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Something beyond visible universe detected?

Sept. 23, 2008
Courtesy NASA Goddard Space Flight Center
and World Science staff

Sci­en­tists have meas­ured an un­ex­pected mo­tion in dis­tant clus­ters of ga­lax­ies—pos­sibly caused, they say, by the gravita­t­ional pull of some­thing out­side the vis­i­ble uni­verse. “We nev­er ex­pected to find an­y­thing like this,” said lead re­search­er Al­ex­an­der Kash­lin­sky of NASA’s God­dard Space Flight Cen­ter in Green­belt, Md.

The gal­axy clus­ter 1E 0657-56 (known as the Bul­let Clus­ter) lies 3.8 bil­lion light-years away. It's one of hun­dreds that ap­pear to be car­ried along by a mys­te­ri­ous cos­mic flow, astro­phys­i­cists say. (Cred­it: NA­SA/STScI/Mag­ellan/D. Clowe et al.)

can see only those areas of the cos­mos close enough that their light could have reached us dur­ing our uni­verse’s ex­ist­ence. What lies past those lim­its, if an­y­thing, has been un­clear. 

Kash­linksy and col­leagues sug­gest what­ev­er is pulling on the mys­te­ri­ously mov­ing gal­axy clus­ters might lie out­side the vis­i­ble uni­verse.

The clus­ters, how­ev­er, lie much clos­er to us than to those vis­i­ble lim­its. There­fore, it’s not cer­tain that the mys­tery at­trac­tor is out­side that zone. How­ev­er, that’s “quite pos­si­ble,” the re­search­ers wrote, be­cause sup­pos­ing oth­er­wise forc­es the un­likely con­clu­sion that a fair chunk of the cos­mos, our area, is atyp­i­cal. 

A re­port on the find­ings is to ap­pear this week in the elec­tron­ic edi­tion of As­t­ro­phys­i­cal Jour­nal Let­ters.

“The clus­ters show a small but meas­ur­a­ble ve­locity that is in­de­pend­ent of the uni­verse’s ex­pan­sion,” Kash­lin­sky said.

The re­sults are based on da­ta from a NASA sat­el­lite, the Wilkin­son Mi­cro­wave An­i­sot­ro­py Probe. The de­vice takes mea­sure­ments of a sub­tle glow of radia­t­ion per­vad­ing the uni­verse, the cos­mic mi­cro­wave back­ground. It’s be­lieved to be left­o­ver light from the Big Bang, a sort of ex­plo­sion that gave birth to our uni­verse.

Hot, ra­di­at­ing gas in a gal­axy clus­ter scat­ters this back­ground light, as­tro­no­mers say. The scat­ter­ing can be meas­ured to de­tect each clus­ter’s in­di­vid­ual mo­tion, al­though the sig­nal is very weak, mak­ing the meas­ure­ment hard to dis­en­tan­gle from oth­er ef­fects. 

Kash­lin­sky teamed up with oth­ers to iden­ti­fy some 700 clus­ters that could be used to de­tect the ef­fect. The as­tro­no­mers de­tected bulk clus­ter mo­tions of nearly two mil­lion miles per hour, to­ward a 20-degree patch of sky be­tween the con­stella­t­ions of Cen­tau­rus and Ve­la. Their mo­tion was found to be con­stant out to at least about one-tenth of the way to the edge of the vis­i­ble uni­verse.

Hot gas in mov­ing gal­axy clus­ters (white spots) shifts the tem­per­a­ture of cos­mic mi­crowaves. Hun­dreds of dis­tant clus­ters seem to be mov­ing to­ward one patch of sky (pur­ple zone), astro­phys­i­cists say. Cred­it: NA­SA/WMAP/A. Kash­lin­sky et al.)

Kash­lin­sky calls this col­lec­tive mo­tion a “dark flow,” in ana­logy with more fa­mil­iar cos­mo­lo­g­i­cal mys­ter­ies: dark en­er­gy and dark mat­ter. “The dis­tri­bu­tion of mat­ter in the ob­served uni­verse can­not ac­count for this mo­tion,” he said.

