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.
Another explanation
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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