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Gaping “hole” in the cosmos detected

Aug. 23, 2007
Courtesy University of Minnesota
and World Science staff

As­tro­no­mers say they’ve ap­par­ently found a gi­ant hole in the un­iverse—a prac­tic­ally emp­ty zone, called a void, whose gap­ing size is hard to ex­plain.

While past stud­ies had re­vealed oth­er voids, this one dwarfs them all, re­search­ers say, be­ing nearly a bil­lion light-years across. A light-year is the dis­tance light trav­els in a year, some six tril­lion miles or nine tril­lion km.

Diagram of the ef­fect of in­ter­ven­ing mat­ter on the view from Earth of the cos­mic back­ground ra­di­a­tion. Above, the ra­di­a­tion is re­leased short­ly af­ter the Big Bang, with ti­ny rip­ples in tem­per­a­ture due to fluc­tu­a­tions in the ear­ly uni­verse. As this ra­di­a­tion crosses the cos­mos, filled with a web of ga­lax­ies and oth­er mat­ter, it un­der­goes slight per­tur­ba­tions. In the di­rec­tion of a huge, new­found void, the WMAP sat­el­lite (bot­tom left) sees a cold spot; the Very Large Ar­ray, a ra­di­o tel­e­scope (bot­tom right) sees few­er ga­lax­ies. (Cred­it: Bill Sax­ton, NRAO/AUI/NSF, NA­SA)


“We nev­er even ex­pected to find [a void] this size,” said Law­rence Rud­nick, an as­tron­o­mer at the Un­ivers­ity of Min­ne­so­ta in Min­ne­ap­o­lis, Minn. It’s “not nor­mal, based on ei­ther ob­serva­t­ional stud­ies or on com­put­er sim­ula­t­ions of the large-scale ev­o­lu­tion of the un­iverse,” added the un­ivers­ity’s Li­liya Wil­liams. 

She, Rud­nick and a grad­u­ate stu­dent re­port the find­ings in a pa­per to ap­pear in the re­search pub­lica­t­ion As­t­ro­phys­i­cal Jour­nal

Cos­mic voids are ar­eas lack­ing both nor­mal ma­te­ri­al, such as stars, ga­lax­ies and gas, and the mys­te­ri­ous “dark mat­ter” that is al­so com­mon in the un­iverse. Voids seem to be rar­er the big­ger they are, as­tron­o­mers said. 

The new find­ing was based on da­ta from a sky sur­vey of the Na­t­ional Ra­di­o As­tronomy Ob­ser­va­to­ry’s Very Large Ar­ray tel­e­scope in So­cor­ro, N.M. Re­search­ers found a re­mark­a­ble drop in the num­ber of ga­lax­ies in a re­gion of sky in the con­stella­t­ion Erid­a­nus, south­west of the con­stella­t­ion Ori­on.

“We al­ready knew there was some­thing dif­fer­ent about this spot,” Rud­nick said: it was dubbed the “WMAP Cold Spot,” be­cause it stood out as un­usu­ally cold in a map of the back­ground radia­t­ion that per­me­ates the cos­mos. This radia­t­ion—a rem­nant of the Big Bang ex­plo­sion thought to have giv­en birth to the un­iverse—was mapped us­ing a sat­el­lite called WMAP, for Wilkin­son Mi­cro­wave Ani­so­tropy Probe.

In a sense, to ob­serve these back­ground rays is to look at what could be called the sur­face of the Big Bang fire­ball, though the eons since then have dis­tort­ed the view. Faint ir­reg­u­lar­i­ties in the temp­er­a­ture of the radiation across the sky are be­lieved to trace struc­tures that ex­isted in the un­iverse’s in­fan­cy. 

The “cold spot” can now be explained by the dearth of ga­lax­ies in that area, re­search­ers said. “Although our sur­pris­ing re­sults need in­de­pend­ent con­firma­t­ion, the slightly low­er tem­per­a­ture of the [radia­t­ion] in this re­gion ap­pears to be caused by a huge hole de­void of nearly all mat­ter roughly 6-10 bil­lion light-years from Earth,” Rud­nick said.

