Shedding light on the Universe
Things took a cosmic turn here at NAM during the exotic sounding session on the 'epoch of reionisation'. Basically, the early Universe was filled with a fog of opaque hydrogen, an era nicknamed the Dark Ages, which was gradually eroded away by harsh ultraviolet radiation, probably from the first giant, hot stars in the first galaxies. Trouble is, our radio telescopes are not yet sensitive to detect this hydrogen and pin down when it really began to erode away. In one talk this afternoon, Ilian Iliev of the University of Sussex described how simulations could help shed some light on the matter. His simulations showed that the first 20 million proto-galaxies began to emerge at redshift 40 (which is a stone's throw from the big bang in cosmological timescales), and the process of reionisation, in which the hydrogen fog was 'burned' away, could begin. By redshifts of 6 or 7, about 13 billion years ago, the fog had more or less all gone on large scales, which we can see in deep Hubble Space Telescope images.
There's another problem though, highlighted in the talk that followed by Milan Raicevic of the University of Durham. He reckons that any workable model of the epoch of reionisation must include theories of galaxy formation. Think about it. As the ultraviolet radiation reionises the gas, it also heats it. Hot gas does not collapse well into stars and galaxies. There's potentially something of a paradox there, and we've got to figure out how the process manages to balance itself. So by learning about reionisation, we're also learning about the first galaxies!
Image: Artwork of the first stars, courtesy of NASA/WMAP Science Team.