May General Meeting
Seeing Light at the end of the Dark Ages
Stuart Wyithe University of Melbourne.
Like the dark ages of modern history, the cosmic dark age is a period about which little is known. Scientists can't even figure out exactly when or how it ended, but it seems to have run for at least half a billion years. It is a crucial stretch of time, because whatever went on then led to everything we know today -- the rules for star formation and galaxy creation and even the cosmic dust that makes up us were laid out.
Stuart opened his talk by reviewing the present knowledge of cosmic evolution after inflation: the COBE map and its implications; Dark Matter and Dark Energy and how they drastically altered our picture of the cosmos, and the 2dF Survey that led to the Sloan Digital Sky Survey. He followed with a brief review of our theoretical understanding, describing the basic physical ingredients leading to the formation of the first sources of light and their observable properties as well as the expected properties of the sources responsible for the re-ionization of hydrogen. He talked about the present observational constraints and the possible results from ongoing efforts, and finally highlighted the possibilities of going beyond the limits of the Hubble Space Telescope in the next decade with the 7m James Webb Space Telescope after its launch in 2010.
The Cosmic Dark Age initially started out as a theoretical stop-gap to fill the evolutionary void between the imprint of the Cosmic Background Radiation some 300,000 years into cosmic evolution and when the first stars or galaxies started to shine. This duration could, in the absence of any recognisable markers, have been B like the first day in Genesis B any length, but astronomers, by delving relentlessly further and further into the past, have narrowed the gap of the void. By observing distant supernovas and quasars modern telescope giants together with the Hubble Space Telescope have B by using clever innovations such as gravitational lensing B been pushing the birth of starlight to Redshift z~6.8; within a billion years of the beginning of our universe, the so-called Big Bang.
Theories built on the Dark Energy concept as we know it have constrained the time elapsed since the Big Bang to around 13 to 13.7 billion years. About 300,000 years later the Universe had cooled to about 3000K, sufficiently for protons and electrons to combine into neutral hydrogen atoms. As atomic hydrogen absorbs and consequently blocks the passage of light, this must have caused an instant black-out in the early Universe, which lasted millions of years until stars formed and galaxies evolved and started to radiate their brilliant light into space. The ultraviolet component of that light re-ionised the ubiquitous neutral hydrogen, blazing a trail for visible light to follow. The new found high-z quasars are some of the first objects to be seen emerging from the foggy dawn following the Dark Age of the Universe, roughly one billion years after the Big Bang.
Many of the present-day properties of galaxies appear to have been established between redshifts of z ~7 and z ~1, which represents the transition between the Aprimordial'' Universe and the Amature'' Universe seen at z < 1. This is the epoch during which more than half of all the stars in the Universe were formed. However the details of how galaxies were assembled into this period, and the relative importance of different physical processes remain quite unclear. Possible clues and explanations may well be revealed by studying the preceding Dark Age. Thus the period has become something like the final frontier of astronomy.
It is unclear over what length of time the first generation of stars, galaxies and their quasars cores established themselves. Much of the information for research into this field so far, including the most distant quasar yet discovered z~6.6 (7), comes from the Sloan Digital Sky Survey, an $80m international collaboration of telescopes with headquarters in New Mexico Based on the current sampling the ASU researchers estimate that at least 400 million such objects filled the entire universe when the curtain on the Dark Age lifted (to the limit of the Hubble deep space image). These are unlikely to have all formed at once. With NASA's planned 7 metre James Webb Space Telescope it is expected to see many more if not the entire population of these proto-galactic objects and their emergence at the dawn of the new cosmic epoch.
What ended the Dark Ages in our human history? Was it a single event like the Pandects at Amalfi in 1137CE, or a combination of effects? It would be even more fascinating to learn whether stars, quasars or normal galaxies ended the cosmic dark period and opened the beauty of the night sky to our eyes. The kind of research Stuart Wyithe is doing will help answer that question in time, and that answer feeds right back into fundamental cosmology.
"When the stars
threw down their spears
And watered heaven
with their tears
did He smile
his work to see?"
from William Blake, "The Tiger"