Cosmological shadows? Theory predicts that objects between us and the source of the cosmic microwave background should cast them. Specifically, the hot gases found in clusters of galaxies should show a measurable shadow effect produced by that background radiation, and there are reports of such effects from various observers. The scattering of the cosmic microwave background by high-energy electrons is known as the Sunyaev-Zel’dovich effect.

However, a new study from the University of Alabama at Huntsville raises real problems. The first to study the phenomena with data from the Wilkinson Microwave Anisotropy Probe (WMAP), the team reports an oddly sporadic shadow effect for a background thought to be afterglow radiation from the Big Bang.

And that raises questions about the Big Bang model itself. Says physicist Richard Lieu, after an investigation involving 31 clusters of galaxies:

“These shadows are a well-known thing that has been predicted for years. This is the only direct method of determining the distance to the origin of the cosmic microwave background. Up to now, all the evidence that it originated from as far back in time as the Big Bang fireball has been circumstantial.”

And Lieu’s team is reporting that among the clusters studied, some showed the shadow effect and some did not. In fact, the actual shadow effect is about one-fourth of what was predicted. Which is decidedly odd, for the microwave background radiation’s effects ought to be obvious. “If you see a shadow…it means the radiation comes from behind the cluster,” Lieu adds. “If you don’t see a shadow, then you have something of a problem.”

All of which is certain to raise controversy and a spate of new studies using the publicly-available WMAP data. The Huntsville team examined hot ionized gases at the center of galaxy clusters; the free electrons there interact with the background radiation to create the shadow effect. Because the observed shadow effect more or less equals in strength the natural variations already observed in the microwave background, the question becomes acute. If the background radiation is closer than the galaxy clusters under study, what exactly is it and how did it arise? And what does this mean about the Big Bang?

The paper is Lieu, Mittaz and Shuang-Nan Zhang, “The Sunyaev-Zel’dovich effect in a sample of 31 clusters: A comparison between the X-ray predicted and WMAP observed decrement,” Astrophysical Journal, Sept. 1, 2006, Vol. 648, No. 1, p. 176. Now available here on arXiv.