Verschuur was studying data from the first ever all-sky survey of interstellar neutral hydrogen (HI) when he noticed intriguing similarities to the structure observed by the Microwave Anisotropy Probe (WMAP) spacecraft.

WMAP was designed to detect faint variations in the cosmic microwave background, a pervasive signal left over from the Big Bang itself. This anisotropy may represent the first step in the structural evolution of the universe, a middle ground between the ultra-smooth cosmic microwave background and the clusters of galaxies that exist today.

The anisotropy detected with WMAP confirms a discovery made a decade earlier by the Cosmic Background Explorer (COBE) spacecraft. As a result, the COBE scientists won the 2006 Nobel Prize in Physics.

However, if even a small fraction of the anisotropy can be associated with structure in the Milky Way, the cosmological interpretations of the data could be called into question.

Verschuur, a pioneer in the science of radio astronomy, has been studying the properties of the Milky Way using interstellar HI for almost 50 years. According to his recent work, it appears that many of the small-scale structures observed by WMAP are correlated with HI.

To describe Verschuur's discovery as controversial would be an understatement. "I realize that my results may not be readily accepted by traditional cosmologists," Verschuur says, "but I hope they will at least consider the possibility that their data may have been compromised by what appears to be a previously unidentified source of weak radio emission originating from our own Galaxy."

Verschuur points out that the WMAP researchers went to enormous lengths to remove contributions from known physical processes occurring in the Milky Way, but the WMAP signals associated with the HI may result from a previously unidentified process. "No one could have foreseen the existence of this newly discovered relationship," Verschuur continues. "After all, a comprehensive HI survey for the northern sky was only completed in 1997 and the all-sky version became widely available even more recently, in 2005."

It will be challenging for cosmologists to prove that the weak radio signals detected by WMAP are not originating from the Milky Way. Verschuur points out that a more fruitful approach may be to determine whether or not something new can be learned about the physics of interstellar HI structure by considering examples of the close associations between HI and the anisotropy observed by WMAP.

As a student of interstellar HI, Verschuur is excited by his new discovery. "I now treat the WMAP peaks as markers that indicate where to focus my efforts," he says.

Further study of the relationships between HI and small-scale structure observed by WMAP will surely reveal a host of new information about the physics and dynamics of our own Milky Way Galaxy.

The new discovery, if confirmed, means that the structure superimposed on the cosmic microwave background is produced in the Milky Way and does not have a cosmic origin. Thus the cosmic microwave background signal from the early universe may be smoother than anyone expected, which raises new questions as to how structure ever emerged in the universe to create galaxies.

Verschuur holds out the hope that bright young theorists will tackle this problem and that some members of a new generation of radio astronomers will focus on the Milky Way to study interstellar HI using the WMAP signals to target their research.

The first inkling that a Milky Way component was still present in the WMAP all-sky image came while Verschuur was preparing illustrations for his new book on radio astronomy, The Invisible Universe, published in early 2007. He had available a color print of the all-sky image of the WMAP data and another of the HI data (integrated over all velocities frpm �450 to 400 km/s) to the same scale.

He couldn't hellp noticing intriguing similarities between structures in the two maps. That seemed to merit a closer look. At the time he had been working with the HI data as part of an unrelated project and had available HI maps at dozens of velocities that could quickly compared to details in the WMAP structure. Note that due to motion of the gas in space, the HI emission is observed over a wide range of velocities introduced by the Doppler effect, which shifts the observed frequency of the HI emission by slight amounts.

The WMAP data were subsequently made available to him by Wayne Landsman and Gary Hinshaw of the WMAP team based at the Goddard Space Flight Center.

Verschuur has produced over 1,000 maps of HI structure in small areas of sky for comparison with the WMAP data. "These comparisons reveal hundreds of close associations between HI and WMAP features and it will take years to fully report this work. This field is wide open for others to get involved.

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