg­est known struc­ture, so im­mense that a space­ship trav­el­ing at light speed would take some four bil­lion years to cross it. 

Mea­sure­ments show it co­vers an only slightly smaller frac­tion of our sky as does a con­stella­t­ion that lies in front of it—Leo, the Li­on—though it’s mil­lions of times fur­ther off than Leo’s stars.

“We can say quite def­i­nitely it is the larg­est struc­ture ev­er seen,” said Rog­er Clowes of the Uni­vers­ity of Cen­tral Lan­ca­shire in the U.K. “This is hugely ex­cit­ing—not least be­cause it runs count­er to our cur­rent un­der­stand­ing of the scale of the uni­verse.” 

The find­ings by Clowes and col­leagues are pub­lished in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty, and also post­ed on­line here.

The struc­ture is an ag­glomera­t­ion of qua­sars, cores of dis­tant and an­cient ga­lax­ies that dur­ing some pe­ri­ods shine so brightly that they’re vis­i­ble well over half­way across the uni­verse.

As­tro­no­mers have not­ed since 1982 a ten­den­cy for qua­sars to group to­geth­er. Huge con­centra­t­ions of mat­ter such as this are thought to pos­sibly be out­growths of smaller struc­tures that char­ac­ter­ized the uni­verse in its in­fan­cy.

The newly found clump is an es­ti­mat­ed 1,600 times wid­er than the dis­tance be­tween our Milky Way gal­axy and the neigh­bor­ing An­drom­e­da Gal­axy. Such a size, the re­search­ers said, is suf­fi­cient to pos­sibly vi­o­late the “Cos­mo­log­i­cal Prin­ci­ple”—the as­sump­tion ac­cept­ed by most cos­mol­o­gists that the uni­verse, seen at a large enough scale, looks the same from any view­point. Most pre­vi­ous ob­serva­t­ional ev­i­dence sup­ports the prin­ci­ple, but it has nev­er been prov­en.

The Cosmological Principle is a philosophical as well as a scientific state­ment. It’s an out­growth of the con­vic­tion that peo­ple don’t occupy a un­ique place in the grand scheme of things—the very idea that brought sci­ent­ists such as Gali­leo and Cop­ern­i­cus into con­flict with Church auth­or­ities in their time.

 Cal­cula­t­ions based on the mod­ern the­o­ry of cos­mol­o­gy, draw­ing on Al­bert Ein­stein’s work, sug­gest an­ything larg­er than about 1.2 bil­lion light-years across would vi­o­late the Cos­mo­lo­g­i­cal Prin­ci­ple, said Clowes and col­leagues. (A light-year is the dis­tance light trav­els in a year.)

This qua­sar group ranges in size from one-third larg­er than that crit­i­cal size, to more than three times larg­er, de­pend­ing on where its width is meas­ured. It lies about 8.7 bil­lion light-years away. It’s “dif­fi­cult to fath­om the scale” of the struc­ture, Clowes said. But “our team has been look­ing at si­m­i­lar cases which add fur­ther weight to this chal­lenge [to the Cos­mo­lo­g­i­cal Prin­ci­ple] and we will be con­tin­u­ing to in­ves­t­i­gate these fas­ci­nat­ing phe­nom­e­na.”

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Astronomers have found a clump of bright galaxies so huge, it challenges their longstanding belief that the universe is pretty uniform on large scales. And the scientists warn that more such findings may be yet to come. Called the “large quasar group,” the clump is said to be the universe’s largest known structure, so immense that a spaceship travelling at light speed would take some four billion years to cross it. Measurements show it covers an only slightly smaller fraction of our sky as does a constellation that lies in its direction—Leo, the Lion—though it’s millions of times further off than Leo’s stars. “We can say quite definitely it is the largest structure ever seen,” said Roger Clowes of the University of Central Lancashire in the U.K. “This is hugely exciting—not least because it runs counter to our current understanding of the scale of the universe.” The findings by Clowes and colleagues are published in the journal Monthly Notices of the Royal Astronomical Society. The structure is an agglomeration of quasars, cores of distant and ancient galaxies that during some periods shine so brightly that they’re visible well over halfway across the universe. Astronomers have noted since 1982 a tendency for quasars to group together. Huge concentrations of matter such as this are thought to possibly be outgrowths of smaller structures that characterized the universe in its infancy. The newly found clump is an estimated 1,600 times wider than the distance between our Milky Way galaxy and the neighboring Andromeda Galaxy. Such a size, the researchers said, is sufficient to possibly violate the “Cosmological Principle”—the assumption accepted by most cosmologists that the universe, seen at a large enough scale, looks the same from any viewpoint. Most previous observational evidence supports the principle, but it has never been proven. Calculations based on the modern theory of cosmology, drawing on Albert Einstein’s work, suggest anything larger than about 1.2 billion light-years across would violate the Cosmological Principle, said Clowes and colleagues. (A light-year is the distance light travels in a year.) This quasar group ranges in size from one-third larger than that critical size, to more than three times larger, depending on where its width is measured. It lies about 8.7 billion light-years away. It’s “difficult to fathom the scale” of the structure, Clowes said. But “our team has been looking at similar cases which add further weight to this challenge [to the Cosmological Principle] and we will be continuing to investigate these fascinating phenomena.”