Temperature/covering factor as a function of redshift. The symbols represent galaxies in which cold neutral hydrogen has been detected and the arrows represent lower limits. This plot in conjunction with our other work shows that apparently high temperatures are produced when the covering factor is (incorrectly) set to unity. The highest redshift of 3.4 corresponds to a look-back time of 11.6 billion years.

Astronomical observations allow us to look into the past, as the light reaching earth now has been travelling from its source for billions of years. Astrophysicists at UNSW have been using these measurements to discover more about the evolution of the universe.

Neil Crighton, John Webb and colleagues studied a high redshift quasar to determine the abundance of deuterium in the young universe. Deuterium is important because it was produced in the first few seconds of the life of the universe and essentially none has been produced since. The amount of deuterium made depended sensitively on the density of the universe and so determining the deuterium abundance today provides a unique probe of one of the fundamental parameters of cosmology. Interestingly, the value we found seems to disagree with independent estimates from the microwave background. This remains a puzzle at the moment.

In last year’s report John Webb, Steve Curran and collaborators at Cambridge presented the first ever measurement of the cosmological evolution of molecular hydrogen abundance in the earlier Universe. Since then, we find that the temperature of the neutral hydrogen gas in these dense clouds, which intersect the lines-of-sight to distant quasars, does not necessarily increase with look-back time, as is currently believed, and that the observed effect is due to the smaller absorbing cross sections at larger look-back times. This is contrary to the current consensus, but is consistent with hierarchical galaxy formation scenarios, where the population of dwarf galaxies in the early Universe have merged to form the larger, near-by (and hence more recent) spirals.

Dmitriev, Victor Flambaum and John Webb used observations of the helium abundance (4He is produced in the big bang) in conjunction with CMB data to explore whether the laws of physics were the same in the early universe as they are today. This general topic is currently a very active area of physics and astronomy, following our previous tentative measurement of a tiny change in the value of one of the constants of Nature (the fine-structure constant).

John Webb, Steve Curran, Neil Crighton
and Victor Flambaum