How does a void make the background radia­t­ion colder as seen from Earth? The an­swer, re­search­ers said, lies in the so-called “dark en­er­gy,” a force that be­came dom­i­nant in the Un­iverse only re­cently in as­tro­nom­i­cal time. Sci­en­tists don’t know what dark en­er­gy is, but it seems to work op­po­site gra­vity and to speed up an on­go­ing ex­pan­sion of the Un­iverse. (Dark en­er­gy is some­thing dis­tinct from dark mat­ter—anoth­er en­ig­mat­ic sub­stance that as­tron­o­mers rec­og­nize thanks to its ef­fect on oth­er ob­jects, but which they can’t ac­tu­ally find.)

Thanks to dark en­er­gy, radia­t­ion that passes through a large void just be­fore reach­ing us has less en­er­gy than oth­er radia­t­ion does, re­search­ers say. 

With­out dark en­er­gy, rays ap­proach­ing a large mass, such as a clus­ter of ga­lax­ies, would gain en­er­gy from their gra­vity, which draws them in, Rud­nick ex­plained. As the rays leave the ar­ea, the gra­vity pulls back on them, sap­ping their en­er­gy. They wind up with the same en­er­gy with which they started.

But since dark en­er­gy be­came dom­i­nant, he said, rays cross­ing mat­ter-rich space don’t re­turn to their orig­i­nal en­er­gy lev­el—be­cause dark en­er­gy coun­ter­acts gra­vity. Thus, these pho­tons ar­rive at Earth with a slightly high­er en­er­gy, or tem­per­a­ture, than they would other­wise. This phe­nom­e­non does­n’t oc­cur when light rays cross a large void, the sci­en­tists added, so they reach us with less en­er­gy.


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Astronomers say they’ve apparently found a giant hole in the Universe—a practically empty void whose gaping size is hard to explain. While past studies had revealed other holes, or voids, this one dwarfs them all, researchers say, being nearly a billion light-years across. A light-year is the distance light travels in a year, some six trillion miles or nine trillion km. “We never even expected to find one this size,” said Lawrence Rudnick, an astronomer at the University of Minnesota in Minneapolis, Minn. It’s “not normal, based on either observational studies or on computer simulations of the large-scale evolution of the Universe,” added the university’s Liliya Williams. She, Rudnick and a graduate student reported the findings in a paper accepted for publication in the research publication Astrophysical Journal. Cosmic voids are areas devoid of both normal material, such as stars, galaxies and gas, and the mysterious “dark matter” that is also common in the universe. Voids seem to be rarer the bigger they are, astronomers said. The new finding was based on data from a sky survey of the National Radio Astronomy Observatory’s Very Large Array telescope in Socorro, N.M. Researchers found a remarkable drop in the number of galaxies in a region of sky in the constellation Eridanus, southwest of the constellation Orion. “We already knew there was something different about this spot,” Rudnick said: it was dubbed the “WMAP Cold Spot,” because it stood out as unusually cold in a map of the background radiation that permeates the cosmos. This radiation—a remnant of the Big Bang explosion thought to have given birth to the universe—was mapped using a satellite called WMAP, for Wilkinson Microwave Anisotopy Probe. In a sense, to observe these background rays is to look at what could be called the surface of the Big Bang fireball, though the eons since then have distorted the view. Faint irregularities in the view trace structures that existed in the universe’s infancy. The dearth of galaxies in that region explains the cold spot, researchers said. “Although our surprising results need independent confirmation, the slightly lower temperature of the [radiation] in this region appears to be caused by a huge hole devoid of nearly all matter roughly 6-10 billion light-years from Earth,” Rudnick said. How does a void make the Big Bang’s remnant radiation colder as seen from Earth? The answer, researchers said, lies in the so-called “dark energy,” a force that became dominant in the Universe only recently in astronomical time. Scientists don’t know what dark energy is, but it seems to work opposite gravity and to speed up an ongoing expansion of the Universe. (Dark energy is something separate from “dark matter”—another enigmatic substance that astronomers recognize thanks to its effect on other objects, but which they can’t actually find.) Thanks to dark energy, radiation that passes through a large void just before reaching us has less energy than other radiation does, researchers say. Without dark energy, rays approaching a large mass, such as a cluster of galaxies, would gain energy from its gravity, which draws them in. As they leave the area, the gravity pulls back on them, sapping their energy. They wind up with the same energy with which they started. But since dark energy became a dominant, rays crossing matter-rich space don’t return to their original energy level, because dark energy counteracts gravity. Thus, these photons arrive at Earth with a slightly higher energy, or temperature, than they would in a dark energy-free Universe. This phenomenon doesn’t occur when light rays cross a large void, the scientists added, so they reach us with less energy.