FUSE Understanding the Big Bang

In the infancy of the Universe, the extreme conditions present everywhere gave rise to the creation of simple chemical elements out of which all matter was made. The simplest element, hydrogen, consists of a single proton orbited by a single electron, as shown at left (top). In some instances, these hydrogen atoms also have a second particle called a neutron in the nucleus accompanying the proton; this type of hydrogen is called deuterium (left, bottom). More complicated elements consist of atoms having larger numbers of protons and neutrons in their nuclei surrounded by correspondingly higher numbers of electrons.

When atomic nuclei formed in the early Universe, the conditions were so extreme that electrons were unbound to the nuclei and moved about freely. Gas with this property is known as plasma. In this plasma, some of the hydrogen was converted to deuterium, and some of the deuterium was converted to helium. The relative amounts of each element produced by this nuclear fusion of protons and neutrons were very sensitive to the temperature and density of the particles in the plasma at that early time. As the Universe expanded, the plasma cooled, the creation of elements ceased, and the free electrons and nuclei combined to form complete atoms.

It is the sensitivity of the nuclear reactions in the primordial plasma to the initial conditions in the Universe that makes astronomers interested in studying the simple elements today. By measuring the relative amounts of each element, it is possible to infer the conditions present at a time before complete atoms existed! In particular, knowing the ratio of deuterium atoms to hydrogen atoms left over from the Big Bang would allow astronomers to place a strong constraint on how much observable matter there is in the Universe.

Alas, Nature does not reveal secrets such as these so easily -- the abundances of some elements have changed over time. The interior cores of stars are hot enough (tens of billions of degrees) to mimic those conditions in the first few minutes of the Universe and convert deuterium into helium by the addition of another proton to the deuterium nucleus. Astronomers thus believe that the total amount of deuterium in the Universe is decreasing as matter gets cycled through stars, but they do not know how fast it is decreasing or how much deuterium has already been destroyed.

This is where FUSE enters the quest to understand our cosmic origins. Astronomers are using FUSE to search for deuterium in the interstellar medium near the Sun, in gas clouds in the far reaches of the Milky Way, and in distant intergalactic clouds between galaxies. By measuring the amount of deuterium relative to both hydrogen and the heavier elements such as oxygen produced by stars, they will be able to estimate how much deuterium has been destroyed since the Big Bang. This, in turn, will allow them to understand how galaxies evolve and to discover what the Universe was like when it was only a few minutes old.