The paper is highly cited as it offers one of the tightest constraints on the cosmological baryon density. The baryon density is a fundamental cosmological parameter, and is used as an input into solutions across a wide range of astrophysical problems. Also, the number of systems from which the primordial D/H ratio can be measured in the intergalactic medium is, at present, quite small, so each system tends to be given a lot of analysis.

  Does it describe a new discovery or new methodology that's useful to others?

The paper reports the discovery of a new place in the universe where we can constrain the baryon density to high accuracy.

This research is one of the core focus items in our group, and has been so for many years. The baryon density is an important cosmological parameter, and as such has had a large amount of "telescope-time" devoted to it.

One of the fundamental goals in cosmology is to constrain a number of parameters which describe the properties of the universe as a whole. One of these parameters is the density of normal matter, such as protons and neutrons, also known as the baryon density. One way to measure the baryon density is to determine the relative amounts of atoms which were made very early in the lifetime of the universe (at times approximately 1 minute after the Big Bang) and which have been left alone since. The process through which these atoms are made is sensitive to the baryon density, so if we can measure the amounts of these atoms, we can determine the baryon density. This paper reports the measurement of the amount of Deuterium, an isotope of Hydrogen, relative to Hydrogen in the intergalactic medium. Such measurements are to date very rare, since the systems we could measure the D/H ratio in are often too complex to obtain a result. The D/H ratio obtained in this system is then combined with other measurements made previously, and the baryon density as measured by D/H, is reported. The end result is that only 4-5% of the universe is composed of normal matter, leaving the remaining 95% as either dark matter, dark energy, or some combination of the two.

John M. O'Meara
Graduate Student
Center for Astrophysics and Space Sciences
University of California, San Diego