A study of nearby galaxy clusters has failed to detect distortions in the ancient microwave radiation many scientists have linked to the 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 space probe used in the study is not the ideal instrument for detecting the distortions and that the discrepancy is less a reflection of problems with the Big Bang theory than of how little scientists really know about galaxy clusters.
The controversial study, led by Richard Lieu at the University of Alabama in Huntsville, is detailed in the Sept. 1 issue of the Astrophysical Journal.
Big Bang echo
The cosmic microwave background (CMB) radiation is a faint afterglow permeating the universe. Many scientists have hailed CMB observations as strong evidence for the Big Bang.
A shadow effect called Sunyaev-Zel'dovich is a distortion that affects CMB photons inside galaxy clusters. It occurs when high-energy electrons inside the clusters crash into the more sluggish CMB photons, boosting them to higher energy levels.
This shifts the CMB spectrum inside clusters from low to higher energies. The dearth of low-energy microwaves inside the clusters means that instruments like the Wilkinson Microwave Anisotropy Probe (WMAP) should see fewer low-energy CMB photons inside clusters compared to outside.
"That's the shadow effect—that at lower energy you see the CMB has a decrement in the direction of the cluster," explained Niayesh Afshordi, an astrophysicist at Harvard University who was not involved in the study.
The shadow test
According to the standard Big Bang theory, the massive X-ray emitting galaxy clusters near our Milky Way 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 claims the effect they did detect was only about one-fourth of that predicted by theory.
If correct, Lieu's findings would be a serious blow to the Big Bang theory, currently the best model scientists have for explaining the creation of the universe, said astrophysicist David Spergel of Princeton University, who was not involved in the study.
"The CMB is one of the central pillars of the hot Big Bang theory," Spergel told SPACE.com.
But Spergel says he seriously doubts the 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 best instrument for detecting the shadow effect, Spergel said. 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 in other galaxy clusters using not only WMAP, but also with ground-based radio telescopes, which have higher resolution and are thus better able to spot the effect.
Lieu counters that WMAP's resolution might be a problem for far away galaxy clusters, but points out that the clusters he examined were relatively close by, and certainly close enough for WMAP to see a shadow effect if it existed.
"The WMAP's resolution is not an excuse here," Lieu said.
Afshordi, the Harvard astrophysicist, suggested that a more likely explanation for Lieu's findings is that there is something about galaxy clusters scientists don't yet understand.
"I think that even if Lieu were correct, it would teach us about clusters rather than the Big Bang theory," Afshordi said in a telephone interview. "Clusters are complicated things and there'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 is noncosmological and that there was no Big Bang. That would be doomsday."
Lieu said that one unlikely, but possible explanation is that the galaxy clusters he examined are unusually strong emitters of radio waves, which could have prevented the shadows from being seen.
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