The find­ing con­tra­dicts con­ven­tion­al the­o­ries, which de­scribe such mo­tions as de­creas­ing at ev­er great­er dis­tances: large-scale mo­tions should show no par­tic­u­lar di­rec­tion rel­a­tive to the back­ground. 

But a the­o­ry called infla­t­ion of­fers a so­lu­tion, the phys­i­cists said. Infla­t­ion is a brief hy­per-ex­pan­sion that would have oc­curred right af­ter the Big Bang. The re­sult would be that we can see only a lit­tle of the cos­mos, be­cause of how far and fast dif­fer­ent parts of the uni­verse burst away from each oth­er early on.

Da­ta re­leased in 2006 sup­ported the infla­t­ion idea, Kash­lin­sky said. The new find­ings “may give us a way to ex­plore the state of the cos­mos be­fore infla­t­ion oc­curred.”

The next step, the in­ves­ti­ga­tors said, is to sharp­en the mea­sure­ments. “We need a more ac­cu­rate ac­count­ing of how the mil­lion-degree gas in these gal­axy clus­ters is dis­tribut­ed,” said Fer­nan­do Atrio-Ba­ran­dela of the Uni­ver­s­ity of Sal­a­man­ca, Spain, one of the re­search­ers. “We’re as­sem­bling an even larg­er and deepe­r cat­a­log of X-ray clus­ters,” added anoth­er, the Uni­ver­s­ity of Hawaii’s Har­ald Ebel­ing.

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Scientists have measured an unexpected motion in distant clusters of galaxies—possibly caused, they say, by the gravitational pull of something outside the visible universe. “We never expected to find anything like this,” said lead researcher Alexander Kashlinsky of NASA’s Goddard Space Flight Center in Greenbelt, Md. Telescopes can see only those parts of the universe that are close enough that their light could have reached us during our universe’s existence. What lies past those limits, if anything, has been unclear. Kashlinksy and colleagues suggest whatever is pulling on the mysteriously moving galaxy clusters might lie outside the visible universe. The clusters, however, lie much closer to us than to those visible limits. Therefore, it’s not certain that the mystery attractor is outside that zone. However, that’s “quite possible,” the researchers wrote, because supposing otherwise forces the unlikely conclusion that a fair chunk of the cosmos, our part, is atypical. A report on the findings is to appear this week in the electronic edition of Astrophysical Journal Letters. “The clusters show a small but measurable velocity that is independent of the universe’s expansion,” Kashlinsky said. The results are based on data from a NASA satellite, the Wilkinson Microwave Anisotropy Probe. The device takes measurements of a subtle glow of radiation pervading the universe, the cosmic microwave background. It’s believed to be leftover light from the Big Bang, a sort of explosion that gave birth to our universe. Hot, radiating gas in a galaxy cluster scatters this background light, astronomers say. The scattering can be measured to detect each cluster’s individual motion, although the signal is very weak, making the measurement hard to disentangle from other effects. Kashlinsky teamed up with others to identify some 700 clusters that could be used to detect the effect. The astronomers detected bulk cluster motions of nearly two million miles per hour, toward a 20-degree patch of sky between the constellations of Centaurus and Vela. Their motion was found to be constant out to at least a billion light-years away, about one-fourteenth of the way to the edge of the visible universe. Kashlinsky calls this collective motion a “dark flow” in the vein of more familiar cosmological mysteries: dark energy and dark matter. “The distribution of matter in the observed universe cannot account for this motion,” he said. The finding contradicts conventional theories, which describe such motions as decreasing at ever greater distances: large-scale motions should show no particular direction relative to the background. But a theory called inflation offers a solution, the physicists said. Inflation is a brief hyper-expansion that would have occurred right after the Big Bang. The result would be that we can see only a little of the cosmos, because of how far and fast different parts of the universe burst away from each other early on. Data released in 2006 supported the inflation idea, Kashlinsky said. The new findings “may give us a way to explore the state of the cosmos before inflation occurred.” The next step, the investigators said, is to sharpen the measurements. “We need a more accurate accounting of how the million-degree gas in these galaxy clusters is distributed,” said Fernando Atrio-Barandela of the University of Salamanca, Spain, one of the researchers. “We’re assembling an even larger and deeper catalog of X ray clusters to better measure the flow,” added another, the University of Hawaii’s Harald Ebeling.