In part 1 of this series I described why the Higgs boson is called the “God particle,” and why the sterile neutrino may be much more deserving of the title. In this article I will review astronomers’ dogged searches over the past twenty years to discover sterile neutrinos and why these searches, to everyone’s surprise, have come up empty so far. I will also describe how these failures have, nevertheless, infused astronomers with new hope that the discovery of sterile neutrinos is at hand.

I frequently use my three-foot crowbar in home-improvement projects around my house. For several months, however, it was lost. I looked in all the reasonable places: my construction sites under the house and in the attic, the garage, and all the closets. Nothing. It wasn’t until I cleaned out the old toys, books, and games my youngest son had crammed under his bed that I found my missing crowbar. Why, I asked David, did he put my crowbar under his bed? He explained he was having bad dreams and had put the crowbar there for self-defense. For me, a crowbar was a construction tool. I never thought of its utility as a weapon. Likewise, astronomers, in focusing on one means for sterile neutrino production, had ignored another and thus failed in their attempts to find their object.

As with my crowbar, astronomers have had no doubts that sterile neutrinos must exist. Sterile neutrinos explain far too many anomalies in astronomy and physics for them not to exist. Still, astronomers and physicists have yet to produce any positive detection that proves sterile neutrinos’ existence.  

Elusive Particles
The “yet” may be short-lived. While sterile neutrinos do not interact with ordinary matter, they can decay. Back in 1990, two physicists at the University of Pisa in Italy, Riccardo Barbieri and Alexandre Dolgov, calculated that with a half-life of more than the age of the universe, sterile neutrinos could decay virtually at rest into a photon and an active neutrino.¹ Such decay, given the anticipated mass of a sterile neutrino (a few millionths of the proton’s mass), would produce a comparatively sharp spectral line at X-ray wavelengths.

Detecting the spectral line from the decay of sterile neutrinos is not easy. Almost everywhere astronomers point their telescopes the X-ray background radiation is expected to overwhelm the signal from sterile neutrino decay. Moreover, the signal from sterile neutrino decay may not be as strong as astronomers once thought. Many, if not most, of the missing dwarf and subdwarf galaxies are starting to turn up in deep surveys of galaxies.² Stars do form early in cosmic history, but according to the final release of the WMAP of the cosmic microwave background radiation (left over from the cosmic creation event), the epoch of first star formation now measures significantly later than what the WMAP first release indicated.³ Thus, the expected number density of sterile neutrinos in the universe is such that these particles almost certainly do not comprise the major fraction of the exotic matter in the universe.

The most frequently exercised detection technique involves pointing the best existing X-ray telescopes at certain regions of the exotic dark matter halos surrounding galaxies where no other known X-ray sources exist. Initial searches already have been attempted for both nearby⁴ and distant⁵ galaxies. So far, the best attempted measurements were performed on Segue 1, a dwarf galaxy with the highest known dark matter density, by the X-ray telescope onboard the Swift satellite (see figure 1)⁶ and on the ultra-faint dwarf galaxy Willman 1 by Chandra X-Ray Observatory (see figure 2).⁷ The measurements of Segue 1 produced a meaningful upper limit to the density of sterile neutrinos in the 1.6–14 keV mass range, while the measurements on Willman 1 yielded a marginal detection (68 percent confidence of actual detection) consistent with a sterile neutrino mass of 5 keV.

Figure 1: The Swift Spacecraft
Launched into orbit in 2004, this spacecraft is a multi-wavelength observatory. It contains three telescopes: the BAT that makes observations at gamma-ray wavelengths, the XRT that observes at X-ray wavelengths, and the UVOT that observes at ultraviolet and optical wavelengths.
Image credit: NASA

Figure 2: The Chandra X-Ray Observatory
This observatory was launched into orbit in 1999. It is shown here inside the payload bay on Space Shuttle Columbia mission STS-93. It ranks as the biggest and most sensitive X-ray telescope in existence. It is the third of NASA’s four Great Observatories residing in space.
Image credit: NASA

Looking in the Right Places
For the first time astronomers have discovered an X-ray signal that qualifies as more than just a possible detection of sterile neutrinos. In a paper published in the December 20, 2010 Astrophysical Journal Letters A,astronomer Dmitry Prokhorov and famous physicist Joseph Silk, author of several books on big bang cosmology,⁸ first point out where astronomers may have gone wrong in their search for sterile neutrinos.⁹ They refer to X-ray observations of dwarf galaxies that apparently rule out measurable production of sterile neutrinos by Barbieri and Dolgov’s proposed mechanism.

Prokhorov and Silk suggest instead that sterile neutrinos are produced from Higgs decays, or more likely, the decay of a very light inflaton. Inflatons refer to particles responsible for generating the scalar field that drove the brief and rapid expansion of the universe between 10-35 and 10-34 seconds after the cosmic creation event—the inflationary era in big bang cosmology. At the very end of 2010, three theoretical physicists submitted a preprint in which they conclude that Silk and Prokhorov’s proposed very light inflaton is probably the axion.¹⁰

For nearly a decade now, both astronomers and theoretical physicists have touted axions as the particles that make up the majority of the universe’s exotic matter. Thus, Silk and Prokorov’s proposal could not only solve the problem of the missing sterile neutrinos but also identify the kinds of particles making up most, if not all, of the universe’s exotic matter (predominantly axions complemented by active and sterile neutrinos).

Next week, I will recount where astronomers may have already found sterile neutrinos without realizing it. In part 4, I will explain the connection between sterile neutrinos and another “God particle,” the axion. I will describe where astronomers can quickly garner confirming evidence for both sterile neutrinos and axions, and conclude with a summary of the theological advances we can expect from these discoveries, both current and emerging.