Somewhere in the cosmos, something is blocking certain wavelengths of starlight from reaching Earth. In a report in the December Astrophysical Journal, scientists say they may have identified the culprit: the simplest and most abundant molecule in the universe, molecular hydrogen.
Astronomers study the composition and motions of stars by spreading their light into bright smears of color--spectra. But they believe that dark bands in the spectra, known as diffuse interstellar bands (DIBs), come not from the stars but from something in interstellar space. These mysterious atoms or molecules must have energy levels spaced so that photons of starlight can boost electrons to higher levels and thus get absorbed. One suspect has been clouds of carbon compounds, either chains or ringed structures known as polycyclic aromatic hydrocarbons.
But astronomers may have the wrong suspects, contend Peter Sorokin and James Glownia, laser spectroscopists at IBM's Thomas J. Watson Research Laboratories in New York. In a presentation on 28 October at a Council for the Advancement of Science Writing briefing in Baltimore, Sorokin argued that clouds of molecular hydrogen could be creating the bands when they are bathed in both intense ultraviolet light from the star and visible light. By absorbing an ultraviolet photon along with a visible one, a hydrogen molecule can make the jump to a higher energy level. Based on the known energy-level structure of the hydrogen molecule, the IBM scientists calculated that two-photon absorption could produce many sharp absorption bands corresponding very accurately to known DIBs.
Their complex scenario for how hydrogen creates the bands has left many astronomers skeptical. According to Alexander Tielens of NASA Ames Research Center, "we don't really see a correlation between DIBs and the abundance of [hydrogen]," which would be expected if the theory were true. "They might get it right eventually," adds Theodore Snow, director of the Center for Astrophysics and Space Physics at the University of Colorado, Boulder, "but they don't have it right yet."