A study of nearby galaxy clusters has failed to detectdistortions in the ancient microwave radiation many scientists have linked tothe creation of our universe.
The finding could cast doubt on the entire Big Bang theory,but other experts have serious misgivings about the results. They say the spaceprobe used in the study is not the ideal instrument for detecting thedistortions and that the discrepancy is less a reflection of problems with theBig Bang theory than of how little scientists really know about galaxyclusters.
The controversial study, led by Richard Lieu at the University of Alabama in Huntsville,is detailed in the Sept. 1 issue of the AstrophysicalJournal.
Big Bang echo
The cosmicmicrowave background (CMB) radiation is a faint afterglow permeating theuniverse. Many scientists have hailed CMB observations as strong evidence forthe Big Bang.
A shadow effect called Sunyaev-Zel'dovichis a distortion that affects CMB photonsinside galaxy clusters. It occurs when high-energy electrons inside theclusters crash into the more sluggish CMBphotons, boosting them to higher energy levels.
This shifts the CMB spectrum inside clusters from low to higherenergies. The dearth of low-energy microwaves inside the clusters means thatinstruments like the WilkinsonMicrowave Anisotropy Probe (WMAP) should see fewer low-energy CMB photonsinside clusters compared to outside.
"That's the shadow effect—that at lower energy yousee the CMB has a decrement in the direction of the cluster," explained Niayesh Afshordi, anastrophysicist at Harvard Universitywho was not involved in the study.
The shadow test
According to the standard Big Bang theory, the massive X-rayemitting galaxy clusters near our MilkyWay galaxy should all display this shadow effect.
However, of 31 nearby galaxy clusters examined by Lieu's team,only some of them showed evidence of the distortion. Furthermore, the team claimsthe effect they did detect was only about one-fourth of that predicted bytheory.
If correct, Lieu's findings would be a serious blow to the BigBang theory, currently the best model scientists have for explaining thecreation of the universe,said astrophysicist David Spergel of Princeton University, who was not involved inthe study.
"The CMB is one of the central pillars of the hot Big Bangtheory," Spergel told SPACE.com.
But Spergel says he seriously doubtsthe conclusions reached by Lieu's team are correct for a number of reasons.First, WMAP, one of the instruments used by Lieu's team, is not the bestinstrument for detecting the shadow effect, Spergelsaid. The shadow effect "occurs on small angular scales predominately,while WMAP is designed to look at large scales across the sky," he said.
Secondly, other astronomers have confirmed the shadow effect inother galaxy clusters using not only WMAP, but also with ground-based radiotelescopes, which have higher resolution and are thus better able to spot theeffect.
Lieu counters that WMAP's resolutionmight be a problem for far away galaxy clusters, but points out that theclusters he examined were relatively close by, and certainly close enough forWMAP to see a shadow effect if it existed.
"The WMAP's resolution is not anexcuse here," Lieu said.
Afshordi, the Harvard astrophysicist,suggested that a more likely explanation for Lieu's findings is that there issomething about galaxy clusters scientists don't yet understand.
"I think that even if Lieu were correct, it would teach usabout clusters rather than the Big Bang theory," Afshordisaid in a telephone interview. "Clusters are complicated things andthere's still a lot to learn about them."
Lieu concedes this is a possibility. "That I do buy,"he said. "I myself am not at this point prepared to accept that the CMB isnoncosmological and that there was no BigBang. That would be doomsday."
Lieu said that one unlikely, but possible explanation is thatthe galaxy clusters he examined are unusually strong emitters of radio waves, whichcould have prevented the shadows from being seen.
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