In 1994, I wrote an article for Christianity Today describing how astronomers had found “the Holy Grail of cosmology.”1 That is, they had determined the proportion of the universe comprised of ordinary matter (protons, neutrons, and electrons) by analyzing high-sensitivity maps of the temperature fluctuations in the cosmic microwave background radiation (CMBR). This determination yielded the best scientific evidence to date that the universe arose from a big bang event, a scenario that aligns with ancient biblical descriptions of the origin of the cosmos.2
In light of the fact that scientists have values for the cosmic expansion rate and the fraction of ordinary matter in the universe (and that improved CMBR maps have produced improved calculations), recent attempts to obtain these same values from measurements of primordial helium and deuterium may seem unnecessary. However, primordial abundance measurements provide astronomers with a much-needed second set of calculations, based on different physical principles and using different detection instruments, with which they can independently check the values of these cosmological parameters and eliminate possible instrumental biases.
Deuterium, a nucleus comprised of one proton and one neutron, especially interests cosmologists. Unlike helium, deuterium was produced only during the first few minutes of cosmic history when hydrogen (the one existing element) was partly fused into helium-4, helium-3, deuterium, and a trace amount of lithium-7. The mass fraction of helium-4 produced depends on how rapidly the universe expanded from the cosmic creation event, while the mass fraction of deuterium depends on the proportion of ordinary matter in the universe.
Observing the primordial deuterium abundance has proven to be a very tricky business. Deuterium spectral lines are much weaker than helium spectral lines—plus, stellar burning has consumed a substantial amount of primordial deuterium. Astronomers must observe gas clouds far enough away that the look-back times predate any significant stellar burning—yet such great distances mean that deuterium spectral lines will be extremely weak. But now, by using the world’s largest telescopes, a dedicated group of astronomers has overcome the hurdles.
Challenges to Observing Deuterium
The primordial abundances refer to the quantities of helium and deuterium extant before any significant stellar burning occurred. Once stars start burning nuclear fuel, they produce helium and consume deuterium. Thus, measurements of the primordial abundances yield the values of important cosmological parameters while measurements at various epochs since star formation onset give astronomers a detailed picture of the history and locations of star formation and stellar burning for the universe in general and the Milky Way Galaxy in particular. Such a picture has important ramifications for establishing the degree of galactic fine-tuning needed for the existence of advanced life.
Initially, astronomers attempted to measure the primordial deuterium abundance in distant quasars. It became clear, however, that even in distant quasars stellar burning was not a trivial factor. Astronomers then turned to damped Lyman-alpha (DLA) systems, huge clouds of dense gas in the earliest stages of collapsing to form galaxies. Such systems typically contain only a small number of newly formed stars and thus consumption of deuterium by stellar burning is close to zero. The problem here is that DLA systems emit undetectable light levels, which limits astronomers to observing DLA systems in absorption (that is, observing the degree to which a DLA system absorbs light from a background quasar).
First Observation Attempts
The first serious attempt to measure the primordial deuterium abundance was made in 2001. Using the Hubble Space Telescope, astronomers Max Pettini and David Bowen observed a DLA system 10.60 billion light-years away, which yielded Ωbh2 = 0.025±0.001,3 Ωb where is the fraction of the universe comprised of ordinary matter and h = 100 divided by the value of Hubble’s constant (the cosmic expansion rate) in kilometers per second per megaparsec (1 megaparsec = 3,258,000 light-years). However, Pettini and Bowen faced uncertainties in determining the abundance of elements in the system heavier than lithium (a measure of the amount of stellar burning that has occurred).
A decade later, Pettini and astronomer Ryan Cooke used the Very Large Telescope (the world’s largest optical telescope) to measure deuterium in a DLA system 11.61 billion light-years away in which the density of elements heavier than lithium was just one percent of the solar value,4 putting the loss of deuterium by stellar burning at an insignificant level. Pettini and Cooke’s observations produced a value for Ωbh2 = 0.0223±0.0009.5
Success at Last
Now, in 2014, Cooke and Pettini, together with three other colleagues, have published results of their observations of deuterium in a DLA system 11.70 billion light-years away in which the density of elements heavier than lithium is a scant 0.13 percent of the solar value.6 Their derived value for Ωbh2 = 0.02202±0.00046.7 This is the first time that a measurement of Ωbh2 based on deuterium has delivered comparable precision to those based on CMBR maps.
The best measures of Ωbh2 from maps of the CMBR are 0.02205±0.00028 from the Planck Collaboration8 and 0.02243±0.00055 from the WMAP team.9 Using the best-determined value of h = 68.30±0.81,10 these measures translate into the following estimates of the proportion of ordinary matter in the universe: 4.72 (Cooke et al.), 4.73 (Planck), and 4.81 (WMAP) percent.
The remarkable consistency in these measurements delivers yet another evidence for big bang cosmology, which, in turn, strengthens the case for the biblical account of creation. The Bible describes several key characteristics of a big bang universe, including a singular beginning of matter, energy, space, and time, ongoing cosmic expansion, and constant laws of physics including a pervasive law of decay, but leaves many other details, such as the rate of cosmic expansion and the mass of the universe, for us to measure. Once again, research continues to affirm the correspondence between the biblical description of the universe and the best scientific understanding of the universe.