The early Universe had no stars and so there were no sources of light — this is known as the Dark Ages. The formation of the first stars is actually quite a complex and interesting problem. There are no heavy elements in the Universe at this time (as they are formed in within stars) and this limits the ability of gas to cool down and condense into the molecular clouds from which stars form. Nevertheless stars do eventually form in high-density regions that will go on to form clusters of galaxies today. The effect of these first stars on their surroundings is of great interest: they could induce other stars to form around them, or their radiation may suppress star-formation in their neighbourhood. We are investigating this process by undertaking simulations of the regions containing these first stars.
The first star clusters first began to ionise the Universe around them. Subsequently, stars in larger systems and emission from supermassive black holes added to the ionising flux, until the Universe became almost fully-ionised. Radio telescopes are now being built that will be capable of detecting this transition from neutral to ionised. LOFAR (already running) will be able to detect only the largest structures corresponding to the final stages of reionisation, but the Square Kilometre Array (SKA) will be able to map the distribution of neutral and ionised hydrogen in the very first structures, to redhsifts of 50 or so — far beyond the reach of any other telescope. We are uundertaking simulations of the reionisation process so as to be able to extract the maximum amount of useful information from these observations.
Galaxy and cluster formation:
One of the major outstanding problems in Astronomy is that of glaxy formation. We have a basic understanding, but the simplest models predict galaxies that are far too large. This necessitates strong feedback of mass and energy from both supernovae and massive black holes, heating gas and expelling it from the galaxy. This is evidenced by the existence of metals in both the intracluster and intergalactic media, and by the requirement of strong heating to expel gas from the centres of clusters of galaxies. We are modelling this process by taking the best analytic models of galaxy formation and inserting them into simulations of clusters of galaxies.
Large-scale structure and Baryon Accoustic Oscillations (BAOs)
The distirbution of galaxies on the largest scales relects both the fluctuations embedded in the cosmic microwave background radiation, and their subsequent growth as the Universe ages. Together, these provide interesting probes of the nature of the Dark Energy that today dominates the mass density of the Universe. Sussex is involved in some of the major observational surveys to measure BAOs, and also in undertaking simulations of very large-scale structure so as to be able to relate those observations to the underlying properties of the Dark Energy.